Lava dome growth and gas-and-steam emissions during August-November 2022

Mayon, located in the Philippines, has steep upper slopes capped by a small summit crater. Recorded eruptions date back to 1616 CE and are characterized by Strombolian eruptions, lava flows, pyroclastic flows, and mudflows. Eruptions mostly originated from a central conduit. Pyroclastic flows and mudflows have commonly descended many of the approximately 40 drainages that surround the volcano. The most recent eruption occurred during January 2018 through October 2019 and included phreatic explosions, gas-and-ash plumes, lava fountaining, and pyroclastic flows (BGVN 43:04). Based on ground deformation parameters from EDM, Precise Leveling, electronic tilt and continuous GPS monitoring, slight inflation has been observed since 2020. More recent activity has been relatively low, consisting of small thermal anomalies, gas-and-steam plumes, and some inflation (BGVN 44:11). This report covers a new eruption during June through November 2022 using daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Low activity was reported during June; minor inflation, particularly on the NW and SE flanks have been recorded since April 2022. According to PHIVOLCS, changes in morphology of the summit lava dome emplaced in 2018 resulted in the increase in the Volcano Alert Level (VAL) from 0 to 1 (on a scale of 0-5). Minor extrusion was estimated at about 40,000 cubic meters during 6 June through 20 August, based on daily visual and camera monitoring data. Seismicity was at baseline levels for most of 2022, though short-lived spikes in the number of low-frequency volcanic earthquakes were recorded on 26 May and 20 June. PHIVOLCS reports that based on this data, dome growth was likely the result of gas pressurization at shallow depths.

Activity during August 2022 mostly consisted of moderate gas-and-steam emissions that rose as high as 900 m above the crater and drifted in different directions. Weather clouds often obscured the summit, thus crater incandescence could not be determined on a majority of days during early-to-mid-August. Sulfur dioxide emissions were measured on 12 August and were an average of 688 tons/day (t/d). During 25-31 August faint crater incandescence was reported based on telescope observations. Sulfur dioxide emissions decreased to 195 t/d based on measurements made on 30 August. Similar low activity continued during September; moderate gas-and-steam emissions rose 100-300 m and drifted in multiple directions. Faint crater incandescence was visible based on telescope observations on clear weather days. A single volcanic earthquake was detected on each day during 6-8, 13, 23-24, and 29 September. Sulfur dioxide emissions were measured on 6, 7, 13, and 24 September and ranged between 339 t/d and 599 t/d. Three volcanic earthquakes were detected on 26 September.

During October, moderate gas-and-steam emissions rose 200-800 m above the crater and drifted in different directions. Sulfur dioxide measurements were made on 1, 12, 13 October with values that ranged from 260 t/d to 391 t/d. Beginning on 2 October, re-mobilized, light-colored ash was deposited in Miisi Gully (S flank), which was likely derived from lava fragmentation as the lava dome continued to grow, according to PHIVOLCS. Three volcanic earthquakes were detected on 3 October. Daily visual and camera monitoring data showed that the lava dome grew an additional 48,000 cubic meters during 20 August through 4 October. One volcanic earthquake was detected each day on 5, 6, 17, and 23 October. Occasional faint crater incandescence was visible in clear weather and sometimes when using a telescope. On 7 October an aerial survey was conducted and reported that freshly extruded lava was visible at the base of the summit lava dome (figure 51). As a result, the VAL was raised to a 2 (on a 0-5 scale). There were two volcanic earthquakes detected each day on 19 and 22 October.

Figure 51. Aerial photos showing the summit lava dome at Mayon on 7 October 2022 with fresh lava effusions at the base (yellow arrows). Courtesy of PHIVOLCS.

Faint crater incandescence persisted on clear weather days during November. Gas-and-steam emissions also continued, rising 200-250 m above the crater and drifting NE, WNW, WSW, W, and SW. Three volcanic earthquakes were detected each day on 1 and 12 November. Sulfur dioxide emissions were measured on 3 and 23 November with values that ranged from 276 t/d to 788 t/d. A single volcanic earthquake was detected each day on 7, 10, 14, and 28 November.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

July 1968 (CSLP 06-68)

Increased activity

Information Report 28 (01 July 1968) Increased activity

The following report was received [at] 1847 GMT on 01 July 1968. "24-hour alert with increasing activity of Mayon and new eruption possibly in offing. Observance of increase in amount of steam emission from summit and more apparent crater glow. Also reported increase in tremors recorded by seismograph at site."

Information Contacts: Philippine Volcanology Commission via American Embassy, Manila, Philippines.

October 1968 (CSLP 06-68)

Steam and ash emissions followed by mudflows down all flanks

Card 0106 (08 October 1968) Steam and ash emissions; high seismicity

The following report was received [on] 2 October 1968. Reddish-brown ash smoke shot up from Mayon's peak at 1240 today, as the volcano stepped up its steaming activity. Mayon started emitting steam and white smoke at 0900, according to Vicente Belbis, COMVOL observer at the Mayon resthouse.

Seismograph recordings for the past 24-hour period ending at 0600 today showed that Mayon is still giving off volcanic tremors. The seismic level of Mt. Mayon remains high and its crater's emission shows that its activity was above normal. Mayon also puffed out ashes at noon yesterday.

What worried the volcanologists was the accumulated mud which could cascade down the volcano's side in the event of strong rains. Because of the threat of mudflows, they urged the safety measures remain in effect.

Card 0112 (09 October 1968) Decreased activity

The following reprt was received [at] 1300 on 8 October 1968. "Light increase level seismic activity and marked decrease in volume crateral steam emission characterized behavior Mayon for past 24 hours. Apparent that Mayon now at low point activity, which, as in past, usually characterized by ash puffs, crater glow and increase volume steam emission, expected within short period."

Card 0129 (14 October 1968) Moderate steam emissions from crater

The following report was received [on] 14 October 1968. "Mayon Volcano, Philippines. As of 12 October [at] 1100. Moderate crater steam emission persists. Low seismic activity. As of 13 October [at] 1300 steam activity continues in crater. Lull in seismicity. As of 14 October [at] 1200 steam emission now mild, but renewed volcanic microseisms of appreciable magnitude."

Card 0139 (15 October 1968) Moderate steaming and low seismicity; mudflow hazard

The following report was received [on] 15 October 1968. "Mayon status as of 15 October [at] 1415. Continuing moderate steaming in crater. Low seismic activity. Intermittent heavy rains, continuing for third day, soaking old and hardened mudflow to fluid masses. Additional mud and boulders coming from upper slopes. Together they pose greater danger to those inhabiting area. Expect new advances by old mudflow aside from possible fresh flow of mud and boulders from upper slopes."

Card 0142 (16 October 1968) Increased seismicity; mudflows down all flanks

The following report was received [on] 16 October 1968. "16 October 0800 Z Mayon exhibited marked increase seismic activity admidst down rushing of mudflows. Mudflows triggered by intermittent heavy rains seem to have affected all sectors around volcano yesterday afternoon. Initial OMCC report indicated that all known critical areas in the SW, S, and SE sectors hit again.

"NE sector, now only beginning to feel effects of mudflow, had its big river channel near Santo Domingo nearly filled by mud and boulders. Mudflow level now only about a meter below concrete bridge which previously stood 10 meters above riverbed. More mudflows expected as result of continued rain. OMCC and COMVOL repeat warning against mudflows. Concerned should follow maximum precaution."

Card 0144 (17 October 1968) Seismicity continues to rise; most stream channels filled with mud

The following report was received [on] 17 October 1968. "17 October 0500Z. Continued build-up in seismic activity seems to indicate Mayon pulling out of trauma to begin another period of peak activity. Low, heavy rain clouds blanket volcano. Apparent most stream channels already nearly filled with mud and boulders. Continue maximum precaution. Watch for possible mudflows."

Information Contacts: Card 0106 (08 October 1968) Vicente Belbis, COMVOL Observer, Legazpi City, Philippines.
Card 0112 (09 October 1968) Philippine Volcanology Commission via American Embassy, Manila, Philippines.
Card 0129 (14 October 1968) Philippine Volcanology Commission via American Embassy, Manila, Philippines.
Card 0139 (15 October 1968) Philippine Volcanology Commission via American Embassy, Manila, Philippines.
Card 0142 (16 October 1968) Philippine Volcanology Commission via American Embassy, Manila, Philippines.
Card 0144 (17 October 1968) Philippine Volcanology Commission via American Embassy, Manila, Philippines.

January 1969 (CSLP 06-68)

Steaming from summit area; faint night glow

Card 0324 (08 January 1969) Steaming from summit area; faint night glow

The following report was received from Kurt Fredriksson, who is at the site of the volcano, on 7 January 1968. "Walked to ash-mud flow south of rest house and climbed to 7,000 feet. All ashes have washed down. Only in protected places, a little left but not clear if material water reworked. Some induration by iron-oxide precipitation. Plenty steam from summit plus many vents mostly south and a little lower. One smoke hole about 100 m below summit on north side. No seismic activity except micro-tremors as recorded by seismograph at rest house. Observed faint glow late at night."

Information Contacts: Kurt Fredricksson, Smithsonian Institution.

November 1977 (NSEB 02:11) Cite this Report

Summit crater glow seen in early November; several new steam vents

Summit crater glow was observed between 2002 and 2125 on 6 November from COMVOL observation stations. Glow was continuous and deep red for the first 3 minutes, then became intermittent and yellowish in color. The next day, steam emission, under increased pressure, occasionally varied in volume and intermittently changed from the normal white color to brown. Several new steam vents had formed outside the crater, on the upper flank. Glow was again observed during the night of 8-9 November and volcanic tremor was recorded.

Reference. Moore, J.G., and Melson, W.G., 1969, Nuées ardentes of the 1968 eruption of Mayon volcano, Philippines: BV, v. 33, no. 2, p. 600-620.

Information Contacts: G. Andal, COMVOL, Quezon City.

February 1978 (SEAN 03:02) Cite this Report

Glow and harmonic tremor continue

"Crater glow at the summit, observed on 6 November, was followed by increased volcanic tremor 2 days later. Aerial investigations disclosed bluish fumes in the SW portion of the crater. At 0116 on 12 November an imperceptible volcanic earthquake with a rather large double amplitude was recorded, lasting more than one minute. Drying of vegetation on the upper flank (about 1,700 m elevation) was noted on 18 November. On 22 December, 13 large-amplitude volcanic tremors were recorded, accompanied by increased steam emission forming a cauliflower-shaped cloud. Glow, accompanied by volcanic tremor, was continuing as of late January."

Information Contacts: G. Andal, COMVOL, Quezon City.

May 1978 (SEAN 03:05) Cite this Report

Moderate eruption: lava flow, ash clouds, and evacuation

"An eruption started at about 2030 on 7 May and gradually increased in intensity, reaching a maximum on 22 May. The event was characterized by weak lava flow extrusion at the start, and at the height of the eruption there was a fascinating night display as incandescent basaltic [andesite] lava flowed down the SW flank. Strong earthquakes were felt at the Mayon Rest House Observatory (on the N flank, elevation 760 m, figure 1) on 26 May, accompanied by voluminous ejection of ash-laden clouds and spattering of incandescent pyroclastic materials. From then on, activity began to decline, punctuated at first by short lulls, which became longer each day until ash ejections, rumblings, and volcanic tremor ceased on 29 May. Small amounts of lava continued to be extruded as of 2 June. This decline in activity suggests that the critical period of the eruption has apparently passed.

Figure 1. Location map of Mayon volcano showing ground deformation and seismic network, after Corpuz (1985).

"No casualties were reported during the eruption, but 8,000 people on the SW sector of the volcano and within an 8 km radius had to be evacuated. An additional 15,000 people evacuated voluntarily due to their fear of the eruption, even though they were not within the declared danger zone." Activity briefly intensified early on 7 March, when ash-laden clouds and a little incandescent material were ejected.

The following information was provided by Chris Newhall. The new aa lava flow emerged from a breach in the summit crater wall and traveled down the SW flank directly over the 1968 flow. By 26 May, the nose of the new lava was about 200 m beyond the end of the 1968 flow. The breach grew to about 1/3 of the crater's circumference and about 100 m depth before it began to fill in during the later stages of the eruption. Maximum eruption cloud height was about 3 km above the summit. Ash contents were low and maximum ashfalls were only a few millimeters. Harmonic tremor was nearly continuous during the period of maximum activity and shallow explosion earthquakes were also recorded.

Information Contacts: G. Andal, COMVOL, Quezon City; C. Newhall, Dartmouth College.

August 1978 (SEAN 03:08) Cite this Report

Lava extrusion ends, but small ash explosions continue

Lava extrusion ended on 6 July, more than a month after activity began to decline. Harmonic tremor had accompanied the extrusion, increasing in magnitude when the lava flow rate increased. Hollow rumbling sounds were occasionally heard. Some ash puffs continued to be ejected, the most recent at 1727 on 21 August, saturating a nearby seismograph. Chemical analysis of the ash and petrographic analysis of the lava both indicated a basaltic andesite composition. COMVOL recommended the return of the last of the evacuees on 7 July, after recommending a partial end to the evacuation several weeks earlier.

Further Reference. Peña, O., 1978, Notes on the Mayon eruption from May 3-July 4, 1978 and COMVOL's role: COMVOL Letter, v. 10, no. 3-4, p. 1-3.

Information Contacts: G. Andal, COMVOL, Quezon City.

August 1979 (SEAN 04:08) Cite this Report

Seismic activity and crater glow

Seismic activity and crater glow were observed at Mayon 14-28 July. The same type of activity preceded the 1978 eruption. COMVOL has established a close watch on the volcano and will monitor any changes.

Information Contacts: G. Andal, COMVOL, Quezon City; J. Wolfe, Pan Asean Technical Services, Manila.

August 1980 (SEAN 05:08) Cite this Report

Harmonic tremor

Short-duration harmonic tremor began to be recorded at Mayon on 16 August. Bursts of tremor continued, but have become less frequent since the 16th. Similar seismicity preceded the 1978 eruption and accompanied crater glow in July 1979.

Information Contacts: O. Peña, COMVOL, Quezon City.

November 1980 (SEAN 05:11) Cite this Report

Occasional tremor episodes through November

Occasional tremor continued through November, and as of the 30th, 214 episodes had been recorded.

Information Contacts: O. Peña, COMVOL, Quezon City.

December 1980 (SEAN 05:12) Cite this Report

Steam emission, crater glow, and seismicity

A moderate quantity of dirty white steam rose weakly to 200 m above the crater rim on 4 December at 1247, accompanied by short-duration harmonic tremor on the Mayon Resthouse Observatory seismograph. Faint crater glow was first noted at 2315 the same day. Additional steam emission was observed 12 and 14 December. Episodes of tremor and discrete earthquakes continued through December.

Information Contacts: O. Peña, COMVOL, Quezon City.

June 1981 (SEAN 06:06) Cite this Report

Mudflows from typhoon rains

At about 2000 on 30 June, mudflows triggered by continuous rains accompanying Typhoon Daling swept villages in the S and E sectors of Mayon. Preliminary estimates set casualties at about 100 persons with many more missing [but see 6:7].

Information Contacts: O. Peña, COMVOL, Quezon City.

July 1981 (SEAN 06:07) Cite this Report

Mudflow casualties updated

Updated casualty figures indicate that the S- and E-flank mudflows triggered 30 June by typhoon Daling killed 40 persons, injured nine, and left seven missing. Other reported casualties were caused by the typhoon itself and associated flooding.

Further Reference. Gianan, O., 1982, A volcano disaster preparedness plan: mechanics of implementation of "Operation Mayon": Proceedings of the First Seminar Workshop on Philippine Volcanoes and Volcanic Terranes, Quezon City, Dec. 1982, p. 88-97.

Information Contacts: O. Peña, COMVOL, Quezon City.

August 1984 (SEAN 09:08) Cite this Report

Eruption clouds to 15 km; pyroclastic flows; lava flows

Slow summit lava production started 10 September at 0821, after at least 12 hours of harmonic tremor [but eruptive activity started the previous evening (9:9)]. Activity was dominantly Strombolian 10-11 September. On the 10th, aerial observers reported that lava from the summit crater was slowly spilling over the rim. Incandescent blocks rolled 600 m down the NW flank, destroying or incinerating trees in their path. Ash-laden clouds rose several hundred meters above the summit. PHIVOLCS recommended the evacuation of people living in a danger zone within 8 km of the crater. The next day, ash-laden steam clouds were ejected to heights of as much as 3 km at intervals of 1-5 minutes. The strongest explosion, accompanied by loud detonations, began at about noon, producing a steam and ash column that rose 3 km and deposited as much as 2 cm of ash. Two lava flows about 200 m wide advanced at about 3 m per minute to about 2 km from the summit crater by evening.

Activity intensified and became more Vulcanian in character beginning on the 12th. Pyroclastic flows reached several kilometers from the summit. Nine explosions occurred within 3 hours starting around 1100, sending ash to 14.5 km altitude. The day's strongest explosion, at 1553, sent ash to nearly 15 km altitude. Incandescent tephra was evident in the eruption columns. Smaller explosions took place at 1-5-minute intervals. Sixteen additional "fairly strong" explosions occurred in a 17-hour period ending midmorning 13 September. Activity on the 13th was characterized by persistent ash emission to 3 km above the crater but no large eruption columns were observed. A total of 26 strong explosions ejecting incandescent tephra were recorded in a 12-hour period ending the morning of 14 September. Rain falling on nearby towns deposited 7.5 cm of wet ash. Three lava flows continued to advance down the NW, N, and SW flanks.

More than 16,000 people were evacuated from 36 villages, most of which were within the 8-km danger zone. Nine people were reported killed, eight by burial in volcanic debris and one by hot steam [but note that PHIVOLCS reported in 9:9 that no casualties were directly attributed to the eruption and mudflows].

Information Contacts: R. Punongbayan, PHIVOLCS, Quezon City; UPI; AP; DPA.

September 1984 (SEAN 09:09) Cite this Report

Explosive activity reintensifies; 73,000 evacuated

Quoted material is from PHIVOLCS.

"Eruptive activity started 9 September at 1923. Initial activity was dominantly Strombolian, with incandescent spattering at the summit and production of small lava flows. A mound of solidified lava inside the crater blocked the 1968 notch at the SW rim, so the small lava flows and initial pyroclastic flows (see below) moved predominantly NW.

"A fairly strong eruption 10 September at 2300 marked the start of Vulcanian activity. Ash-laden steam clouds rose 5 km above the summit and a pyroclastic flow moved down to the NW, reaching 700 m elevation. Stronger explosions 11 September reopened the notch at the SW rim, so more of the later lava and pyroclastic flows moved SW than NW. The eruption continued to intensify, peaking 13 September. Cauliflower-shaped, ash-laden steam clouds accompanied by rumbling sounds reached a maximum height of 15 km before drifting SW, W, and NW. Continuous volcanic tremors with increasing amplitude were recorded, punctuated by explosion earthquakes. Two lava flows emerged through the SW breach. One reached 500 m elevation, adjacent to and W of the 1978 flow. The other, a little farther W, advanced to 1,400 m elevation (figure 2). The new lava is porphyritic augite-hypersthene andesite.

Figure 2. Map of Mayon showing the extent of the 1984 lava flows, after Corpuz (1985).

"Activity gradually declined 14-21 September. A mild eruption 22 September at 0502 was accompanied by a volcanic earthquake felt at intensity II on the MRF scale at the Mayon Resthouse Observatory. A relatively quiet period followed. A very strong explosion 23 September at 0433 ejected voluminous ash-laden steam clouds that reached 10 km in height. Incandescent tephra rose 2 km above the summit and spread in all directions, covering the summit area with red-hot tephra to about 1,500 m elevation. A large notch was formed in the SE rim of the crater and a smaller one in the E rim. Subsequent pyroclastic flows were directed predominantly SE and E, although some moved in other directions along gullies. Ash fell within about 50 km SW, W, and NW of the summit. Areas E and NE of the volcano received most of the fine airfall tephra generated by pyroclastic flows. The eruption continued to intensify until the 24th. Voluminous ash emission, sometimes sustained for 5 minutes, occurred at intervals of 2-15 minutes, accompanied by strong detonations and at times by electrical discharges. Maximum height of the eruption clouds was 15 km. On 24 September at 1614, a nuée ardente [almost] reached the nearest village. A large volume of pyroclastic flow material was deposited on the SE flank. The eruption started to decline 25 September. By 5 October activity was limited to weak steaming and faint to moderate crater glow, accompanied by volcanic tremors and discrete earthquakes."

Press sources reported reintensification of the eruption 6 October. Ash-laden steam clouds rose as much as 1.7 km above the summit and lava flowed 1 km from the crater.

"Mudflows generated by rain destroyed three sections of the Legaspi-Santo Domingo highway roughly 8 km SE of the volcano. Larger mudflows on the 27th overran the same portion of highway. Two bridges were destroyed along the Malilipot-Santo Domingo highway, roughly 8 km E of Mayon."

As of 30 September, press sources reported that 6,500 hectares of farmland had been covered by mudflows.

"Implementation of the Mayon preparedness plan was fairly smooth. On 10 September, the area within 6 km of the summit was declared off-limits and all residents were recommended for evacuation. On 12 September, the danger zone was extended to 8 km from the summit on the S and SW flanks. About 26,000 people were evacuated during the first phase of the eruption. On 23 September, the danger zone was expanded again, to 10 km from the summit on the SE side and 8 km from the summit around the rest of the volcano. All residents of that area were recommended for evacuation and the number of evacuees swelled to more than 73,000 at 50 centers. No casualties were attributed directly to the eruption or mudflows."

Reference. Corpuz, E., 1985, Chronology of the September-October 1984 eruption of Mayon volcano, Philippines in Punongbayan, R.S. (ed.), 1985, Special issue on the 1984 eruption of Mayon: Philippine Journal of Volcanology, v. 2, p. 36-51 (9 papers, 205 p.).

Further Reference. Umbal, J., 1987, Recent lahars of Mayon volcano in Geologic hazards and disaster preparedness systems: National Science and Technology Authority, p. 56-76.

Information Contacts: PHIVOLCS, Quezon City; DPA; AP.

February 1985 (SEAN 10:02) Cite this Report

Eruption clouds from 23 September seen on satellite images

Yosihiro Sawada observed a series of plumes from the September 1984 eruption of Mayon (table 1) on images from the GMS satellite. Problems with the scanning system of the GMS limited images to every 6 hours during the spring and summer, and at times prevented data returns from its southern zone of coverage.

Table 1. Series of plumes from the September 1984 eruption of Mayon observed on images from the GMS satellite. Plume lengths and widths were measured from images rather than digital data. Courtesy of Yosihiro Sawada.

Eruption clouds from Mayon's intense activity 23-24 September appeared much larger and denser on satellite imagery than those from the early- to mid-September activity. A moderate plume on 23 September at 0800 had grown much larger 6 hours later (figure 3, left and right) and plumes remained large and dense through 1400 the next day. Declining activity remained visible until 26 September at 1400.

Figure 3. GMS infrared satellite images with arrows pointing to eruption clouds from Mayon, 23 September 1984 at 0800 (left) and 1400 (right). Land areas are outlined with fine white lines. Courtesy of Yosihiro Sawada.

Further Reference. Sawada, Y., 1987, Study on analysis of volcanic eruption cloud image data obtained by the Geostationary meteorological Satellite (GMS): Technical Reports of the Meteorological Research institute (Japan), no. 22, 335 p.

Information Contacts: Y. Sawada, Meteorological Research Institute, Tsukuba, Japan.

August 1988 (SEAN 13:08) Cite this Report

Lahars and faint crater glow

Since late 1987, white emissions from Mayon have remained volumetrically moderate, obscuring 20-50% of the crater. Bluish fumes accompanying steam emission were first noted on 26 July and were also seen on 1, 2, and 9 August, moving downslope to the NE and E. On 19 July, there was one report of rumbling noises. A lahar on 14 August (at about 2100) moved SE down Basud, Matanag, and Mabinit gullies and was recorded on the Santa Misericordia Station (SMS) about 8 km E of the crater (figure 4). Field observations indicated that the lahar eroded 1.3 m of the Matanag gully near the Buyuan-Matanag road. Rainfall measured 56.0 mm at the SMS station.

Figure 4. Map of Mayon showing gullies affected by the 14 August 1988 lahars and relative positions of seismograph stations. Modified from a PHIVOLCS hazard map, 1984 eruption.

Faint crater glow was first seen during the night of 15 August from the SMS station and from Bogtong, Legaspi City (12 km SE of the crater). From 16 to 22 August, four high-frequency and 18 low-frequency events were registered by seismographs around the volcano. The water tube tiltmeter readings at the Mayon Resthouse Observatory (3.5 km N of the crater) have indicated an inflationary trend since March 1988.

Information Contacts: PHIVOLCS.

September 1988 (SEAN 13:09) Cite this Report

Crater glow; new areas of steaming

Faint crater glow was observed at Mayon almost nightly during September. White steam emission, occasionally accompanied by bluish fumes, remained moderate, occupying 1/2 of the vent area. When PHIVOLCS volcanologists climbed the volcano on 8 September, the crater was 40-50 m deep and new areas of steaming had developed near the summit. A temperature of 285°C was measured at one of several fumaroles. Strong static charges at the summit caused hair to stand on end. On 29 September, a mudflow with a velocity of 2.3 m/s traveled down Basud gully, along the volcano's SE slope.

During September, 36 high-frequency and 31 low-frequency earthquakes were recorded plus 129 low-frequency and 16 high-frequency tremor episodes. From 1 to 9 October, 19 high-frequency and seven low-frequency earthquakes were detected in addition to 71 harmonic tremor episodes.

Information Contacts: PHIVOLCS.

November 1988 (SEAN 13:11) Cite this Report

Crater glow and steam emissions continue; mudflow damage

Faint crater glow continued to be observed during nights with good visibility in October and November. Moderate steam emission covered 30-50% of the crater and blue volcanic fumes sometimes moved slowly downslope.

In October and November, 77 high-frequency events, 59 low-frequency events, and 119 harmonic tremor episodes were recorded. Mudflows were detected (by seismographs and by a lahar mapping team) along the SE slopes on 12 and 23 October, and along the SE and SW slopes on 4, 6, 20, and 21 November. The 12 October lahar moved at 2.5 m/s down Basud gully between 2347 and 0141 the next morning. On the 23rd, three lahars with velocities of about 3.2 m/s were triggered by a typhoon. The Basud River was incised by 4 m (near the Lidong-Basud boundary), and a jeep caught by a flow surge along the river was partially buried by debris. Continuous undercutting caused 3-4 m of a cement road to collapse and a house was carried away by a flow. No casualties were reported.

Information Contacts: PHIVOLCS.

January 1993 (BGVN 18:01) Cite this Report

Explosion generates pyroclastic flow that kills 68 people; activity continuing

An explosion at 1311 on 2 February generated a pyroclastic flow that traveled 6 km SSE down the Bonga gully and spread out over most of the fan built by 1984 pyroclastic flows. A cauliflower-shaped cloud rose 4.5 km above the summit, and ash fell near Camalig, about 9 km SW. A seismograph 4 km N of the summit recorded the explosion earthquake, which lasted for 6 minutes and 40 seconds, and corresponded with a booming sound. The eruption lasted for about 30 minutes.

Prior to the 2 February eruption, PHIVOLCS conducted visual observations, and seismic monitoring from three permanent stations around the volcano: Mayon Rest House Observatory (MRHO), on the N slope 4 km from the summit; Sta. Misericordia Observatory (SMO), in Sto. Domingo on the E slope 7 km from the summit; and the Lignon Hill Observatory (LHO), in Legaspi City on the SE slope 12 km from the summit. Ground deformation measurements were done at MRHO using a water-tube tiltmeter, a precise leveling line on the N slopes, and two electronic distance meter lines on the N and E slopes. White steam emission varied from wispy to moderate through 1992 and into 1993, with no increase or discoloration prior to the explosion. Seismicity over the same period was usually low, with 0-10 events/day, again with no significant increase before the 2 February event. No inflation or crater glow was observed. PHIVOLCS has since installed six additional seismic stations, with three telemetered seismic stations planned. Teams were also deployed to make ground deformation measurements.

A part of the SE crater rim and/or a block of the wall of the Bonga gully slumped into the gully 1900-1930 on 2 February. There were a few small ash emissions on 3 February. A degassed plug of lava was also growing in the crater, causing incandescent rocks to tumble into Bonga gully. There were two small explosions on 6 February at 0400 and 1600. The one at 0400 produced a small pyroclastic flow. A NOTAM advising all pilots to avoid flying over the area was issued on 4 February. Eruptions larger than the initial explosion occurred on 12 February at 1127 and 1230. The first eruption produced an ash cloud that rose about 1.5 km, and a pyroclastic flow 4 km down the Bonga gully. The second sent an ash cloud to 3 km height and a pyroclastic flow 5 km down the Matanag gully, also on the SE flank of the volcano.

COSPEC measurements from a helicopter detected 1,415 metric tons/day (t/d) SO₂ on 3 February. Additional measurements 6, 7, and 8 February were 700, 800, and 900 t/d, respectively.

Press sources have reported at least 68 dead and over 100 injured, almost all resulting from the 2 February pyroclastic flow. No casualties were reported from the 12 February eruptions. An evacuation order has been issued for the area within 6 km of the summit, already off-limits for settlement. Most of the dead were farmers tending crops within the 6 km danger zone. A zone within 10 km on the SE side of the volcano has also been evacuated. The evacuated population was about 60,000 on 17 February.

Information Contacts: PHIVOLCS; AP.

February 1993 (BGVN 18:02) Cite this Report

Eruption continues; pyroclastic flows; lava extrusion

Eruptive activity ... continued following the 2 February eruption that killed at least 70 people (18:1). The pyroclastic flow associated with that event traveled 6 km SSE down the Bonga Gully and covered approximately 4 km² (figure 5). The total volume of the flow, roughly 1-2 x 10⁶ m³, is much less than that of the 1984 pyroclastic flows (30 x 10⁶ m³). As much as half of the volume of the 2 February flow may have been pre-1993 rock that was scoured from the gully as the flow descended. Calculations based on eyewitness accounts suggest an early speed for the flow of >120 km/hour. Up to 5 mm of ash fell 9 km SW of the volcano.

Figure 5. Sketch map showing the extent of the 2 February 1993 pyroclastic deposits. Courtesy of PHIVOLCS.

Between 2 February and 19 March, activity consisted principally of slow lava extrusion, occasional ash puffs (100-500 m high), explosions (figure 6), and small pyroclastic flows generated by the collapse of lava deposits on the steep-sloped summit (table 2). Triggered by explosions or gravity, the collapses resulted in incandescent blocks rolling down the Bonga Gully. When a larger block (10³-10⁵ m³) spalled or slumped from the front of the slowly advancing lava flow, it formed a small collapse-type pyroclastic flow that traveled as far as 4 km. Ash columns from the pyroclastic flows rose as high as 1 km above the crater unless strong winds sheared them off at a lower elevation. Such flows occurred on 12-13 February, 28 February-3 March, 10 March, and 15-17 March. On 15 March the number of explosion-type seismic signals increased and a new lava flow was observed at the summit.

Figure 6. Explosion-type earthquake count recorded at Mayon, 1 February to 18 March 1993. Courtesy of PHIVOLCS.

Table 2. Significant events at Mayon, 6 February-19 March 1993. Courtesy of PHIVOLCS.

COSPEC measurements of SO₂ began on 3 February (figure 7). Temporary decreases to 200-400 t/d on 12-13 February and 28 February were followed by small collapse-type pyroclastic flows. Most measurements after 7 March were ground-based instead of air-based. The ground-based measurements may not record all the SO₂ that air-based measurements record because no available roads allow complete, rapid circumnavigation of Mayon. On 10 March, however, the two techniques were cross-checked and their values agreed.

Figure 7. COSPEC measurements of SO2 emission at Mayon, 31 January-18 March, 1993. Courtesy of PHIVOLCS.

During the last half of February, moderate to heavy rains on the SE flank remobilized the new pyroclastic deposits and generated small lahars. All lahars remained essentially confined to the channels.

There were numerous reports of a 2-3 m drop of the water table in the vicinity of Mayon in late 1992 and January 1993 as evidenced by water wells drying or decreasing production. Local observers also stated that similar drops occurred before the 1968, 1978, and 1984 eruptions, but that those drops were not as pronounced as the current one. A climber observed an increase in rockfalls and deepening of the Bonga Gully 6 days before the 2 February eruption but did not report his observations until after the eruption. Just prior to the 1984 eruption of Mayon he had reported a bulging of the normally concave crater floor into a convex dome. A PHIVOLCS observer reported that the temperature of a crater fumarole increased from 97°C in May 1991 to 150°C in July 1992.

Stephen O'Meara observed Mayon from 27 February to 7 March and reported the following.

Three N-S-trending vents or fissures in the summit crater were visible from the ruins near the village of Cagsaua (10 km SE of the summit) on 27 and 28 February. The N vent (also the highest) was the most active. At times the vents were clearly visible; at other times the entire throat of the summit crater was filled with a large, white, cauliflower-shaped cloud that did not rise above the crater. The steam cloud appeared to contain ash on 28 February.

Winds on the morning of 1 March blew the steam plume to Legazpi City, 14 km SE of the summit. It contained a dark gray tinge and smelled of sulfur. At 0700 a slow, silent pyroclastic flow traveled down the Bonga Gully. Simultaneously, a smaller flow moved S down the Miisi Gully. The gray cloud from the Bonga flow appeared to be composed of many segments produced by frequent bursts, but this may have been an illusion caused by strong winds. Ash fell on the S flank of the volcano.

At Arimbay (approximately 2.5 km N of Legazpi City), the smell of sulfur was noticed about 5 minutes prior to the arrival of a lahar at 1530 on 3 March. The knee-deep lahar was warm but not boiling. Approximately 1.5 m of mud, stones, and boulders deposited by previous lahars blocked traffic on the Legazpi-Tabaco road in Arimbay. Also on 3 March, a pyroclastic flow, larger and darker than the one seen on 1 March, "casually" rolled down the Bonga Gully.

On 4 March a reddish brown ash cloud traveled down the Bonga Gully, but did not develop into a strong pyroclastic flow. The cloud remained confined to the upper half of the gully. At 0100 on 5 March a faint crater glow was noticed with just a "spark" coming from the N vent. One small gray ash puff occurred between 0530 and 0730 and was confined to the crater. A reddish-brown ash puff seen in the afternoon also remained in the crater. Crater glow was observed before sunrise on 6 March and a reddish-brown ash puff similar to that observed on 5 March occurred in the early morning. At approximately 0800 on 7 March a larger, darker, gray ash eruption lasted 5 minutes. Nothing traveled down the Bonga, but a tiny pyroclastic flow may have traveled S down the Miisi Gully.

Information Contacts: PHIVOLCS; S. O'Meara, Sky & Telescope, Belmont, MA; Chris Newhall, USGS.

March 1993 (BGVN 18:03) Cite this Report

Strombolian eruption; activity wanes

Lava fountaining began on 21 March after almost 2 months of intermittent precursors, including a small, but deadly, phreatomagmatic eruption on 2 February. Following the 2 February eruption, sluggish and intermittent production of lava continued until 19 March when its extrusion rate increased. Several tens of small collapse-type pyroclastic flows on both 19 and 20 March resulted from portions of the flow front detaching and rolling down the steep upper slopes of the volcano. On the night of 21 March, lava fountaining to heights of several hundred meters began, forming a small cone at the head of Bonga Gully where it descends from the summit crater. Most of the lava fell back into the crater and around the vent, but eventually flowed SSE down Bonga Gully. By 26 March the flow front was 4.5 km from the summit, and the estimated volume of lava extruded was 60 x 10⁶ m³, more than half the volume of the 1984 flow. Ash-bearing steam clouds from the fountains rose 2-3.5 km above the crater and deposited a few millimeters of ash on nearby towns. This was less ash than resulted from the pyroclastic flows, which stopped when fountaining began.

The fountaining episodes typically lasted 20 minutes; the longest lasted 50 minutes. They were separated by repose periods lasting 30 minutes to 1 hour. Some episodes were followed by 10-20 minutes of intermittent 2-Hz tremor, the amplitude of which varied greatly suggesting that each tremor episode consisted of a series of tremor events. The tremor did not correlate with any visible steaming. Continuous, strong gas jets, glowing "like a blowtorch" and emitting a continuous "jet plane sound," were visible from Legazpi city, 14 km SE of the summit. They appeared to be in the summit crater, 100-200 m upslope from the vent.

COSPEC measurements of SO₂ flux increased from 4,000 metric tons/day (t/d) on 1 March, to 5000 t/d on 24 March. On 26 March, the SO₂ flux measured in the morning was 3,920 t/d rising to 7,600 and 8,800 t/d in the afternoon (two sets of measurements).

By 2 April, lava fountaining had ceased, and little or no new material was feeding the flow. Seismicity was low to moderate and dominated by small explosion-type earthquakes. Ash puffs were rare and weak. A single small pyroclastic flow occurred on 1 April, originating in the crater. The glow from the crater persisted, but was considerably dimmer and the gas jets burning in the crater had disappeared. However, SO₂ emission remained high and variable, 3,000-8,000 t/d, and the volcano was not deflating.

The lava flow front, still about 5.4 km from the vent, was not expected to advance much farther, having moved only a few meters on 1 April. The flow was confined to Matanag Gully except for a small finger that reached the lower Bonga Channel. Lateral levees and pressure ridges were well defined.

On 9 April a dirty-white steam plume rose only 50 m above the crater rim. At night, a faint glow from the crater was visible. Small "explosion-type" earthquakes, continued; 57 were detected in the 24-hour period beginning at 1700 on 8 April. Most, however, were associated with incandescent materials detaching from the lava deposits in the Bonga Gully. The rate of SO₂ emission was 2,272 t/d.

More than 45,000 people fled their homes during the early stages of the eruption, from 2 February to 19 March, filling 43 evacuation centers. An additional 12,000 evacuated their homes as the eruption entered its Strombolian phase on 19-21 March. Since the 2 February event, which killed 75 people, no deaths directly attributable to the eruption have been reported.

Information Contacts: PHIVOLCS; Chris Newhall, USGS; Reuters.

August 1995 (BGVN 20:08) Cite this Report

Crater glow and steam emissions

On the evening of 23 August, staff at the Lignon Hill Observatory in Legazpi observed moderate to intense glow from the crater of Mayon. Moderate steam emissions rising ~300 m above the summit preceded the observation of glow. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) increased the Alert Level to 2, indicating that volcanic activity had increased slightly. PHIVOLCS also recommended strict compliance with the 6-km-radius permanent danger zone, an area restricted to regular human activity, especially below the Bonga channel on the SE flank. The PHIVOLCS Quick Response Team was dispatched to augment monitoring staff at the Lignon Hill and Mayon Resthouse observatories.

The summit crater continued to exhibit glow on 25 August, with varying intensity, and there was moderate steam emission. COSPEC measurements of SO₂ levels in the steam plume were ~630 metric tons/day (t/d), well above the 100-200 t/d measured during quiet periods. No unusual seismicity was detected. The last sighting of crater glow was on 2 September, although it was not until 12 September that SO₂ measurements by COSPEC decreased to near background.

As of mid-September, the dominant seismicity consisted of occasional high-frequency volcanic earthquakes (<5 events/day); they were large enough to be located, and occurred within the E and N parts of the edifice. Observations of the crater area disclosed that some multi-phase events were due to large lava blocks detaching from the vent. The vent is open to the SE as a result of the 1993 explosions. A preliminary investigation of a water well in Malilipot (ENE of the summit) on 25 August revealed a slight decrease in water level, also an indicator of volcanic unrest. However, further measurements of water wells on the SE and S margins of lahar fans around Mayon, where most wells are located, did not show measurable or significant changes. PHIVOLCS therefore concluded that little ground deformation was taking place.

Information Contacts: Ernesto G. Corpuz, Chief of Volcano Monitoring Division, Philippine Institute of Volcanology and Seismology (PHIVOLCS), 5th & 6th Floors Hizon Building, 29 Quezon Avenue, Quezon City, Philippines.

June 1999 (BGVN 24:06) Cite this Report

Explosion on 22 June sends a plume to ~10-12 km altitude

At 1658 on 22 June Mayon emitted an ash column that rose 7-10 km above the vent (figure 8). The emission was recorded by the seismic network of the Philippines Institute of Volcanology and Seismology (PHIVOLCS) as an explosion that lasted for 10 minutes. No volcanic earthquakes nor other visible signs of abnormal activity were observed before the explosion. During May, however, low-frequency volcanic earthquakes had been recorded intermittently, accompanied by faint crater glow.

Figure 8. A column of steam and ash rising from Mayon's crater and a pyroclastic flow descending its SE flank during its sudden isolated explosion on 22 June. Photograph courtesy of PHILVOCS.

The explosion represented an isolated event as activity immediately declined to typical incidents of weak steaming without measurable seismicity. Faint glow was seen the next day at the summit crater. An aerial survey noted a new explosion pit at the summit; the small diameter pit was later described as a deep hole lined with sulfur deposits (figure 9). The presence of sulfur suggested that lava had not yet ascended to the surface.

Figure 9. The crater of Mayon as it appeared after the 22 June explosion. A new, circular explosion pit developed on the crater floor; the shadow formed along the rim of this pit can be seen in this NW-looking photo shot through the breech. Courtesy of PHILVOCS.

Beginning at 0700 on 25 June there was a slight increase in seismicity and SO₂ emission. The COSPEC measured an SO₂ flux of 4,800 tons/day, compared with 4,200 tons/day the previous day. SO₂ fluxes normally average 500 tons/day. A short interval of high-frequency tremor was also recorded.

Tremor, light steaming, low-frequency volcanic earthquakes, and elevated SO₂ fluxes continued for several days. Also, deformation surveys conducted with laser-ranging EDM equipment indicated sustained inflation on the SE slope.

PHIVOLCS maintained an alert status of "Level 1," advising the public not to venture within 6 km of the summit area (figure 10). In particular, residents were advised to avoid the Bonga pyroclastic fan, an area on the SE side of the volcano that contains a deep canyon and lies directly below the crater rim notch. This fan was the site of most of the fatalities in the 1993 eruption and is considered the area most vulnerable to future pyroclastic flows.

Figure 10. Details on Mayon and vicinity taken from a volcanic hazards map (PHILVOCS, 1999). The legend describes some effects of the 1993 eruption. The solid black circle represents the 6-km-radius safety zone currently in effect. An additional 1-km-wide precautionary zone lies to the SE of the volcano below the Bonga Pyroclastic Fan. Some local cities and river drainage are also shown. Courtesy of PHILVOCS.

Reference. Philippine Institute of Volcanology and Seismology (PHIVOLCS), 1999, 1999 Mayon permanent danger & high susceptibility areas (map), URL: http://www.philonline.com/~seismo/Volcanoes/Mayon/MayonHazMaps.htm.

Information Contacts: Raymundo S. Punongbayan and Ronnie Torres, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia St. Diliman, Quezon City Philippines (URL: http://www.phivolcs.dost.gov.ph/).

January 2000 (BGVN 25:01) Cite this Report

Summit-crater dome growth and escalating eruptions herald evacuations

Mayon stratovolcano, in SE Luzon, entered a period of increasing unrest in May 1999. This has led to significant ash eruptions, lava flows, and pyroclastic flows-and associated rises in the hazard status. On 22 June 1999 an eruption sent an ash plume 10 km above the summit and the volcano continued to exhibit activity that could signify additional eruptions (BGVN 24:06). After this event the hazard status was raised from Alert Level 1 (slight unrest) to Alert Level 2 (moderate unrest, no eruption imminent). Another ash explosion on 5 January 2000 produced a 5-km-high ash column.

A lava dome was spotted growing in the summit crater on 12 February. Accordingly, on the 15th the hazard status was raised to Alert Level 3 (intensifying unrest, magma close to the crater, eruption within weeks). Dome theodolite measurements showed continuing growth and, as of 23 February, the dome was ~74 m wide by ~24 m high. On 19 February, portions of the dome began to glow, presumably as magma forced cracks to open. At the time of this report, the dome already overlapped the SE side of the crater, which was unconfined and open. On 20 February some lava fragments detached from the dome and incandescent rockfalls descended the SE flank, traveling within a large channel called the Bonga Gully.

As of 23 February, PHIVOLCS had recommended evacuation to at least 7 km from the summit in the SE and to at least 6 km elsewhere. The former is a permanent danger zone. Activity continued to increase during the night of 23-24 February, with minor explosions and lava fountaining, prompting a change in the hazard status to Alert Level 4 (hazardous eruption imminent, possible within days) at 0300 on 24 February. No additional evacuation was recommended, but people residing within 8 km of the summit were advised to prepare for evacuation. Later that morning, at 0829, a pyroclastic flow descended the SE flank towards the Bonga Gully. This caused the hazard status to rise to Alert Level 5 (hazardous eruption in progress). Eruptive and seismic activity continued into early March. Additional details will be provided in next month's Bulletin.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia St. Diliman, Quezon City Philippines (URL: http://www.phivolcs.dost. gov.ph/).

February 2000 (BGVN 25:02) Cite this Report

Strong explosions, lava flows, and pyroclastic flows following dome growth

Volcanic unrest that began in May 1999, and intermittent explosive eruptions beginning in June 1999, eventually led to growth of a lava dome on 12 February 2000. By 23 February PHIVOLCS had recommended evacuation to 7 km from the summit in the SE and to 6 km for the rest of the volcano. The latter is a permanent danger zone.

At 2206 on 23 February the seismic network detected an explosion-type earthquake that coincided with rumbling and minor ejection of lava fragments from the summit. This earthquake was followed shortly by bright incandescence, indicating that lava emission and ejection had intensified. Low-frequency volcanic earthquakes then occurred beginning at 2217 and lasting until about 2326 when the seismographs began to record harmonic tremor. The tremor became pronounced at about 0034 on 24 February and was accompanied by minor lava fountaining to 50 m above the summit lava dome. The hazard status was raised to Alert Level 4 (hazardous eruption imminent, possible within days) at 0300 on 24 February. No additional evacuation was recommended, but residents within 8 km of the summit were advised to prepare for evacuation.

At 0826 on 24 February another explosion-type earthquake was recorded by the seismographs at Anoling, Sta. Misericordia, and Mayon Resthouse Observatory. The summit was obscured, but at 0829 a pyroclastic flow descended SE towards the Bonga Gully with a run-out distance of ~7.2 km, reaching the distal end of the Bonga fan. The hazard status was then raised to Alert Level 5, (hazardous eruption in progress). Because pyroclastic flows could continue to sweep down along well-incised gullies and channels, especially the Bonga Gully, PHIVOLCS recommended extension of the danger zone to 8 km along the SE sector of Mayon Volcano. Likewise, ashfall was expected mainly W, SW, and NW of the crater.

The SO₂ emission rate increased on 24 February to 4,070-5,700 metric tons/day (t/d). Ground deformation measurements showed that the volcanic edifice swelled significantly in the previous two days, consistent with the growth of the lava dome.

By the morning of 25 February activity was mainly lava extrusion, with a flow channeled along the Bonga Gully. COSPEC readings conducted on 24 February reached 13,500 t/d. The abrupt increase in this value may be attributed to the series of highly gas-charged ash ejections comprising the volcanic plume.

Following a quiet interval that started at 1420 on 26 February, more vigorous activity resumed on the evening of the 27th. Seven ash-and-gas explosions occurred between 1950 and 2237, the most significant of which (at 2144 and 2237) were accompanied by lava fountaining with ejection of volcanic bombs. Large incandescent fragments were ejected to ~500 m above the crater rim. Ground deformation measurements showed that the edifice remained inflated. COSPEC readings of SO₂ flux remained significantly above normal at 4,900 t/d. Explosion earthquakes and harmonic tremor accompanied the lava fountaining and persisted even when the activity had apparently subsided.

Explosive eruptions during 28 February-1 March. Mayon had another series of explosive eruptions during 0700 to 2100 on 28 February, with the most significant eruptions occurring at 1641, 1732, and 1940. The first explosion produced a 5-6-km-high eruption column and generated a large pyroclastic flow that descended the W portion of the Bonga Gully on the SE flank and entered the Mabinit channel to the S. This was followed by voluminous eruption clouds beginning at 1732 that rose to ~10 km above the summit and generated multiple pyroclastic flows to the SW, S, and SE. Vigorous explosions sustained the eruption column and discharged large volcanic fragments that splattered the upper portions of the cone. Thick ash clouds hovered around the volcano and created frequent lightning discharges.

Most of the ash clouds were eventually carried to the SW and W, affecting Ligao, Guinobatan, and Camalig. However, the pyroclastic flows did not travel beyond the present danger zones. The ash clouds contained high concentrations of sulfur dioxide, with COSPEC-recorded emission rates of 13,000 t/d, as expected for an eruption cloud. Aircraft were warned to avoid lingering ash clouds to the W of the volcano. The E side, towards the Legaspi airport, remained free from volcanic ash, debris, and SO₂ emissions.

Electronic distance measurements revealed that the volcano's edifice remained inflated. Such inflation was thought to be caused by the ascent of magma as indicated by the near-continuous seismic tremor associated with active magma transport.

The series of major ash ejections and subsequent pyroclastic flows that occurred along Bonga Gully, Mabinit, and Miisi Channels started at 1641 on 28 February 2000. The maximum height estimated for the vertical ash plume was 12 km during the 1732 event. The approximate runout of the pyroclastic flows reached to ~5-6 km downslope. Severe ashfall occurred in the SW sector of the volcano, especially at Barangay Tumpa in Camalig and Barangays Maninila and Masarawag in Guinobatan. Lava fountaining with ballistic bombs was also frequently observed starting at 1732 with maximum heights estimated at 1 km.

After the vigorous activities late in the afternoon to early evening on 28 February, only quiet effusion of lava was noted during times when the summit was not obscured through the morning of 29 February.

Another series of ash ejections began at 1211 on 29 February. The largest event occurred at 1501 and produced a 14-km-high eruption column. This event also generated several pyroclastic flows that descended all sides of the volcano. Pyroclastic flows that were channeled by gullies in the SW, S, and SE reached up to 5-6 km from the summit. Smaller pyroclastic flows that followed gullies in other sectors stopped ~2-3 km from the crater. Ash from the tall eruption column and from pyroclastic flows drifted to the W and SW. The ash ejections were generally accompanied by rumbling sounds. Vigorous lava fountaining began at 1531 and ballistic projectiles fell within 1.5 km of the crater. Lava flows were observed on 1 March to have reached the 1,000 m elevation, or about 2.3 km from the summit.

COSPEC measurements on 29 February were hampered by thick ash cover. Ground deformation measurements made the morning of 29 February along the Buang and Masarawag EDM lines showed that the volcano edifice remain inflated. Significant potential was noted for lahars along major tributaries draining from the NW due to the presence of ash and pyroclastic-flow deposits, which could be eroded and remobilized during heavy rainfall.

Mayon exhibited another series of eruptions that began on 1 March and produced dense and highly convective ash columns that rose up to 7 km above the summit. Part of the eruption column would occasionally collapse to produce pyroclastic flows that traveled along major gullies around the volcano. Pyroclastic flows were observed along the main gullies facing Anoling. The largest of these pyroclastic flows occurred along Bonga Gully and traveled ~6 km from the crater, while smaller flows at other gullies descended some 4 km downslope. Explosive eruptions produced lava fountaining with discrete ballistic volcanic fragments hurled out to ~500 m above the crater rim. Frequent rumbling accompanied the explosions, which lasted until 1609. By the end of this episode of explosive activity, quiet lava extrusion followed and continued to be observed up to the present. Areas SW and W of the volcano were severely affected by ashfall with the most significant deposition in Camalig, Guinobatan, and Ligao. Minor ash and steam were continuously being generated by lava deposits from the summit crater and Bonga Gully and drifted to the SW and W areas by prevailing winds.

Lava emission phase. Mayon was relatively quiet during 2 March as the seismic network recorded short-duration harmonic tremors and some discrete low-frequency volcanic earthquakes. This departs from the continuous tremor recorded in the past days during periods of relative quiet. The volcano has apparently entered a phase of lava emission with sporadic episodes of minor ash puffs. Ash and steam emission from both the summit crater and new lava flow deposits produced a haze over the SW sector, particularly in the municipalities of Camalig, Guinobatan, and Ligao. SO₂-flux measurements on 2 March yielded a value of 14,500 t/d. Ash clouds derived from the new lava flow deposits apparently produced a significant portion of this emission rate. Ground deformation measurements indicated that the volcano deflated slightly following the 1 March ash ejections.

Lava emission with sporadic episodes of minor ash puffs dominated the eruptive activity on 3 March. This relatively quiet state was reflected in the low-level but significant seismicity comprised by short-duration harmonic tremors and some discrete low-frequency volcanic earthquakes. Thick clouds covered the summit area, but below the cloud line and on the middle and lower slopes of the volcano ash clouds and steam emanated from the new lava flows and pyroclastic-flow deposits. A high emission rate of 8,900 t/d SO₂ was measured by COSPEC. Much of the ash and steam clouds resulting from this degassing drifted to the W and SW sections of the volcano due to prevailing winds. The haze produced by fine ash suspended in the air temporarily precluded ground deformation measurements.

Potential exists for hot lahar flows due to the presence of highly erodible pyroclastic deposits, which may be remobilized during heavy rainfall. Gullies with confirmed pyroclastic-flow deposits in their headwaters, which may therefore be sites for future lahars, are the Mabinit and Matanag river channels in Legaspi City, Miisi channel in Daraga, Basud-Lidong channel in Sto. Domingo, San Vicente and Buang channels in Tabaco, and the Bulawan channel in Malilipot.

Short-duration harmonic tremors and low-frequency volcanic earthquakes continued on 4 March. This type of seismicity indicated that eruptive activity was limited to quiet lava emission. Ground deformation measurements showed that the volcano was still inflated in its lower portion, while the SO₂ emission rate was determined to be at a minimum of 12,100 t/d. Preliminary estimates of the volume of deposits emplaced by the eruptions yielded at least 40 million cubic meters of lava flow and pyroclastic flow deposits. Lava flow deposits account for the major proportion of this estimate.

Activity for the next day was mainly characterized by gentle outpouring of lava. During cloudbreaks the night of 5-6 March, intense glow from the crater and from some portions of the advancing lava flow along the upper and middle Bonga gully were evident. Rockfalls and minor collapses along the length of the flow contributed to some localized ash and steam emission. However, the majority of the thick volcanic plume came from the summit crater which emitted about 8,300 t/d of SO₂. The PHIVOLCS seismic network continued to record short-duration harmonic tremors and low-frequency volcanic earthquakes. Ground deformation measurements showed some slight inflation of the volcano on the lower NW flank. The very high sulfur dioxide emission rate, occurrence of tremor and volcanic earthquakes associated with magma ascent, and slight swelling of the Mayon edifice indicate that some ascent of magma is still ongoing. Due to cessation of explosive eruptions, the sky W and SW of the volcano was generally clear of ash.

During 6 March the volcano exhibited quiet lava effusion accompanied by intense crater glow and rolling incandescent materials along the upper and middle reaches of the Bonga Gully. Moderate to strong emission of steam drifted generally to the N from the summit crater. The high steam output also yielded an elevated SO₂ emission rate of at least 8,800 t/d. Seismic activity consisted of 11 low-frequency volcanic earthquakes and 25 episodes of short-duration tremors. Slight inflation of the lower NW flank of the volcano continued.

At 0746 on 7 March, a parallel collapse of the new lava flow deposit in the upper middle slopes produced a voluminous secondary pyroclastic flow. The billowing ash cloud descended the Bonga Gully to the SE.

The seismic network recorded low-frequency volcanic earthquakes and short-duration harmonic tremors on 7 March. The measured SO₂ gas emission rate of 3,900 t/d, although low compared to recent measurements, was still well above the volcano's baseline level. Likewise, ground deformation surveys showed that the edifice was slightly inflated. At night on 7-8 March, when the volcano's summit area was visible, intense crater glow continued.

A PHIVOLCS report on the morning of 9 March noted that since the last eruption of 1 March, a waning trend in Mayon's overall activity has been evident. The number of volcanic earthquakes decreased and remained at unremarkable levels. In addition, tremor associated with emission of lava from the crater ceased. Seismic activity only reflected sporadic surface disturbances such as occasional rockfalls caused by oversteepened slopes. The Electronic Distance Meter (EDM) and precise leveling surveys also showed a return to the baseline levels, indicating a probable deflation of the edifice. Mayon continued to vent a large amount of steam, but the SO₂ component measured by COSPEC had decreased. Although the summit and isolated spots on the new lava flow deposits continued to glow at night, this incandescence was attributed to residual heat.

In view of these recent developments at Mayon, PHIVOLCS lowered the volcano status to Alert Level 4. On 9 March the 8-km-radius extended danger zone in the SE quadrant was reduced to 7 km. PHIVOLCS emphasized that the 6-km radius Permanent Danger Zones should remain evacuated at all times because of instability of new pyroclastic and lava deposits that may be dislodged towards the lower slopes with resultant secondary explosions and life-threatening secondary pyroclastic flows.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia St. Diliman, Quezon City Philippines (URL: http://www.phivolcs.dost. gov.ph/).

April 2000 (BGVN 25:04) Cite this Report

Decreasing activity; small eruptions, lava flows, secondary pyroclastic flows

Volcanic unrest that began at Mayon during May 1999 led to growth of the lava dome beginning on 12 February 2000, and continued in the form of explosive eruptions from 23 February through 1 March (BGVN 25:02). Since the 1 March 2000 eruption, observations have indicated that activity is declining.

After 1 March activity was relatively quiet with the largest event being an ash puff that was produced on 12 March when hot lava at the summit came in contact with surface water. The ash puff rose to a height of ~1 km and drifted to the NW. After 1 March there was moderate seismicity, high volcanic gas outputs, no increase in ground deformation, continuing glow of the summit, and new lava effusion. These conditions were associated with very gradual return to repose. Therefore on 16 March PHIVOLCS reduced the alert status of Mayan volcano from Alert Level 4 (hazardous eruption imminent, possible within days) to alert Level 3 (less probability of a hazardous eruption).

On 17 March at 1254, 1350, 1609, and 1619 partial collapse of the new lava flow on the volcano's upper middle slopes produced voluminous secondary pyroclastic flows, with the associated ash clouds blown to the SW. On 19 March at 0138, 0203, and 0300 similar secondary pyroclastic flows occurred with ash clouds that blew NE. The billowing ash clouds did not originate from the crater, but from the side of the lava flow that failed on the volcano's upper slope, and from descending detached lava fragments along Bonga Gully. Scientists expected the series of secondary pyroclastic flows due to the instability of volcanic material deposited on steep ground.

After 25 March the number of low-frequency volcanic earthquakes was relatively high (up to 40 per day), presumably due to strong jetting of hot gases emanating from the crater. In addition, relatively high levels of SO₂ emissions (up to 9,000 metric tons per day) also occurred after 1 March that were associated with degassing of residual magma. Since the volcano showed no signs of an imminent eruption, on 1 April PHIVOLCS reduced the Alert Level to 2 (the probability of hazardous explosive eruption is minimal). The volcanic system is expected to continue producing earthquakes and to vent a large amount of gas because fresh magma still resides along the whole length of the volcanic conduit and near the summit. Due to the possibility of sudden explosions caused by the release of gas from localized pockets within the magmatic system, and the threat of secondary pyroclastic flows, rockfalls, and ash fallout, PHIVOLCS maintains off-limit zones around the volcano up to 7 km in radius.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia St. Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost. gov.ph/).

May 2001 (BGVN 26:05) Cite this Report

April 2000-May 2001 summary; dome growth beginning in January 2001

Since the last report (BGVN 25:04), activity was variable at Mayon. The following report covers activity during April 2000-May 2001, but does not include the event that began on 24 June 2001; details of that eruption will appear in a subsequent issue. This report was compiled from reports posted on the Philippine Institute of Volcanology and Seismology (PHIVOLCS) website.

April-June 2000. Mayon's hazard status remained at 2 (on a scale of 0-5) as of 2 April. At that time, no entry was allowed within the 6-km-radius Permanent Danger Zone (PDZ) and the 7-km-radius Extended Danger Zone (EDZ) in the SE sector. Low-frequency (LF) and high-frequency (HF) earthquakes, and short-duration HF tremors, were recorded. Around this time, SO₂ flux increased from 3,600 metric tons/day (t/d) to 6,210 t/d. The summit crater emitted a weak to moderate steam plume which drifted WSW. Faint crater glow was observed during the evening. Similar activity continued through the end of April, although the SO₂ emission rate had decreased to 4,061 t/d as of 26 April.

Seismicity during 2-3 May included seven LF earthquakes with relative amplitudes of 55-56 mm, but there was no other variation in activity. On May 3 PHIVOLCS raised the Alert Level from 2 to 3. The next Mayon volcano bulletin, issued on 1 June, noted that SO₂ flux on 21 May was 680 t/d, slightly above the baseline of 500 t/d.

By 1 June the hazard status had been decreased to Alert Level 0. Seismicity had also decreased markedly; only two HF events and two short- duration HF tremors were reported on 1 June. Crater illumination resumed the same day. SO₂ flux readings were not available for the month.

July 2000. On 16 July at 0629 a phreatic explosion occurred that was visible only from the E due to thick clouds on the other sides. The explosion produced a small volume of gray ash as well as steam clouds that rose ~1 km above the summit before drifting NNE. Mayon Volcano Observatory at Ligñon Hill (MVO) seismographs recorded an explosion-type seismic signal that lasted for 1.5 minutes. Tiltmeters at Buang and Mayon Resthouse stations did not, however, detect significant ground movement, which suggested that the explosion was caused by shallow activity.

On 30 July at 1315, Mayon produced a mild ash ejection. MVO reported a small ash plume that rose 1 km. Seismicity associated with the event lasted for about 1 minute. As with the 16 July event, other monitoring, including SO₂ flux readings, did not indicate further activity. Mayon's Alert Level was undisclosed for the month.

August-December 2000. A mild ash ejection at 1432 on 31 August sent a small gray ash cloud ~1 km above the summit. An activity update on 1 September noted that small explosions similar to those in July had occurred in the previous weeks. PHIVOLCS suggested that these shallow explosions were probably due to rainwater seepage into the February-March 2000 lava deposits (BGVN 25:04). No further reports were issued in 2000.

January 2001. A resurgence of activity was observed as of [8] January. MVO reported an apparently growing lava dome which emitted voluminous gases from its summit. During the previous week there had been increases in both the number of earthquakes and in tilt, presumably due to magma ascent. [These] events led PHIVOLCS to set the Alert Level to 2.

On 10 January aerial observers noted that the dome appeared to have a spiny, blocky surface, which resulted from the crater floor being pushed upward by rising magma. Slight incandescence was also emanating from the crater. Correlation spectrometer (COSPEC) measurements detected an elevated SO₂ emission rate of 2,300 t/d. Seismicity also remained elevated. Ground deformation measured on the N flank continued to indicate tilting. Over the next week, activity remained high. Crater glow, however, was weak, and only visible from a distance with a telescope.

Activity escalated further after 19 January. Sixty seismic events occurred on 20 January, and a high number of earthquakes continued to occur. SO₂ flux spiked up to ~8,070 t/d. A brown steam puff rose from the lava dome at 0932 on 22 January. This brief emission of ash-laden steam coincided with a volcanic earthquake. A second ash emission occurred later the same day. Alert Level 3 became effective as of 25 January. Five ash emissions rose from Mayon's summit on 28 January followed by two more the next day. Plumes rose ~500 m and generally drifted WNW or NW. The earthquakes associated with these late January events were noticeably larger than those in previous weeks. Inflation of the edifice was also detected.

February-May 2001. The Alert Level remained at 3 for the entire period; high seismicity and moderate steaming prevailed. Inflationary trends were shown by tiltmeter readings through the end of March, when uplift tapered off slightly. On 24 February a small ash-and-steam plume rose 250 m and was blown ENE. SO₂ flux decreased through February with a reading of 2,889 t/d on the 28th. Crater glow was observed rarely during February, and not at all during March.

On 2 April the SO₂ flux rose to 7,205 t/d, but then dropped to 444 t/d two days later. SO₂ emission rates ranged from ~2,000 to 4,000 t/d during the rest of April. Low-intensity crater glow was observed sporadically during the month. On 7 May more intense crater glow was observed. A small ash emission occurred at 1752 on 11 May and sent material 50 m above the summit.

On 12 May a series of explosions were detected by a seismometer S of the summit. Ash ejection occurred, and late in the day the SE portion of the dome partially collapsed, causing a small lava avalanche that reached ~300 m down into Bonga Gully. Following the avalanche, MVO workers noted incandescence at the dome and continuing rockfalls into the gully. Workers speculated that active magma transport upward toward the crater was increasing.

Rockfalls due to molten lava fragments rolling down from the dome dominated activity during 13-14 May. When conditions cleared briefly on 14 May observers saw that the partial dome collapse had produced a V-shaped gash; this breach was the source of the outpouring lava. Avalanches had reached 500 m downslope as of this date.

Rockfalls and lava emissions ceased on 15 May but resumed the following day. Fresh lava began to refill the previously formed gash. SO₂ flux remained high, and tiltmeters detected consistent inflation through 31 May. Similar activity, accompanied by elevated seismicity that included rockfall-induced signals, continued through the month.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia Avenue, U.P. Diliman, 1101 Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

June 2001 (BGVN 26:06) Cite this Report

Eruption escalates; pyroclastic flow on 24 June

The following report covers activity during 28 May through most of June 2001, and discusses the high-energy event that began 24 June. This report was compiled from those posted on the Philippine Institute of Volcanology and Seismology (PHIVOLCS) website. Until the evening of 23 June the five-step PHIVOLCS hazard status system for Mayon stood at Alert Level 3, a status that implies a rapid rate of magma supply and that an explosive eruption may occur within weeks. This projection proved true as both the monitored parameters and the vigor or eruptive events rose significantly in late June. A pyroclastic flow on 24 June stimulated the rise to Alert Level 5, and this status remained for all or most of the month. Tables 3 and 5 summarize SO₂ flux and seismic data; table 4 describes the qualitative scale of crater glow intensity.

Table 3. SO₂ fluxes for Mayon during 28 May through June 2001; questionable values that were ambiguously referred to in the daily report appear in parentheses. Mayon's stated baseline values have been ~ 500 metric tons per day (tons/day). Values were measured by COSPEC. Taken from reports posted on the PHIVOLCS website.

Table 4. Qualitative scale of the intensity of crater glow used at Mayon. Through mid-June, crater glow fell into one of the first three categories; heightened activity led to stronger glow and Intensity IV was introduced; it was first reported for the evening of 23 June. Crater glow was often mentioned in daily reports, sometimes with descriptions of the incandescent part(s) of the dome or lava flows. Courtesy of PHIVOLCS.

Table 5. Mayon seismic data at Upper Anoling station as posted on daily reports in June, with the relative amplitudes shown in parentheses where clearly stated. Dashes are used to represent undisclosed values. "Tremor" refers to short-duration high-frequency harmonic tremor linked to rockfalls. Some intervals of continuous tremor appeared in late June as noted in the comments. Courtesy of PHIVOLCS.

Activity during 1-8 June 2001. During this time period, seismic instruments registered generally increasing numbers of tremors (table 5). Many of these tremors were of high frequency but short duration and inferred to be associated with mass-wasting of lava-dome fragments that descended from the volcano's SE rim. Other kinds of tremor were seen later in the month (see table 5).

The summit lava dome glowed brightly (Intensity III, table 4) during cloud breaks on the night of 1 June. During 2-8 June crater glow held steady at a Level II intensity except for 4 and 6 June when it varied between Level II and Level III. Incandescent materials occasionally rolled down from Mayon's summit, traveling along the SE slopes in the upper Bonga Gully. Glow came from detached zones of extruding, pasty lava at the dome's W base and SE face. On 3 and 6 June moderate to weak steaming issued from the summit crater.

Activity during 9-16 June 2001. As observed from Legazpi City and vicinity, lava fragments frequently detached from the summit dome and slid or rolled into the Bonga Gully to the SE and deposited a pyroclastic fan on Mayon's middle to upper slopes. Nearly continuous rockfalls produced distinct ground tremor with high-frequency spectra. PHIVOLCS noted that recordings of these multiple rockfall events from the reference station in Upper Anoling graded into each other, indicating more vigorous extrusions and rockfall events than those recorded by the station.

Ground-deformation surveys using EDM (Electronic Distance Meter) instruments were unable to make readings due to weather during 2-8 June. The previous reading, made on 28-29 May 2001, found universal inflation (i.e. displacements along the line LHO-Lower Slope measured -9 mm and the line Buan-MRHO, -6 mm). Ground deformation recorded on 10 June again indicated a minor degree of inflation (the line Buang-MRHO, -1 mm).

At 1819 on 12 June, part of the summit lava dome collapsed and heralded a period of vigorous rockfalls from the lava dome; however, no lava flow formed. Bright glow (Intensity III) occurred at a point in the mid-portion of the dome where extruding pasty lava squeezed out.

On 10 June moderate steam emission at the summit correlated with an SO₂ flux of 4,115 metric tons/day (t/d) (table 3). At this point in time, Mayon was still considered to be in a mild state of eruption with magma only slowly intruding the summit. On 11 June PHIVOLCS noticed an increase in the overall tempo of unrest, including days with elevated numbers of rockfall-induced tremor.

At 1347 on 11 June the dome partially collapsed and produced a small pyroclastic flow that descended along the Bonga Gully. The flow reached about 1,480 m elevation and produced a thin ash cloud, which drifted E. Similarly, on 12 June at about 1819 the summit lava dome again partly collapsed, spawning vigorous, continuous emissions of lava fragments until about 1930.

Activity during 17-23 June 2001. On 23 June mild explosive activity and lava fountaining took place. Prior to that, a significant change in the pace of unrest was indicated by the appearance of tremor at 0405 on 19 June. A lava flow spotted during a cloud break from 1008-0152 enabled observers to see an intense glow emitted by the dome and the margins of a newly emplaced lava flow, which extended to about 500 m below the summit dome (to ~1,800-1,900 m elevation). The tremor so dominated the seismic record that discrete rockfall counts dropped. Only 76 rockfall-related tremors were registered, although extrusive activity had clearly increased. The lava flow signified that hotter, more fluid, and more voluminous lavas were being extruded. The new lava corresponded to a sudden increase in sulfur dioxide emissions from 1,700 metric tons/day (t/d) the previous week to nearly 6,000 t/d on 19 June.

By 20 June the volcanic edifice had inflated slightly as recorded by ground-deformation surveys. Tiltmeters midway up on the NE edifice, at the Buan-Mayon Resthouse station, registered accelerating inflation. During 1209-1218 on 20 June a portion of the lava dome collapsed, generating brownish dust clouds along the Bonga Gully.

On 21 June lavas were seen exiting from two points of the dome. Two lobes descended, both on the SE side (in the general direction of the settlements of Buyuan and Mabinit). Magma ascent through the uppermost levels of the volcano's conduit appeared to be associated with high-frequency harmonic tremor at all five seismic stations in the vicinity of the volcano. Magma intruding the summit area also exerted pressure on the edifice and influenced ground tiltmeters. The COSPEC instrument measured the highest SO₂ flux of the June episode: ~9,000 t/d.

The main lava flow moved SE in the general direction of Mabinit on 21 June, and the lowermost toe of the lava flow descended 300 m farther, to ~1,500 m elevation. On 22 June the lava flow reached 1,200 m elevation; by 23 June, it had descended 3.4 km from the summit to reach 600 m elevation.

At 1909 on 23 June, lava fountaining in the summit crater ejected material at least 50 m above the rim, with the bulk of pyroclasts falling to the SE (into the upper Bonga Gully). As lava flows continued to travel SE they generated high-frequency tremor. Activity was still dominated by relatively rapid but quiet effusion of lava. At this point the seismicity lacked clear explosion signals and deformation measurements lacked inflation signals; it was believed that such signals would presumably accompany a major explosive eruption (if one were to occur).

Activity during 24-30 June 2001. At 2000 on 23 June the Alert Level was raised from 3 to 4 when the already substantial lava extrusions changed from quiet effusions to more explosive, but nonetheless non-destructive, Strombolian outbursts. The latter were first observed in the crater at 1909 on 23 June. Small explosions in the crater sent molten lava up to 50 m above the rim.

At 0317 on 24 June, a series of strong explosions were audible as far as Lignon Hill Observatory, 12 km SSE of the volcano. Accompanying ash columns reached 1 km above the summit. Visible molten lava fragments were thrown to 300 m in height. Lofted ash blew N and ash fell in the barangays (settlements) Amtic and Tambo of Ligao City and barangays San Vicente, San Antonio, Quinastillojan, Bantayan, Tabiguian, and Buang of Tabaco City.

At 1245 on 24 June a pyroclastic flow descended the Bonga and Buyuan Gullies to ~600 m elevation, about 4 km from the summit. An explosion from the crater also produced a 5-km-high column. Ash associated with the pyroclastic flow ascended to ~2.4 km altitude. The two ash-laden clouds then drifted NE, in the general direction of Malilipot (a town 10 km away on the coast).

The 24 June pyroclastic flows signaled the start of explosive eruptions with tall columns. At 1300 the hazard status was raised from 4 ("Hazardous Eruption Possible Within Days") to 5 ("Hazardous Eruption in Progress"). Concomitant with Alert Level 5, the previously delineated 7-km-radius Extended Danger Zone in the SE sector was extended to a radius of 8 km. People within these new zones evacuated. Areas to the E and NE of the volcano were considered prone to heavy ashfall due to prevailing winds.

Another major eruption sequence began at 1444 on 24 June, characterized by strong explosions, multiple pyroclastic flows around the volcano, and lava flows into SE-flank gullies. Following drainages, the pyroclastic flows passed the settlements of Basud, Buyuan, Mabinit-Bonga, Miisi, Anoling, Maninila, Nabonton, and Buang, all within the 6-km-radius Permanent Danger Zone (PDZ).

The main eruption cloud discharged from the crater rose to about 10 km altitude and moderate-to-heavy ash blew mainly NE towards Malilipot. Residents ~5 km N of Malipot (in Tabaco) along the coast also experienced light ashfalls. Lava flows and dilute ash clouds dominated activity after 1541. Activity waned in the early morning of 25 June. Beginning at 0037 on 25 June seismicity diminished from continuous tremors into discrete events.

On 26 June Mayon lapsed into an apparently quiet state; however, SO₂ flux remained high at 4,640 t/d and reflected active degassing from both the crater as well as from newly extruded lavas covering the summit area. Lava still flowed SE from the summit area along Bonga Gully on the 26th, but its lowermost portions moved slowly. The lava by then extended ~4.3 km from the summit. Its flow front constantly shed incandescent boulders that released gases and ash, burning vegetation along its path. However, the crater's diminished extrusion rate led PHIVOLCS scientists to conclude that the lava flow was unlikely to reach populated areas.

Although outward quiet prevailed for most of 24-30 June, several explosion signals occurred during 26-27 June. One explosion sent an ash cloud to about a kilometer above the summit and caused small lava avalanches in the upper Bonga Gully. Lava continued to trickle from the summit towards the SE along the Bonga Gully. From this time through at least 29 June crater glow stood at Intensity II and lava continued to descend from the summit crater.

Heavy rains fell on the night of 27 June. A team dispatched to the Padang area watched the river channel for lahars. Only a muddy stream flow was observed and rains eventually abated after about an hour. The swollen, muddy streams after this time meant that smaller amplitude volcanic earthquakes were often obscured by the seismic noise produced by the streams. Ground deformation measurements employing EDM instruments and electronic tiltmeters continued to indicate inflation of the edifice. Observers also noticed small rockfalls, and vigorous steaming of the hot lava deposits.

At 1605 and 1702 on 30 June, explosions generated pyroclastic flows that swept the upper and middle slopes within the Bonga Gully and produced billowing ash clouds to about 4 km altitude. Their runout distance reached ~3 km from the summit (in the general direction of Matanag). During the eruption an undisclosed portion of the volcano's E sector also collapsed along the Upper Basud Gully.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia Avenue, U.P. Diliman, 1101 Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

August 2001 (BGVN 26:08) Cite this Report

Two main episodes in 2001; quiet seen in late August

Mayon has undergone two eruptive episodes thus far in 2001. The first episode began in January 2001 and involved a period of unrest that culminated in explosive eruptions on 24 and 29 June. The second episode took place on 20 July, climaxing on 26 July. Low-level lava spattering and active degassing continued for days after the latter climax but activity dropped in early August.

The stratovolcano was last reported on through 31 May 2001 (BGVN 26:05); the present report covers through mid-August 2001. The volcano's Alert Levels are discussed in more detail in the last section.

Precursors and minor explosive activity. Unrest during the year 2001 was first recognized on 8 January when the Lignon Hill Observatory (LHO) in Legaspi City (11.5 km SE of the summit) reported a blocky lava dome growing on top of the summit. Lava dome extrusions occurred before an explosive eruption the previous year, so the January 2001 dome was an ominous sign of renewed activity. From January to April 2001, the dome slowly grew and sporadic ash explosions accompanied or followed periods of seismic unrest. The hazard status was set at Alert Level 2, signifying the ascent of magma.

During the second week of May, LHO staff noticed that the growing summit lava dome overlapped the unconfined side of the SE crater rim. At 1752 on 11 May a minor explosion ejected ash and vapor to 50 m above the summit. A series of similar small explosions followed on 12 May that were likely triggered by magma intruding into the dome. As a result, the SE portion of the dome partially collapsed.

Subsequently, the SE flank of the dome facing the observatory glowed conspicuously and lava fragments began to detach from the summit lava dome. Rockfalls were episodic at first and it was not clear initially whether detaching lava was caused by instability of the growing dome or due to the effects of increased internal pressure.

In time, observations from Bonga, ~8 km SE of the summit, indicated that incandescent rockfalls were apparently caused by slowly ascending magma entering the dome. The magma was degassed but hot, presumably a remnant of magma erupted during 2000. PHIVOLCS later postulated that ascending magma punched an exit point on the SE flank of the growing lava dome. This material then spilled into the Bonga Gully, with hot lava boulders as big as trucks falling, rolling, and sliding to form a pyroclastic apron on slopes at 1,800-2,000 m elevation. Rockfall activity, monitored via the seismic network, progressively increased in frequency until magma discharge was sufficient to form a stubby lava flow on 17 June. By 20 June, the seismograms displayed more or less merging codas of high-frequency tremor, which suggested that lava extrusion dominated earlier rockfall activity. As seen earlier, the lava flow was thought to represent relatively fresh but still degassed magma.

Lava fills crater then extends 5 km. By 22 June, lava had already buried the summit dome and partially filled the crater. Lava was no longer exiting from a single patch at the side of the dome but from the whole breadth of the SE summit.

Episodes of conspicuous summit glow began on 23 June, and intensified to a pulsating light-yellow incandescence by early evening. The summit did not stay quiet for long because the crater began to vent voluminous gases and to shower spatter around the summit. COSPEC readings indicated an SO₂ flux of ~7,000 metric tons per day (t/d), well above the baseline of ~500 t/d. At about 1909 on 23 June, a period of low-level lava fountaining began to feed lava flows that eventually descended from the summit elevation to ~500 m elevation-a distance of ~5 km.

When lava fountaining commenced the Alert Level rose from 3 to 4. This status meant that PHIVOLCS considered a hazardous eruption imminent, within hours to days. The corresponding Level 4 Bulletin carried with it a recommendation to evacuate areas within the 6-km-radius Permanent Danger Zone (PDZ) and a 7-km-radius Extended Danger Zone (EDZ) in the SE sector. The EDZ provided a buffer zone to the Bonga Gully, which descends from near the crater mouth to the lower mid-slopes (~600 m elevation) to the SE, a distance of ~4 km. By 0100 on 24 June the PDZ and EDZ were fully evacuated through the efforts of a group called "Task Force Mayon," a military and civilian organization charged with implementing the evacuation of the danger zones. Temporary shelters received ~25,000 people.

At 0317 on 24 June a series of explosions fed an ash column that rose to ~1 km above the volcano's summit. A thin blanket of ash fell mainly on the northern half of the volcano in the vicinity of barangays (hamlets) Amtic and Tambo of Ligao City and San Vicente, San Antonio, Quinastillojan, Bantayan, Tabiguian, and Buang of Tabaco City.

First substantial pyroclastic flows. Although lava fountaining and small ash puffs signaled the start of explosive activity, it was not until 1245 on 24 June that the first major pyroclastic flow occurred. It followed the eastern branch of the Bonga Gully in the general direction of Barangay Buyuan. PHIVOLCS promptly raised the status to the highest Alert Level, 5, first verbally to provincial disaster-mitigation officials shortly after 1245, followed by an official bulletin released by 1300. Alert Level 5 provided a reminder that hazardous eruptions were taking place. Although the 1245 pyroclastic flow was short-lived and ran down to the middle slopes only (~700-1,000 m elevation), this again-elevated status emphasized that more explosive eruptions were expected.

At 1444 on 24 June, large explosions commenced and generated multiple pyroclastic flows around the cone. Ash clouds from the eruption column and pyroclastic flows enveloped the volcano in ash and rose to ~10 km altitude. Although the volcano seemed to disappear within its own eruption clouds, giving the impression of massive explosions that might have threatened the lowlands, the pyroclastic flows and lava flows were all contained within the PDZ, with maximum runouts to only ~5.5 km.

Considerable airfall ash blanketed the northern areas, particularly the cities of Ligao and Tabaco, but this was chiefly a function of wind velocity and direction, because the wind mostly comes from the SW this time of the year.

Eruptions continued until 1921 on 24 June when seismographs began to record diminishing eruption intensity as indicated by decreasing harmonic tremor amplitudes. However, sporadic explosive eruptions continued throughout the evening as LHO noted light ashfall in Legaspi up to about 2135 that day. Thereafter, during 25-28 June, Mayon remained quiet, although Alert Level 5 was maintained in anticipation of more explosions.

At around 1605 and 1702 on 29 June, Mayon erupted again and sent relatively small pyroclastic flows down the Bonga Gully to the SE. Over the period 30 June to 19 July, Mayon's apparent activity waned and the hazard status was eventually lowered to level 3 (which states that an eruption may still be expected within the coming weeks). Observations in support of reduced activity included a general deflation of the edifice, decreased seismic activity, lowered gas emission rates, and the disappearance of summit incandescence. The first eruptive episode ended and scientists inferred that intrusions into the cone had ceased.

Activity during late July 2001. Mayon's eruptive episode during July 2001 was essentially a continuation of June's activity. On 20 July seismographs around the volcano recorded high-frequency, short-duration tremor associated with rockfalls. The number of seismically detected rockfalls had already declined from the pre-June 24 eruption level of more than 200 events per day to (by 19 July 2001) a post-eruption level of less than ~10 events per day. The latter number was attributed to unstable, freshly deposited lavas on steep upper slopes.

Scientists were alerted when the S-flank seismic station at ~800 m elevation registered an abrupt increase, from 5 rockfall events on 19 January to 48 events on 20 January. Over the same time period an upper seismic station (at 1,700 m elevation) recorded a jump from 25 to 142 events. Incandescent rockfalls became persistent.

Other striking changes soon occurred. On 21 July the SO₂ flux tripled, to 7,400 t/d. The uppermost electronic tiltmeter (at 1,700 m elevation) fluctuated by ~20 µrad. Crater glow increased and rockfall occurrences peaked.

PHIVOLCS inferred that Mayon had again entered a mild eruptive stage. The character of unrest resembled activity observed between mid May and 20 June, prior to explosive eruptions on 24 June. Scientists recognized that an explosive and hazardous eruption could occur anytime. By 23 July, PHIVOLCS gave the Albay provincial government a notice of increasing unrest and by 25 July, the Municipal Mayors were informed of reactivation and possible explosive eruption of Mayon.

Overall, unrest was accelerating. On the morning on 25 July, the bulletin also added that the current extrusion of lava was clear evidence of eruption and that more explosive eruptions were expected. At 0418 on 25 July seismometers detected more or less continuous high-frequency tremor. Although clouds shrouded Mayon, volcanologists believed these signals indicated that a lava flow had extruded from the dome, an idea confirmed when observers saw a short lava tongue draping the SE slope just below the summit crater.

During 0219-0315 on 26 July, LHO staff saw mild lava fountaining that reached to ~70 m high. This prompted the return to Alert Level 4 at 0400 on 26 July and a rapid evacuation. During quiet times, farmers work portions of land within the 6-km-radius PDZ, but at Alert 4, people in this zone are required to evacuate as quickly as possible. As in the previous 24 June eruption, a 7-km-radius SE-flank EDZ was also declared (to include river gullies upstream of barangays Mabinit, Bonga, Buyuan and Matanag). But, lava fountaining declined at about 0400 and the volcano seemed quiet. This led some people to be initially lax, and some farmers viewed the lull as an opportunity to gather their livestock near the Bonga Gully. PHIVOLCS firmly advised not to proceed. This warning proved justified when at 0538 a brief burst from the crater sent an ash cloud to ~500 m above the summit. This was accompanied by a low-frequency type earthquake that lasted for about a minute. A lack of urgency towards evacuating may have been widespread. Legaspi City Mayor Rosal made the following admission, which appeared in The Philippine Star the next day. "We were surprised by its sudden explosion. We were told to evacuate last night but we did not know it would explode so fast."

At 0745 on 26 July there occurred another ash explosion with similar seismic signature. In retrospect, sequences of low-frequency seismic events were detected by the Mayon Resthouse station (780 m elevation) before the onset of explosive eruptions at 0756 on 26 July. These events were not detected at other stations or were obscured by high-frequency tremor associated with both lava flowing out at the uppermost elevations and lava fragments detaching from the advancing lava flow.

The 0756 eruption produced a turbulent head of steam and ash, followed by a column of roiling dark-gray ash clouds. The column convected to ~10 km altitude while pyroclastic flows descended the Bonga (SE flank) and Basud (E flank) gullies. Upper-level winds conveyed the topmost eruption column to the SW. Lower-level winds carried fine ash lofted upwards (elutriated) from pyroclastic flows to the SE. Accordingly, the main ashfall deposit reached ~7 mm or more in thickness to the SW (in Camalig); it included scoria up to 10 cm diameter and perhaps larger. Most scoria fragments broke up upon impact with hard surfaces such as concrete and asphalt, but scoria clasts that landed on softer ground were preserved. A second ashfall deposit occurred to the S, SE, and ESE (in Legazpi, Daraga, and Lidong, respectively), amounting to ~5 mm thickness during this initial eruption. Additional lighter ashfalls occurred to the S (in Daraga) and to the SW (in Guinobatan).

A brief helicopter flight over Albay Gulf looking at Legaspi and Santo Domingo showed the dark curtain of ash progressively blanketing these localities. Pyroclastic flows remained well within the PDZ, a fact used to conclude that additional areas were not endangered. Only small-volume pyroclastic flows were seen descending the S-flank regions (Mi-isi and Anoling gullies).

The eruption that began on 0756 on 26 July lasted for about an hour. Ash clouds remained suspended throughout the day, even when Typhoon Feria's rains swept over Mayon. At 1420 that day another episode of eruptions began. Although the suspended ash and rain clouds covered Mayon, harmonic tremor and booming sounds signified explosive discharge until about 1500. A third and final eruption episode occurred from 1749 until 1810. Like the second period of eruptions, ash and rain clouds obscured much of the volcano from Legaspi. From Santo Domingo, however, pyroclastic flows were seen descending the Basud Gully. A ground survey to Bonga, facing this gully in the SE indicated that very small pyroclastic flows were passing here, yet there were large pyroclastic flows to the E.

When the eruption cleared the following day, observers recognized that the septum between the Bonga and Basud Gullies near the summit had breached. It is therefore very likely that late-stage pyroclastic flows during the third eruptive episode were funneled through Basud and little material was channeled along the Bonga Gully. This demonstrates the high probability that subsequent flows will also affect the eastern sector and not just the SE. Fortunately, flow runouts remained within defined danger zones.

On 27 July Mayon entered an effusive state as lava from the summit fed a flow that eventually reached ~3.75 km to the SE at an elevation of ~650 m. This was smaller than the lava flow extruded in June; it traveled farther and eventually reached ~5.5 km down the SE slope at ~500 m elevation. Hazy conditions in the SE foothills were caused by ash-and-steam plumes from the summit and from pyroclastic-and lava-flow deposits. Seismicity remained active, with signals from sporadic explosions and persistent background tremor related to lava flows and other surface events. Numerous (206) discrete rockfall signatures, for example, were detected by the seismic network and many of these were visually confirmed from LHO. The resumption of rockfalls was interpreted to not result from another intrusion but from loosened lava debris on steep slopes.

The SO₂ flux at 6,450 t/d remained very high on 27 July and even on the following days, SO₂ emission rates varied between 3,265 and 9,915 t/d. Voluminous degassing coincided with loud roaring from the crater, which caused some residents of Santo Domingo, at least 8 km E of the crater, to evacuate. According to residents, the last time they heard the crater degas loudly was prior to the resurgence on 23 September 1984, so that they were troubled when they heard another explosive eruption after 26 July 2001. The concern was not at all unfounded. Although incandescence of the summit already diminished to faint conditions as observed from LHO, some low-level fountaining became evident on video cameras with night vision. The cameras clearly showed blobs of lava thrown 100 m above the crater rim. This new observation, along with elevated seismic and SO₂ levels, and other monitored parameters, kept the alert status at Level 5.

Waning activity. It was not until there were clearer signals of gradual decline of activity that PHIVOLCS lowered the Alert Level 5 status to Level 4. A bulletin on 9 August 2001 explicitly noted the cessation of explosive eruptions.

After 10 August seismic activity decreased. Background tremor associated with active magma transport had stopped and rockfall occurrences had become insignificant. The number of low-frequency volcanic earthquakes occurring daily was still above baseline, up to 22 events, but this is not unusual after an eruption of Mayon and was probably related to shallow magma degassing. The SO₂ fluxes, up to 6,600 t/d, were still very high, presumably for the same reason. Electronic tiltmeters supported the idea of substantial degassing, showing a general deflation episode following the 26 July eruption. In summary, while various monitoring parameters continued to show significant unrest of Mayon, the general trend was one of declining activity. This information may be used to eventually lower alerts over the volcano and allow the return of evacuees to their homes by the end of August 2001.

June and July eruptions compared. The eruptions in June appeared to be more voluminous and produced more lavas than tephra. The estimated volume of 15 x 10⁶ m³ was in the ratio 2/3 lava and 1/3 pyroclastics. The June eruptions also produced pyroclastic flows that ran through many gullies radiating around the cone. The 26 July eruption produced roughly similar proportions of lava and tephra (namely, 5 x 10⁶ m³ lava; 6 x 10⁶ m³ tephra).

When the 26 July pyroclastic flows poured down the SE and E flanks, the low-altitude SE winds caused Legaspi City to be enveloped in ashfall. Legaspi City generally remains ash-free due to seasonal wind patterns. Not fully prepared to cope with ashfall, many residents panicked even though the threats to life were virtually nil. Phone lines jammed and vehicle traffic was backed up for several kilometers on the highway from Rawis, Legaspi City to Padang, and Santo Domingo. Busy communication networks also prevented PHIVOLCS from relaying real-time information by telephone to the central office in Quezon City. Fortunately, anticipation of explosive eruptions earlier that day meant that warnings to local and national authorities were already sent out. A notice to the Volcanic Ash Advisory Center in Tokyo was also made that morning.

Another marked difference between the June and July 2001 unrest was the time interval between perceived disquiet to the day of explosive eruption. The 24 June eruption was preceded by over a month of seemingly increasing rockfall activity. In a sense, rockfalls were an indicator of magma-discharge rates and the number of rockfalls per day progressively increased up until lava-flow extrusion. In contrast, the period between the onset of rockfalls and the 26 July eruption was barely a week, so that magma-discharge rates jumped abruptly before the onset of lava extrusion and explosive discharge.

Background provided by PHIVOLCS. The towering Mayon stratovolcano is famous for its highly conical shape and its symmetry. It is the most active volcano in the Philippines, with 47 historical eruptions since 1616. The typical eruption episode lasting from a few days to about a month produces a sequence of basaltic andesite lava flows, pyroclastic flows, and tephra falls. Based on geological studies on the nature and extent of deposits, a 6-km-radius "Permanent Danger Zone" (PDZ) has been defined to discourage people from permanently occupying hazardous areas.

Table 6 shows the Mayon warning scheme devised by PHIVOLCS. It is similar to the one employed at Pinatubo. Six alert levels provide the general activity status.

Table 6. A simplified version of the current warning scheme used at Mayon. Courtesy of PHIVOLCS.

It has been suggested that Mayon erupts every 10 years, referring to the eruptions of 1928, 1938, and 1947. Then there were the eruptions of 1968 and 1978 as well as the interval between 1984 and 1993 events. Yet in recent years, it seems that this general periodicity has changed. The Millennium eruption, 24 February to 7 March 2000, occurred just 7 years after the 1993 outbursts. A similar period of repose is evident in the interval 1978-84. In fact, close inspection of the historical record suggests other intervals with eruption repose periods of less than 10 years.

Information Contacts: Ernesto Corpuz, Philippine Institute of Volcanology and Seismology, C.P. Garcia Ave., Univ. Philippines Campus, U.P. Diliman, 1101 Quezon City.

April 2002 (BGVN 27:04) Cite this Report

Declining activity prompts PHIVOLCS to lower Alert Level to 0

Eruptions at Mayon in June and July 2001 were followed by a decrease in seismic activity beginning on 10 August. Low-frequency volcanic earthquakes and SO₂ fluxes were still high and were probably related to shallow magma degassing. While various monitoring parameters continued to reflect significant unrest, the general trend was one of declining activity (BGVN 26:08).

Volcanic activity remained low during August. There was relatively little seismicity, slight inflation, occasional observations of incandescence at the summit, and a moderate amount of steam emission. SO₂ flux remained well above the baseline of 500 metric tons per day (t/d) (table 7). SO₂ emission rates reflected continued degassing of cooling magma, and ground-deformation data continued to indicate the absence of magma intrusion. On 21 August the Alert Level was lowered to 3 and, following a continued decrease in activity, on 19 October it was lowered to 1.

Table 7. Earthquakes, tremor, and SO₂ flux at Mayon during 13-30 August. Differences in reported daily and weekly data during 20-26 August could not be resolved by press time. Courtesy PHIVOLCS.

News reports on 21 November stated that lahars were generated after several days of heavy rainfall mixed with unconsolidated material on the volcano's slopes. According to the civil defense, flooding caused more than 4,800 families to be evacuated from their homes.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that since 19 October 2001, when the Alert Level was lowered to 1, all measured parameters had continued to decrease to near-baseline levels. Ground deformation data from electronic tiltmeters continued to indicate the volcano's deflated condition, and SO₂ emission rates yielded relatively low values of 450-900 t/d. The observations implied that no active magma intrusion was occurring beneath the active cone. Although incandescence was still visible at night, PHIVOLCS suggested that it was likely due to still-hot magma beneath the crater. As a result of the low activity, on 5 February PHIVOLCS lowered the Alert Level to 0, but reminded the public to avoid the 6 km Permanent Danger Zone, and residents near major river channels emanating from the volcano were advised to be on alert during heavy rainfall because loose pyroclastic deposits could be remobilized as life-threatening stream flows and lahars.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia Ave., Univ. Philippines Campus, U.P. Diliman, 1101 Quezon City; Associated Press.

March 2003 (BGVN 28:03) Cite this Report

Small ash puff on 11 October 2002; explosions on 17 March and 5 April 2003

Until 11 October 2002, no significant volcanic activity had been reported since eruptions in June and July 2001 (BGVN 26:08). Subsequent deflation, combined with declining seismicity and sulfur dioxide flux, resulted in the Alert Level being lowered to 0 (no eruption is forecast in the foreseeable future, but entry in the 6-km radius Permanent Danger Zone (PDZ) is not advised because phreatic explosions and ash puffs may occur without precursors) in February 2002 (BGVN 27:04).

Mayon remains intermittently active, with tremor episodes, a small ash puff in October 2002, steam emission in January 2003, and an explosion and ash plume in March 2003. Small ash explosions on 5 May and 6 April will be described in the next Bulletin.

Activity during October 2002. At 0635 on 11 October 2002 the volcano produced a small ash puff that reached 500 m above the summit crater. The small ash cloud from this minor explosion quickly diffused and drifted E without noticeable deposits on the slopes. The ash puff followed a series of imperceptible volcanic tremors that began in the early hours of 22 September and occurred sporadically until the last tremor was recorded on 9 October. The 11 October report from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) also noted that slight swelling of the volcano's edifice was detected by an electronic tiltmeter on the S flank. However, the Alert Level remained at 0.

A 30 October notice from PHIVOLCS indicated that the number of volcanic earthquakes, although imperceptible, remained significantly above background levels since the ash emission of 11 October. Another notable observation was the occurrence of small volcanic tremors and consistent inflation detected by electronic tiltmeters, which suggested that magma was intruding into the volcano. Gas output from the summit had increased from recent emission rates of ~950 metric tons per day (t/d) to ~2,200 t/d on 29 October. Because of these consistent increases in monitored parameters, PHIVOLCS raised the Alert Level to 1. Although a major explosive eruption was still considered unlikely at this stage, the persistent unrest over the previous weeks clearly indicates a shift from its former period of repose. Alert Level 1 is meant to call attention to increased volcanic activity specifically an increased likelihood for steam-driven or ash explosions to occur with little or no warning. During the last week of October PHIVOLCS augmented its monitoring network around Mayon with additional personnel and equipment.

Activity during January 2003. A brief period of vigorous steam emission occurred at 1753 on 31 January after an episode of volcanic tremor the previous day. The steam ejection lasted for about a minute and produced a dirty white steam cloud that rose ~500 m above the summit crater. A low-frequency, short duration, harmonic tremor coincided with the steam venting. The sulfur dioxide emission rate increased slightly to 764 t/d on 31 January from the previous reading of 441 t/d taken on 21 January, which followed several episodes of low-frequency volcanic tremor during the previous weeks.

Activity during March 2003. An explosion from the crater at 1819 on 17 March sent ash and steam ~1 km above the summit before it was blown WNW by winds. The explosion was recorded as a high-frequency seismic signal, indicating a sudden release of pressure. No significant seismicity was apparent prior to the event. Measurements of SO₂ flux within the emission plume between 0900 and 1100 earlier that morning averaged ~890 t/d, which is more than the usual 500 t/d typical during periods of repose. Electronic tiltmeters on the N and S flanks indicated slight inflation of the edifice beginning on 13 March. Due to the increased possibility of additional ash ejections, the hazard status was raised to Alert Level 1.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs. dost.gov.ph/).

May 2003 (BGVN 28:05) Cite this Report

Three small ash-and-steam explosions during April-May 2003

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported small ash and steam explosions from the Mayon volcano on 5 April, 6 May, and 14 May 2003. The alert status for the area around the volcano remained at Alert Level 1 on a scale of 0-5 (indicating an increased likelihood for steam-driven or ash explosions to occur with little or no warning). PHIVOLCS reminded the public to continue avoiding entry into the 6-km-radius Permanent Danger Zone (PDZ), especially in the sectors where life-threatening volcanic flows might be channeled by gullies.

Activity during April 2003. Following a small ash explosion on 17 March 2003 (BGVN 28:03), a brief burst of ash and steam occurred at about 0600 on 5 April. The ash column rose to ~1.5 km above the summit crater before being blown SW. The explosion was recorded as a low-frequency volcanic earthquake, signifying a shallow source. Prior to the explosion, the volcano's seismic network had detected three small low-frequency volcanic earthquakes and three low-frequency short-duration harmonic tremors in the past 24 hours. Electronic tiltmeters indicated continuing slight inflation of the edifice. The increases in activity strongly indicated the likelihood of sudden ash explosions. Although no major eruption was expected immediately after the explosion of 5 April, there was growing evidence that magma was ascending the volcano's conduit.

Activity during May 2003. A small explosion from the crater at 0721 on 6 May produced a brownish ash-and-steam column that rose to ~450 m above the summit crater and was blown SW. The ash-and-steam column rose slowly with minimal noticeable force and was not detected by the volcano's seismic network, indicating a very shallow source. No significant seismicity occurred prior to the explosion. However, electronic tiltmeters on the N and S flanks continued to show inflation. Likewise, a precise leveling survey on 24 April 2003 showed a general inflation of the N flank. Alert Level 1 remained in effect.

At 1813 on 14 May, a small ash puff was emitted from the summit crater. This very brief explosion caused a small volume of ash and steam to rise less than 100 m above the crater and to later be blown NW. The Mayon Resthouse and Sta Misericordia seismic stations recorded the ash puff as a small-amplitude event. Prior to the ash explosion, one short-duration tremor was recorded. Volcanic gas outputs were notably moderate in volume, and the sulfur dioxide emission rates increased from the previous 1,824 metric tons per day (t/d) to ~3,088 t/d. The seismic characteristics associated with the ash and steam emission appeared similar to, though smaller than, previous explosions since 22 October 2002, indicating that this ash puff was very minor. This assessment was also consistent with the smaller volume of ash produced.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost. gov.ph/).

September 2003 (BGVN 28:09) Cite this Report

Elevated sulfur-dioxide flux after mid-September; crater glow in October

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported on 18 September 2003 that earthquake activity at Mayon had been within background levels (< 5 events/day) since 14 August with no volcanic earthquakes over the previous five days and moderate volcanic gas outputs. However, the sulfur dioxide (SO₂) flux at 1,237 metric tons per day (t/d) was above baseline levels, having increased from 829 t/d since 5 September. In view of increased SO₂ gas emissions, and recent significant earthquake occurrences, PHIVOLCS set the hazard status at Alert Level 1 (on a scale of 0-5).

For the period 29 September-5 October, 16 low-frequency volcanic earthquakes (19.0 mm amplitude), five high-frequency volcanic earthquakes (26.0 mm amplitude), and four high-frequency short-duration volcanic earthquakes (2.5 mm amplitude) were recorded, accompanied by weak to moderate steaming and no visible crater glow. During 6-12 October, 29 low-frequency volcanic earthquakes (14.0 mm amplitude), four high-frequency volcanic earthquakes (6.2 mm amplitude), and two high-frequency short duration volcanic earthquakes (2.0 mm amplitude) were recorded, with moderate steaming and faint crater glow.

PHIVOLCS reported on 9 October that a faint glow had been seen by telescope at the inner E portion of the summit crater between 2330 on 8 October and 0048 on 9 October, and again between 1630 and 1650 on 9 October. Low-frequency volcanic earthquakes occurred four and six times, respectively, during 8 and 9 October. Steam emission remained moderate, with visible plumes barely rising above the crater rim. Mayon's SO₂ flux on 9 October rose to 2,386 t/d from 1,616 t/d on 1 October.

On 11 October PHIVOLCS noted persistent and significant incandescence inside the summit crater, apparently from lava in the E portion of the volcano's conduit. Seismicity over the previous 24 hours was relatively low (three low-frequency volcanic earthquakes). The Alert Level was raised to 2, signifying instability that may lead to ash explosions or a magmatic eruption.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs. dost.gov.ph/).

December 2004 (BGVN 29:12) Cite this Report

Minor activity in June, July, and September 2004; reported ash emission

An explosion-type volcanic earthquake detected by the Upper Anoling seismic station on the afternoon of 3 June 2004 was not visually observed due to thick clouds covering the summit. Residents closer to the Upper Anoling Seismic Station and Mayon Resthouse did not notice any unusual activity. No traces of ash or changes in the crater wall were observed. Sulfur dioxide (SO₂) emission rose from 1,169 metric tons/day (t/d) on 12 May to 2,521 t/d on 4 June, then decreased to 1,514 t/d on 18 June. Precise leveling measurements showed a slight but deflation of the edifice. The number of low-frequency volcanic earthquakes and low-frequency short-duration harmonic tremors increased in June to almost twice the number recorded in May. In addition, faint crater glow continued to be observed at the summit, as it had since 7 October 2003.

Another explosion was recorded on 22 July 2004. According to news reports, ash from that event was deposited in two local villages.

Sulfur dioxide (SO₂) emissions remained only slightly above baseline at 829 metric tons per day as of 6 September 2004. On the evening of 12 September 2004 the very faint glow at the summit of Mayon intensified slightly. The brighter incandescence, observable from Lignon Hill Volcano Observatory and in Legaspi City proper, coincided with a slight increase in the overall background tremor detected by seismographs around the volcano. However, there were no significant changes in ground deformation or SO₂ measurements. A news report also noted that volcanic material emitted from the crater that day set fire to grass on the volcano's slopes.

The hazard status remained at Alert Level 2, indicating a low level of volcanism. PHIVOLCS reminded the public to refrain from venturing into the 6-km Permanent Danger Zone because life-threatening volcanic flows may occur with little or no warning.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, Univ. of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Associated Press (URL: http://www.ap.org/); The Australian (URL: http://www.theAustralian.news.com.au/).

March 2006 (BGVN 31:03) Cite this Report

Eruptions resume in February 2006 after a 2-year hiatus

Since the previous report in December 2004 (BGVN 29:12) Mayon had remained quiet until 21 February 2006. On that day the Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that a minor explosion at 0941 produced an ash plume that rose ~ 500 m above the volcano's crater and drifted SW. Ash was deposited on the upper slopes of the volcano. The ash emission was accompanied by a small explosion-type earthquake, recorded only by seismographs around the volcano.

Prior to the explosion PHIVOLCS had seen an increase in seismicity at the volcano. Between 1545 on 20 February and 0520 on 21 February, there were 147 low-frequency earthquakes recorded, a number considerably above the five or fewer events per day normally detected. Seismicity also indicated some minor rockfalls, which probably resulted from lava blocks detaching from the summit. Steaming was observed. No incandescence was visible at the crater due to clouds obscuring the volcano.

PHIVOLCS reported that about nine earthquakes related to explosive activity took place at Mayon around 23 February. Cloudy conditions prevented visual observations, but the seismic events detected probably signified minor ash explosions. This was supported by reports from local residents who heard rumbling. The seismic network also recorded two low-frequency earthquakes associated with shallow magma movement. The SO₂ flux averaged 1,740 metric tons per day (t/d), similar to values obtained during the last measurements on 28 November 2005. The flux was well above the usual 500 t/d measured at the volcano. Mayon remained at Alert Level 2, with a 6-km-radius Permanent Danger Zone in effect. At this point the possibility of more violent eruptions triggered warnings to tourists and the public in general to remain outside of the danger zone.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, Univ. of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

July 2006 (BGVN 31:07) Cite this Report

New eruptive pulse starting 13 July; lava flows; thousands evacuated

Mayon was last reported on in March 2006 (BGVN 31:03), discussing an eruption in February 2006. Low-level activity and seismicity prevailed through early July. This report covers an eruptive pulse that began on 13 July 2006 and continued through August 2006. On 13 July there were phreatic eruptions that produced light ashfall in the areas of Calbayog and Malilipot. At 2200 on 14 July, authorities raised the Alert Level from 1 to 3 due to moderate white steam drifting NE and lava flows extending 0.7-1.0 km from the summit onto the SE slopes. On 15 July, the lava flow continued its SE progression towards Bonga gully.

On 16 July, the 6 km radius hazard zone known as the Permanent Danger Zone (PDZ) established around the SE area, was extended to 7 km and during the period covered by this report the radius of the danger zone around the southern sector was extended to 8 km. On 18 July, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that the lava flow had reached 1 km in length and incandescent boulders had rolled 3 km towards the Bonga gully. Seismicity, reported SO₂ fluxes, and posted alert levels appear in table 8.

Table 8. Mayon's reported seismicity, SO₂ fluxes, and alert levels during 15 July 2006 to 24 August 2006. "?" indicates information not available. Courtesy of PHIVOLCS.

Pyroclastic flows on the SE slopes prompted approximately 100 families to evacuate on 20 July. On 22 July, lava flows advanced NE towards the Mabinit channel. By 24 July, lava flows had traveled SSE, ~4 km from the summit toward Bonga gully, and branched off to the W and E. Incandescent blocks shed from the toe and margins of the flows traveled SE and were visible at night. Additionally, on 24 July seismographs recorded more than 324 tremor episodes and 11 volcanic earthquakes. SO₂ emissions from the summit crater reached 7,000 metric tons per day, several times larger than fluxes reported earlier.

PHIVOLCS reported lava flow advance in terms of straight-line distances, which progressed as follows: 26 July, 4.45 km; 27 July, 4.7 km; and 29 July, 5.4 km. During this time, SO₂ rates remained high (table 8), suggesting fresh magma at shallow levels in the volcano. The number of tremor episodes and earthquakes also remained high. Tremor was thought to indicate near-continuous lava blocks detaching from the lava flows. Volcanic earthquakes were thought to reflect ascending magma. Figure 11 shows the lava flow front on 29 July.

Figure 11. Photograph taken on 29 July 2006 at Mayon showing the lava front as it continued to advance down the Mabinit channel. Courtesy of C. Sagution, PHIVOLCS.

On 29 July, light ash accumulation was reported about 12 km S and SE, in Daraga municipality and Legazpi City and vicinity, respectively. Emissions of sulfur-dioxide reached ~ 12,500 tons per day on 31 July, a record high for this reporting interval. By 1 August, in the SE sector of the Bonga gully, lava flows had advanced ~1.35 km, and in the SSE sector they had advanced a maximum distance of 5.8 km from the summit.

According to a Philippine Information Agency (PIA) press report, military and police checkpoints were set up on 2 August around the 6-km-radius PDZ to prohibit entry. A large lava deposit had grown on the SE flanks. The lava which faced Legazpi and Daraga, had piled up during the initial two weeks of the eruption and threatened to cross the PDZ. PHIVOLCS had reported that the advancing incandescent front of the lava flow was ~20 m high and 50 m wide (figure 12). PHIVOLCS estimated that the lava front could breach the 6-km-radius PDZ within two to three days.

Figure 12. On the evening of 3 August 2006, lava advancing down the Mayon's Mabinit channel formed this impressive front. For scale, note tree at right. Although the government had issued an evacuation warning, many tourists flocked to the scene to watch the lava flows. Courtesy of Romeo Ranoco (Reuters).

An overflight of Mayon on 6 August revealed that lavas discharging from the summit crater extended along the Mabinit channel and spilled into the Bonga gully, E of the Mabinit channel. Due to the decreased supply of lava to the Mabinit channel, the flow there was expected to cease a short distance beyond the 6-km-radius PDZ. Six ash explosions sent ash columns up to 800 m above the summit, prompting PHIVOLCS to raise the alert level from 3 to 4, indicating an eruption is imminent. According to the Manila Bulletin Online, as many as 50,00 people in the Albay province were evacuated.

On 7 August, an advancing lava flow crossed 100 m beyond the 6-km-radius PDZ. According to the Manila Standard Today, authorities warned residents of more lava and fires as the lava flows crept along the Mabinit and Bonga gullies.

During 9-15 August, explosive activity continued at Mayon after a brief respite on 8 August. Based on interpretations of seismic data, minor explosions during 9-11 and 13-15 August were accompanied by lava extrusion and collapsing lava flow fronts that released blocks and small fragments. A drop in SO₂ emissions on 9 August worried volcanologists that something had blocked the flow of magma in Mayon's conduit and could therefore cause a build up in pressure resulting in a larger eruption. Visual observations were commonly obscured by clouds. On 11 August an ash plume was seen drifting ESE. On 12 August, four explosions occurred; one produced a pyroclastic flow that traveled over the SE and E slopes and generated a plume that rose to an altitude of 500 m and then drifted NE. On 15 August, a brief break in the clouds allowed for a view and confirmed the presence of fresh pyroclastic deposits from activity in the previous days. Approximately 40,000 people remained in evacuation centers and authorities maintained an Extended Danger Zone at 8 km from the summit in the SE sector.

PHIVOLCS reported that explosions from Mayon continued during 16-19 August. On 17 August, ash-and-steam plumes drifted at least 5.3 km NE and reached the town Calbayog, where light ashfall was reported. Lava extrusion continued and on the SE slopes lava-flow fronts shed blocks and small fragments. On 18 August, the Mibinit and Bonga gully lava flows reached ~ 6.8 km SE from the summit. PHIVOLCS estimated the volume of erupted materials at between 36 and 41 million cubic meters.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), PHIVOLCS Building, C.P. Garcia Avenue, U.P. Campus, Diliman, Quezon City, Philippines, Reuters Alert Network (URL: http://www.alertnet.org/thenews/newsdesk/MAN212904.htm); The Associated Press (URL: http://www.ap.org/); Manila Standard Today (URL: http://manilastandard.net/); Manila Bulletin Online (URL: http://www.mb.com.ph/).

August 2006 (BGVN 31:08) Cite this Report

Lava extruding but with less vigor

During 6 September to 3 October 2006, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) described lava extrusion and associated rockfalls on Mayon's SE slopes. This continued the previous pattern seen during 23 August-5 September 2006 (BGVN 31:07). Mayon's eruptive vigor generally declined by mid-September into October. Background on Mayon's geography follows (figures 13-15 and table 9).

Figure 13. Satellite image of the SE area of the Philippine island of Luzon showing Mayon volcano and surrounding volcanoes and towns. Courtesy of Google Earth.

Figure 14. Oblique aerial image of Mayon, looking from the N at an eye altitude of 1.78 km. Courtesy of Google Earth.

Figure 15. Contour map of Mayon volcano showing surrounding towns (see table 9 for more detailed list of names of nearby settlements) . The prominent circle around the volcano delineates the 6-km Permanent Danger Zone. Scale is 1:50,000. Courtesy of PHIVOLCS.

Table 9. An alphabetical list including some of the settlements and other place names on and surrounding Mayon volcano, and their bearings and distances from the summit. Taken from the PHIVOLCS map referenced below.

Seismicity and lava extrusion generally decreased during 6-26 September. SO₂ fluxes broadly declined, generally ranging between 1,200 and 3,000 tons per day, although the 25 August and 2 September readings were outliers, ~ 5,400 and ~ 6,600 tons per day, respectively. Ground-deformation measurements showed an overall deflation. On 11 September, the Alert Level was lowered from 4 to 3 (on a scale of 0-5, with 0 referring to No Alert status).

During late September surface activity was characterized by intermittent spalling of incandescent lava fragments and glow from the summit crater. Steaming at the summit was moderate with white plumes drifting NNE and SE. Low-frequency tremor continued to indicate elevated unrest. Alert Level 3 remained in effect, meaning that the new Extended Danger Zone (EDZ) of 7 km from the summit crater in the SE sector and the normal 6 km-radius Permanent Danger Zone (PDZ) for other areas continued. Table 10 lists Mayon's reported seismicity from 25 August-27 September 2006, continuing the list developed in BGVN 31:07.

Table 10. Summary of 25 August-3 October 2006 events observed at Mayon volcano for 24-hour periods ending at 0800 hours on the date indicated. The SO₂ emission rates apply to the gas within the volcanic plume. No data was available for 10, 28, or 29 September. Courtesy of PHIVOLCS.

Reference. PHIVOLCS, (date unknown), Geologic map of the deposits and features of the 1984 eruption of Mayon Volcano: PHIVOLCS, prepared by H.B. Ruelo, scale 1:50,000.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

May 2007 (BGVN 32:05) Cite this Report

Eruption ends on 1 October 2006; typhoon causes deadly lahars

The eruption from Mayon that began on 13 July 2006 (BGVN 31:07) ended in early October. By mid-September 2006, the volcano's eruptive vigor had decreased (BGVN 31:08). According to reports by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) through 3 October, low-frequency harmonic tremor was not recorded after 24 September, crater glow was last seen on 27 September, and tremor episodes were not detected after 30 September. Sulfur-dioxide flux was also low on 26 September, 1,200 metric tons per day (t/d), but could not be measured the following week due to poor weather. On 3 October PHIVOLCS noted that there had been a continuous decline in overall activity since 11 September. Lava extrusion apparently ceased on 1 October 2006, as reported by the Ligñon Hill Observatory. As a result, the hazard status was lowered to Alert Level 2, indicating that the possibility of a hazardous eruption was remote.

Fumarolic activity on 30 September and 1 October produced white steam plumes that drifted ENE. On 11 and 12 October steaming was moderate and one volcanic earthquake was recorded. During the first week of October, six low-frequency volcanic earthquakes and one low-frequency short duration harmonic tremor were recorded. Crater glow was seen again on 4 October. The SO₂ flux measured on 6 October remained low at 1,600 t/d.

On 25 October, PHIVOLCS announced the lowering of the hazard status to Alert Level 1. The 7-km Extended Danger Zone on the SE flank remained in effect. At that time the number of daily volcanic earthquakes was below background levels (1-5 per day), tremor episodes were rare, there was no ground deformation, SO₂ emission was within the typical 1,600-2,650 t/d range, and the observed glow remained at a consistent intensity.

Lahars caused by tropical storm. Typhoon Durian, also called Reming, struck the Philippines on 30 November and mobilized material from the flanks of Mayon that resulted in significant lahars, burying thousands of homes under 1.5 m of volcanic debris, mud, and flood waters. Official government information releases as of 13 December noted that the torrential rains and mudslides, particularly in the area of Mayon, killed 720 people, injured 2,360 others, and left 762 people missing. In addition, 328,218 houses were partially damaged and 214,400 houses were destroyed. A map produced by the United Nations Institute for Training and Research (UNITAR) Operational Satellite Applications Programme (UNOSAT) indicated that about 51 km² on the flanks of Mayon was damaged by the flooding and mudslides.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Associated Press (URL: http://www.ap.org/); Agence France-Presse (URL: http://www.afp.com/); United Nations Institute for Training and Research (UNITAR) Operational Satellite Applications Programme (UNOSAT), Palais des Nations, CH - 1211 Geneva 10, Switzerland (URL: http://www.unosat.org/); United Nations Office for the Coordination of Humanitarian Affairs (OCHA) (URL: https://reliefweb.int/).

February 2009 (BGVN 34:02) Cite this Report

Mild phreatic explosion with ash plume on 10 August 2008

Our last report on Mayon (BGVN 32:05) discussed an eruption from 13 July to early October 2006, along with deadly lahars down Mayon's flanks caused by a typhoon that struck the Philippines on 30 November 2006. On 25 October 2006, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) lowered the hazard status to Alert Level 1 (low level unrest).

The U.S. Air Force Weather Agency (AFWA) reported that an eruption had occurred on 4 June 2007. It sent a steam-and-ash plume seen on satellite imagery up to 4 km altitude, which blew toward the SW.

There were no further reports on Mayon until August 2008. On 10 August PHIVOLCS reported a mild explosion that produced an ash plume that rose to an altitude of 2.7 km and drifted ENE. According to PHIVOLCS, seismic activity during the weeks before the explosion had increased slightly and incandescence at the crater had intensified. Some inflation of the volcanic edifice also was apparent. The seismic network recorded the ash ejection as an explosion-type earthquake that lasted for one minute. Immediately after the explosion, visual observation becomes hampered by the thick clouds. Precise leveling surveys during 10-22 May 2008 compared to 17 February-2 March 2008 showed the edifice inflated.

A news account inThe Philippine Star described the explosion as phreatic and ash bearing, based on discussions with PHIVOLCS staff.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); U.S. Air Force Weather Agency, Public Affairs Office; 106 Peacekeeper Dr., Ste 2NE; Offutt AFB, NE 68113-4039, USA; The Philippine Star (URL: http://www.philstar.com/).

October 2009 (BGVN 34:10) Cite this Report

Increased activity in mid-2009; November 2009 eruption

On 10 August 2008 an explosion and resulting ash plume followed weeks of increased activity and summit incandescence (BGVN 34:02). According to a Philippine Information Agency (PIA) Daily News Reader press release, the 10 August eruption was followed by an M 5.8 earthquake on 15 August and a series of aftershocks that continued through at least 20 August.

On 10 July 2009, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) noted increased activity beginning in June 2009. According to PHIVOLCS, there was a rise in low-frequency volcanic earthquakes, a ground uplift of ~ 1 cm, moderate steam emissions, and summit incandescence. An 8 July overflight discovered the crater contained a "cone-shaped pile of hot, steaming old rocks." The fresh deposits were possibly from a previous eruption, and may have been the source of the glow in the crater. The Alert Level for Mayon was raised from 1 (low level unrest) to 2 (unrest which could lead to more ash explosions or eventually to hazardous magmatic eruptions).

According to a 6 August 2009 article from the Philippine Daily Inquirer, resident PHIVOLCS volcanologist Eduardo Laguerta reported that the number of earthquakes at Mayon had decreased by early August 2009. However, the Inquirer reported that SO₂ emissions had increased, with a maximum of 1,977 tons per day on 6 August, compared to 500 tons per day when there is no activity.

PHIVOLCS reported that 11 earthquakes were detected during 14-15 September, with steam plumes drifting NW and ENE. On 15 September, three ash explosions produced a brownish ash plume that rose 700 m above the crater and drifted SW. On 28 October a minor explosion produced a brownish ash plume that rose 600 m above the crater and drifted NE, preceded by 13 volcanic earthquakes over the previous 24-hour period.

On 11 November 2009 another ash eruption occurred at 0158 that lasted for ~ 3 minutes and ejected incandescent rock fragments seen from nearby villages. The explosion was accompanied by rumbling sounds and light ashfall in surrounding areas to the SW, W, and NW. According to the Inquirer, a second explosion was recorded at 0702, with an ash plume reaching 300 m above the crater. The Inquirer reported that residents in Daraga township to the S were ordered to evacuate early, but that further mass evacuations would not be ordered until the Alert Level was raised to Level 3. An aviation ash advisory from the Tokyo VAAC noted continuous ash erupting in MTSAT-IR satellite imagery at 0800 on 11 November.

A 21 November 2009 article from Vox Bikol confirmed that as of 17 November, Mayon continued to exhibit summit incandescence and emit fluctuating amounts of SO₂. Due to the continuing unrest PHIVOLCS installed additional seismic monitoring equipment, including three sets of broadband instruments from the Japan International Cooperating Agency (JICA).

A news article from Vox Bikol stated that PHIVOLCS did not observe summit incandescence during 2-3 December due to heavy cloud cover, but as of 4 December 2009 ground deformation and moderate steam emissions were continuing. PHIVOLCS continued to enforce the 6-km-radius Permanent Danger Zone (PDZ) and the 7-km-radius Extended Danger Zone (EDZ) on the SE flank, and urged residents to avoid river channels that are prone to lahars.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Philippine Daily Inquirer (URL: http://www.inquirer.net/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Vox Bikol (URL: http://www.voxbikol.com/); Philippine Information Agency (URL: http://www.pia.gov.ph/).

December 2009 (BGVN 34:12) Cite this Report

December 2009 eruption causes evacuation of more than 47,000 people

After erupting in September and November 2009 (BGVN 34:10), monitoring of Mayon by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) continued. Renewed eruptions began on 14 December 2009 with ash columns reaching as high as 1,000 m above the summit, incandescent materials rolling downslope from the crater, and a lava flow descending SE from the summit (table 11). More than 47,000 people were ordered to evacuate for nearly three weeks (figure 16), requiring them to abandon homes and farms (figure 17). International news attention was acute, highlighting evacuations, the volcano's grandeur, and glow over substantial areas in long-exposure night photos.

Table 11. Daily summaries of observations reported at Mayon, including seismicity, SO₂ emission rates, and other observations (including Alert Levels) during 14 December 2009-12 January 2010. Numbers of events represent counts from the seismic monitoring network over a 24-hour period prior to the stated reporting date/time (except as noted). Rockfall events are related to the detatchment of fresh lava fragments at the volcano's upper slopes. Ash explosions and other observation are based on actual sightings. SO₂ emission rates, measured by FLYSPEC, are for the day before the reporting date. Courtesy of PHIVOLCS.

Figure 16. A Mayon map variously showing volcanic hazard (3-, 6-, and 8-km radius danger zones), evacuation centers, Albay Province census data (shaded areas defined on legend), and the actual evacuated population (44,637 people). The locations evacuated (table at upper left) were within the 6-km Permanent Danger Zone and SE Quadrant High-Risk Zone (7-8 km radial distance) and came from eight municipalities and their 32 subdivisions (barangays). The table shows both the targeted number of evacuees and the actual number as of 1100 on 21 December. Courtesy of the United Nations OCHA, 21 December 2009.

Figure 17. A farmer tills the soil while Mayon steams in the background. Many residents in threatened areas were reluctant to leave their homes and livestock. Date and photographer unknown. Courtesy of AFP.

At the onset of the eruption, after a minor ash explosion at 0740 on 14 December, five more minor ash explosions occurred at the summit crater. These explosions produced brownish to grayish ash clouds which were blown by strong winds WSW and WNW. The explosions lasted for ~3 minutes and were registered on the seismograph as explosion earthquakes. Twenty-three volcanic earthquakes were also recorded from 0800 to 1600. During the morning of 14 December the sulfur dioxide (SO₂) emission rate measured by FLYSPEC [a miniature, light-weight ultraviolet correlation spectrophotometer (Horton and others, 2006)] was 757 metric tons/day (t/d). At 1800, incandescent materials originating from the summit crater were seen rolling downslope SE ~3 km in the direction of Bonga, Buyuan, and Mabinit channels.

On 14 December 2009 PHIVOLCS raised the hazard status to Alert Level 3 (meaning that magma is close to the crater and a hazardous explosive eruption is possible). The alert was again raised, to Level 4 (meaning a hazardous explosive eruption is possible within days), on 20 December. After decreased activity the Alert was lowered to Level 3 on 2 January 2010.

Satellite observations and measurements. Figure 18 shows a satellite image of Mayon captured on 15 December 2009. NASA's Jesse Allen noted that "A small plume of ash and steam is blowing west from the summit. Dark-colored lava or debris flows from previous eruptions streak the flanks of the mountain. A ravine on the southeast slope is occupied by a particularly prominent lava or debris flow."

Figure 18. A natural-color image of Mayon taken 15 December 2009 (N to the top; for approximate scale, the distance from the summit to the coast is ~10 km). Image acquired by the Advanced Land Imager on NASA's Earth Observing-1 (EO-1) satellite. Courtesy of NASA.

MODIS/MODVOLC satellite thermal alerts were measured nearly daily during 14-31 December 2009; alerts were absent after 31 December 2009 and at least as late as 12 January 2010. It is noted that during 3 passes of the MODIS satellite (on 24 December at 1715 UTC, 25 December at 1330 UTC, and 28 December at 1400 UTC), 11-pixel alerts occurred each pass that gave some idea of the area covered by the thermal anomaly. Prior to this period, alerts were measured only during an eruption of Mayon from 15 July-25 September 2006 (BGVN 31:07, 31:08, 32:05, and 34:02).

Evacuation. The alert status rose from Level 3 to 4 (on a scale of 1-5) on 20 December (table 11). According to a news article by Sophia Dedace at GMANews.TV on 14 January 2010, between 14 December and 2 January, the threatening eruption prompted the provincial government to evacuate more than 47,000 residents located within Mayon danger zones.

Reference. Horton, K.A., Williams-Jones, G., Garbeil, H., Elias, T., Sutton, A.J., Mouginis-Mark, P., Porter, J.N., and Clegg, S., 2006, Real-time measurement of volcanic SO₂ emissions: validation of a new UV correlation spectrometer (FLYSPEC): Bull. Volc., v. 68, no. 4, p. 323-327 (doi:10.1007/s00445-005-0014-9).

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Philippine Daily Inquirer (URL: http://www.inquirer.net/); Vox Bikol (URL: http://www.voxbikol.com/); Philippine Information Agency (URL: http://pia.gov.ph/); GMANews.TV, 6/F GMA Network Center, EDSA corner Timog Avenue, Diliman, Quezon City, 1101, PHILIPPINES (URL: http://www.gmanews.tv/index.html); Jesse Allen, NASA (URL: https://www.nasa.gov/), and MODIS/MODVOLC Thermal Alerts, Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Agence France Presse (URL: http://www.afp.com/).

September 2011 (BGVN 36:09) Cite this Report

Brief seismic crisis in May 2011, low activity follows

Mayon volcano (figure 19) underwent an eruptive crisis in late 2009 into early 2010 and a seismic crisis in May 2011. This report provides some final remarks on the late 2009 to early 2010 eruptive crisis, and summarizes activity through 23 September 2011. Our previous report summarized the heightened activity in December 2009, which culminated in the evacuation of 47,000 people from their homes (BGVN 34:12). The eruption waned following the evacuation, and, accordingly, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) lowered the Alert Level from a high of 4 to 3 (on a scale from 0 to 5) on 2 January 2010. At that point, ~2,000 evacuees were still unable to return to their homes. On 13 January, the Alert Level was lowered from 3 to 2, and, according to Sophia Dedace (GMANews), enabled the remaining evacuees to return to their homes. Dedace reported that Governor Joey Salceda estimated the damage to agriculture and infrastructure from Mayon's 2009-2010 eruption at 26.2 million Philippine pesos (~$600,000 USD).

Figure 19. Maps showing geographic location (star on index map) and shaded relief map of Mayon volcano. Index map courtesy of Ginkgo Maps; shaded relief map courtesy of United Nations Institute for Training and Research's (UNITAR's) Operational Satellite Applications Programme (UNOSAT).

On 2 March 2010, amid continually declining activity at Mayon, PHIVOLCS lowered the Alert Level from 2 to 1. As of 23 September 2011, the Alert Level remained unchanged, indicating, as stated by PHIVOLCS, "low level unrest" and that "no eruption [is] imminent." At Alert Level 1 (and all higher levels), access is prohibited within the 6-km-radius Permanent Danger Zone.

With the exception of May 2011, seismicity (figure 20) typically consisted of no more than a few volcanic earthquakes and rockfall events per day (i.e. less than 50 of either type per month) and relatively low SO₂ flux (averaged per month on figure 20). Mayon typically vented ash-free steam at weak-to-moderate intensities, and crater glow persisted, observable by residents at night.

Figure 20. Reported volcanic earthquakes and seismically detected rockfall events per month (dark and light gray bars, respectively, left axis) and SO2 flux (open triangles and dashed line) averaged per month (right axis) at Mayon from 1 January 2010 to July 2011. Background colors indicate the Alert Level corresponding to the scale to the right of the figure. Little if any data are available from March through December 2010, presumably due to low activity during this interval. Data courtesy of Philippine Institute of Volcanology and Seismology (PHIVOLCS).

During May 2011 there was a significant increase in seismicity, reaching a daily maximum of 38 volcanic earthquakes on 25 May. This increase coincided with a slight increase in the SO₂ flux (averaged per month, figure 20). The increase in both seismicity and SO₂ flux was short-lived, and activity declined to relatively low levels by June 2011.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); GMANews.TV, 6/F GMA Network Center, EDSA corner Timog Avenue, Diliman, Quezon City, 1101, PHILIPPINES (URL: http://www.gmanews.tv/index.html); United Nations Institute for Training and Research's (UNITAR's) Operational Satellite Applications Programme (UNOSAT), Palais des Nations, CH-1211 Geneva 10, Switzerland (URL: http://www.unitar.org/unosat/); Ginkgo Maps (URL: http://www.ginkgomaps.com/).

April 2013 (BGVN 38:04) Cite this Report

Mainly calm during 2009-2013; 7 May 2013 explosion kills five climbers

Mayon's emissions, often small, gas-driven, ash-bearing, and without visible magmatic components, were generally minor during 2009 through early June 2013. After erupting in late 2009, Mayon seismicity generally declined to baseline levels through 23 September 2011 (BGVN 36:09). This report summarizes seismic activity from the end of the last report into early June 2013. The summit crater released a sudden minor phreatic eruption on 7 May 2013 that would have been harmless except for the ejection of some large blocks and the presence of dozens of climbers on the nearby upper slopes; five died.

According to the Philippine Institute of Volcanology and Seismology (PHIVOLCS), small ground and tilt deformations observed since 2 March 2010 were probably due to regional faulting and not magmatic intrusion. A report published by PHIVOLCS on 27 November 2012 noted that precise leveling surveys found slight inflation of the lower N and E slopes; ground tilt changes were not fully consistent with volcanic ground deformation, but rather with incremental motion along a nearby segment of the Philippine fault zone.

That November 2013 report also noted that steaming had waned significantly by 27 November 2012. Steaming from the crater varied, but was, by November 2012, weak and occasionally wispy. The report indicated that crater incandescence had ceased since March 2012. Sulfur dioxide emissions had decreased to below baseline levels of 500 metric tons/day. As a result of diminished activity, PHIVOLCS decreased the Alert Level to 0 on 27 November 2012; however, the public was reminded not to enter the 6-km-radius Permanent Danger Zone.

The next available report on Mayon indicated that a small phreatic eruption occurred on 7 May 2013 lasting in the range of 73-146 seconds. PHIVOLCS observed that a gray-to-brown ash cloud rose 500 m above the crater and drifted WSW. Traces of ash fell in areas WNW, affecting communities up to 19 km away. The seismic network detected a single associated rockfall event. Seismicity and gas emissions remained within background levels and indicated no increase in activity. The Alert Level remained at 0.

Gas-driven explosion and fatalities on 7 May. PHIVOLCS posted a photo of the eruption taken at distance (figure 21). According to news reports, that 7 May event was fatal to climbers who had ventured to [500 m] of the summit, a point well within the 6-km-radius Permanent Danger Zone. Multiple news articles (including those in Interaksyon, The Philippine Star, Associated Press, Sunstar, and GMA Network) noted that the 7 May 2013 phreatic eruption at 0800 ejected large rocks towards climbers, killing five and injuring at least seven. A climber was quoted as saying that their team was resting when they heard a loud rumbling and then saw falling rocks "as big as a living room." A local tour operator said "It rained like hell with stones. It was sudden and there was no warning."

Figure 21. Photo taken at 0800 on 7 May 2013 of a phreatic eruption at Mayon. Dense billowy plume is largely white with areas of brown to gray. News reports said eruptions like this were, according to PHIVOLCS, a regular occurrence. PHIVOLCS reported this plume as 500 m tall. According to news reports, rocks discharged by this eruption at 0800 killed five climbers and injured at least seven others in a region close to the summit and well within an exclusionary zone. Courtesy of PHIVOLCS.

One of the more detailed news reports, a 7 May article by Andrei Medina and Amanda Fernandez in GMA News, said that at least two groups of climbers were on the volcano at the time of the explosion. One of those groups, 20 climbers, incurred all five fatalities. Those fatalities included 4 foreigners [Europeans] and one Filipino tour guide. The article said that (according to Bernardo Rafaelito Alejandro, head of the Office of Civil Defense in Bicol) the foreign nationals and their guide were about half a kilometer from the crater when the 0800 explosion occurred. Another group, consisting of about seven climbers on another trail suffered three injuries (all Indonesians). Other articles raised sometimes inconsistent details about the number, composure, and locations of the various groups on the volcano.

Although authorities had set the Alert Level at the lowest risk (0, at a scale reaching up to 6) at the time of the eruption, and it remained so immediately thereafter, they had previously established the Permanent Danger Zone. In accord with this information, the article said that Albay Governor Joey Salceda said that the mountain climbing activities of the two groups affected were unauthorized. He added the tourist guides also failed to secure a permit from the Albay Public Safety and Emergency Management Office (APSEMO) and the Department of Tourism." The article went further to say that Bernardo Rafaelito Alejandro, head of the Office of Civil Defense in Bicol, said there was no need to evacuate the residents near the volcano, adding such eruptions are expected from an active volcano, and evacuation only occurs during an Alert Level 3.

An 11 May article by Cet Dematera and Celso Amo in The Philippine Star noted that the bodies of the four foreign nationals had been retrieved from Mayon's slopes. Another climber on Mayon during the 7 May eruption, Boonchai Jattupornpong, had lost contact with fellow Thai climbers, but had survived for 4 days by gathering rainwater. He was found, carried out, and brought to a hospital suffering burns, cuts, and a fractured arm. The composite disaster team involved in the search, rescue, and retrieval operations after the 7 May disaster was recommended for awards and commendation, including a possible Bronze Cross medal award or equivalent, for bravery and heroism by the Albay Provincial Governments.

Rockfalls, degassing, and incandescence. On 8 May 2013, PHIVOLCS reported that two rockfalls at Mayon had been detected within the previous 24 hours. Seismicity remained within background levels and indicated no increase in overall volcanic activity.

On 31 May 2013, PHIVOLCS raised the Alert Level to 1 as a precaution because, during the previous 36-hours, a visible but weak and short-lived hydrogen sulfide (H2S) emission was observed, along with a persistent incandescence. PHIVOLCS was concerned that the incandescence might reflect a steady emission of magmatic gas. However, PHIVOLCS also noted that seismicity remained markedly low and sulfur dioxide (SO₂) measurements remained below the normal level. A ground deformation survey indicated slight edifice inflation compared to a 13 February 2013 survey.

According to PHIVOLCS, white to off-white steam plumes drifted in various directions during 5-10 June. Occasionally, bluish fumes were noted. During most evenings during this period, PHIVOLCS observed incandescence from the crater, although cloud cover sometimes obscured the volcano. The seismic network recorded one volcanic earthquake during 5-6 June and another one during 9-10 June. During 6-7 June, a single rockfall signal was detected.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS) (URL: http://www.phivolcs.dost.gov.ph); InterAksyon (URL: http://www.interaksyon.com); The Philippine Star (URL: http://www.philstar.com/); Sunstar (URL: http://www.sunstar.com.ph/).

September 2013 (BGVN 38:09) Cite this Report

Quiet during May 2013-November 2013; super-typhoon and lahars

Following the phreatic eruption on 7 May 2013 that killed 7 climbers (BGVN 38:04), there has been little increase in volcanic activity at Mayon volcano. Seismicity has mostly receded to baseline levels, aside from occasional volcanic earthquakes. These earthquakes occur about once every other day, with minimal earthquakes in June and September. The activity reported by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) in table 12 below represents a continuation of table 11 from a previous Bulletin report (BGVN 34:12). Rockfalls and earthquakes are plotted in figure 22.

Table 12. Almost daily summaries of observations at Mayon, including seismicity and SO₂emission rates during 1 June -23 November 2013. Number of events represent counts from the seismic monitoring network over a 24-hour period prior to the stated reporting date/time (except as noted). For example RF: 1 means 1 rockfall; and VE: 2 means 2 volcanic earthquakes. Rockfall events are related to the detachment of lava fragments at the volcano's upper slopes. No ash explosions were recorded during this time period. SO₂ emission rates, measured by FLYSPEC [a miniature, light-weight ultraviolet correlation spectrometer (Horton and others, 2006)], are for the day before the reporting date. Courtesy of PHIVOLCS.

Figure 22. Graph showing distribution of volcanic earthquakes and rockfalls from June 2013 to November 2013. Created by Bulletin editors from PHIVOLCS reports.

When cloud cover and heavy rain does not inhibit observations, PHIVOLCS had consistently recorded white steam plumes that drifted in various directions from June to November 2013. Bluish fumes, a sign of hydrogen sulfide, were witnessed on 5 and 7 June, 15 and 23 August, and 7 and 28 September. Ground deformation surveys in the second week of August showed that the inflationary trend was continuing. In May 2013, electronic tilt meters measured Mayon's edifice to be slightly inflated compared to January 2010.

Crater glow of Intensity 1 was observed numerous times from June to September. According PHIVOLCS, a crater glow of Intensity 1 is faint, Intensity 2 is more visible to the naked eye, Intensity 3 is bright, and Intensity 4 is intense. Crater glow likely results from incandescence of new lava, or newly exposed lava, reflecting off local crater walls, clouds, or steam.

PHIVOLCS interprets enhanced crater glow as a sign of SO₂ clouds, but there had been little SO₂ fluctuation from June to November. Mayon's Alert status remained at Level 1 following the increase from Level 0 on 31 May 2013. However, PHIVOLCS continues to advise residents and visitors to avoid the 6-kilometer radius Permanent Danger Zone (PDZ) due to hazards such as rockfalls, landslides, sudden ash emissions, and phreatic eruptions. Level 1 is the 2nd value on a scale from 0 to 5, with 5 signifying an ongoing hazardous eruption; level 1 indicates an abnormal condition, but no magmatic eruption is imminent.

On 6 November 2013, PHIVOLCS issued a warning for super-typhoon Haiyan, locally known as Yolanda, indicating that excessive rainfall might trigger landslides and lahars at Mayon. Peak winds during Haiyan were consistently 170 mph, making the storm a super-typhoon as classified by NOAA ("maximum sustained 1-minute surface winds of at least [150 miles per hour]"). With the potential of large-magnitude lahars, the province of Albay started an evacuation of about 103,200 people along areas downstream, as shown in figure 23. According to the news source, Inquirer, adjacent communities, including Guinobatan, Legaspi City, Sto. Domingo, Daraga and Ligao City, were at risk for inundation, burial and washout. PHIVOLCS also issued precautions concerning debris flows from landslides of Mt. Masaraga, an old volcanic edifice N of Mayon.

Figure 23. Super-typhoon Haiyan left destruction in its wake after hitting the Philippines in early November. In this photo from the Associated Press, residents downstream from Mayon were evacuated due to the possibility of lahars engulfing nearby communities (Daily Mail).

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS); Associated Press (URL: http://www.ap.org/); Daily Mail (URL: http://www.dailymail.co.uk); NOAA (URL: http://www.aoml.noaa.gov); and Philippine Daily Inquirer (URL: http://www.inquirer.net/).

March 2016 (BGVN 41:03) Cite this Report

New lava dome appears in summit crater, August 2014; evacuations due to rockfall hazard

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that visual observations of Mayon volcano's summit crater on 12 August 2014 revealed the growth of a new lava dome approximately 30-50 m high. This event was preceded by inferred low-volume intrusions that cause inflation and increased sulfur dioxide (SO₂) emissions (beyond the baseline level of 500 tonnes/day) over the previous two months. The sulfur dioxide flux was 850 t/d on 11 August (table 13). Volcanic earthquakes and rockfalls, however, remained at baseline levels, and almost no crater glow had been observed since May 2014.

Table 13. Measured sulfur dioxide flux (metric tons/day) at Mayon, 11 August-29 September 2014. Baseline level for SO₂ is 500 t/d (during quiescence) based on continuous gas measurements since June 2014. Courtesy of PHIVOLCS.

The emergence of the new dome prompted PHIVOLCS to raise the Alert Level on 15 August from 1 to 2 (on a scale of 0-5) (see table 14). Sulfur dioxide flux remained high over the next month, suddenly rising to over 6,000 t/d on 14 September. On 15 September PHIVOLCS raised the Alert Level to 3 and reported that activity had escalated, including 39 rockfalls during 0500-0800 and 32 low-frequency volcanic earthquakes. Crater glow became visible around 2000, and thermal anomalies were detected in MODIS data by MODVOLC (2 pixels). According to a news article some residents in Guinobatan (11.8 km SW) had already evacuated.

White plumes rose from the crater on 16 September (figure 24), with incandescence seen later and incandescent material descending the upper reaches of Bonga Gully on the SE flank. During the previous 24 hours, there were 32 volcanic earthquakes and 72 rockfall events. Incandescent material moving down the cone was obvious at night on 17 September (figure 25), and a news article indicated that almost 24,000 people from villages within an 8-km radius from the crater had been evacuated.

Seismicity and rockfalls increased over the next few days, though clouds prevented visual observations. During 17-18 September the network recorded 142 volcanic earthquakes and 251 rockfall events. White steam plumes drifting SSW were noted. The network recorded 38 volcanic earthquakes and 277 rockfall events during 18-19 September. During 19-21 September four volcanic earthquakes along with 8-22 rockfall events per day were recorded. White steam plumes drifted ENE and NNE during 20-21 September. During 21-23 September three volcanic earthquakes per day and 13-18 rockfalls per day were recorded; white steam plumes drifted NNE, NE, ENE, and SW.

Figure 24. Mayon volcano emits white plumes as seen from Legazpi, Albay province, SE of Manila on 16 September 2014. Courtesy of Charism SAYAT/AFP/Getty Images via The Weather Channel.

Figure 25. Lava flows from the crater of Mayon volcano seen from Legazpi City, Albay province, southeast of Manila on 17 September 2014. Courtesy of Charism SAYAT/AFP/Getty Images via The Weather Channel.

According to a news article, the Albay Province Public Safety and Emergency Management Office stated that more than 54,000 people were in evacuation shelters as of 30 September. On 12 October the Office of the Governor of Albay issued a statement noting that 12,602 families (about 56,000 people) had been relocated to 46 evacuation centers.

Volcanic earthquakes (up to nine per day) and rockfalls (as many as six per day) continued to be detected at least through 12 October. During an overflight on 12 October 2014, volcanologists from PHIVOLCS observed a 350-m-long lava flow traveling down the SE flank, on the E side of Bonga Gully. The report noted that the small number of volcanic earthquakes and rockfall signals during the previous few days indicated slow lava extrusion from the crater and a slow-moving lava flow downslope. On 14 October a seismic signal indicating a rockfall was recorded and a brief period of incandescence from the crater was observed. During 14-21 October white plumes rose from the crater and drifted NW, W, WSW, SW, and SE, and a few volcanic earthquakes were recorded during 18-21 October. On 19 October weak incandescence from the crater was noted. A new lava flow first observed later that same day was 300-400 m long by 20 October, based on an aerial survey. Weak crater incandescence from the lava dome was again seen on 21 October.

During 22 October-4 November 2014, white plumes rose from the crater and drifted SW, WSW, WNW, and NW. Weak incandescence from the crater was noted at night on 22, 25, 27, and 28 October, and a few volcanic earthquakes and rockfall signals were periodically recorded.

A PHIVOLCS report on 4 November 2014 noted that ground deformation continued to be detected (since the beginning of 2014). Tilt data from the network on the NW flank indicated continuing inflation since August, subsequent to a period of inflation in June and July. Precise leveling measurements also indicated sustained inflation.

White plumes continued to rise from the crater in November and early December 2014, and weak incandescence was noted some nights. As many as five volcanic earthquakes were recorded per day. Data from a deformation study conducted during 9-13 November 2014 indicated deflation relative to 21-28 October, although the volcano remained inflated relative to the baseline.

On 19 December 2014, PHIVOLCS reported a general decline in the overall activity since a seismic swarm last detected on 29 November. Specifically, for the previous three weeks, seismic activity had declined to an average 2-3 mostly volcano-tectonic earthquakes daily, a few low-frequency earthquakes, and a few minor rockfall events. In addition, deformation data did not indicate magma intrusion; sulfur dioxide emissions had declined on 2 October 2014 to below the baseline value; no lava flows had been observed since 19 October.

The Alert Level was lowered to 2 (on a 0-5 scale) on 19 December 2014. Activity continued to decrease; the Alert Level was reduced to 1 (abnormal) on 3 September 2015 and to 0 (no alert) on 12 February 2016.

Table 14. Mayon volcano Alert Levels as revised on 15 August 2014. Courtesy of PHIVOLCS.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Office of the Governor of Albay, Philippines (URL: http://albay.gov.ph/); Philippine Daily Inquirer (URL: http://www.inquirer.net/); Associated Press (URL: http://www.ap.org/); The Weather Channel, 17 Sept. 2014 (URL: https://weather.com/); Inquirer.net (URL: http://newsinfo.inquirer.net/641491/phivolcs-volume-of-magma-moving-up-mayon-similar-to-2009-eruption); BBC News (URL: http://www.bbc.com/news/world-asia-29217253); Daily Mail (URL: http://www.dailymail.co.uk/).

April 2018 (BGVN 43:04) Cite this Report

Explosion on 13 January 2018 begins new eruptive episode; 5-km-high ash plume on 22 January

Steep-sloped and symmetrical Mayon has recorded historical eruptions back to 1616 that range from Strombolian fountaining to basaltic and andesitic flows, as well as large ash plumes, and devastating pyroclastic flows and lahars. A lava dome that grew during August-October 2014 resulted in rockfalls, pyroclastic flows, and lava flows from the summit crater that led to evacuations in nearby communities (BGVN 41:03). Activity declined during November and December 2014 and remained low throughout 2015. By February 2016 the Alert Level was reduced to 0 (on a 0-5 scale) by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) which monitors the volcano. A seismic swarm in August 2016, and the beginning of a new eruption in January 2018 are covered in this report with information provided primarily by PHIVOLCS.

After a brief seismic swarm in August 2016, Mayon remained quiet until a phreatic explosion on 13 January 2018 sent an ash plume 2,500 m above the summit and scattered ash over numerous nearby communities. The growth of a new lava dome sent lava flows down the flanks and ash plumes multiple kilometers above the summit during subsequent weeks. Lava fountaining produced incandescence at the summit for many weeks. Lava collapse events from the flow fronts sent pyroclastic density currents (PDC's) down multiple ravines during January and February 2018. Lava fountaining activity became nearly continuous at the beginning of February but began to taper off by mid-month. Flows had reached as far as 4.5 km down ravines, and lava-collapse generated pyroclastic density currents reached 5 km from the summit crater. The pyroclastic activity continued through February from the gravity-driven collapsing flow fronts even though fountaining and lava effusion had decreased. Brief periods of fountaining and gravity-driven lava flow were noted throughout March 2018, but activity had essentially ceased by month's end.

Activity during 2016-2017. Very low seismicity of 0-2 volcanic earthquakes per day was typical for January and early February 2016; the largest number recorded was 12 on 9 January. On 12 February 2016, PHIVOLCS noted that seismicity had remained at baseline levels of 0-2 earthquakes per day for the previous six months, indicating that rock fracturing associated with magmatic activity had diminished. Ground deformation information suggested a return to pre-2014 eruption positions, and low levels of SO₂ flux had been consistent since November 2015. They reduced the Alert Level to 0.

Increasing SO₂ flux above 1,000 tons/day beginning in July 2016 was accompanied by ground deformation measurements suggesting renewed inflation. A brief swarm of 146 earthquakes was recorded by the Mayon Volcano Observatory's seismic network from 3-6 August; they were located 10 km away on the SE flank. This change led PHIVOLCS to raise the Alert Level back to 1 on 8 September 2016. Seismicity and SO₂ levels remained very low through the end of 2016, but GPS data suggested continued inflation. Slight inflation was recorded throughout 2017. Rare days of small seismic swarms of more than 10 earthquakes occurred during 2017, but otherwise seismicity and SO₂ flux values remained within background levels.

Activity during January 2018. A sudden phreatic eruption at 1621 local time on 13 January 2018 sent a gray steam-and-ash plume 2,500 m above the summit that drifted SW. The activity lasted for a little under two hours. Traces of ash fell on the Barangays of Anoling (4 km SW), Sua (6 km SW), Quirangay (9 km SW), Tumpa (9 km SW), Ilawod (10 km SW), and Salugan (8 km SW) in the city of Camalig and in the Barangays of Tandarora (26 km WSW), Maninila (8 km SW), and Travesia (10 km SW) in the municipality of Guinobatan. Incandescence at the summit crater was first observed a few hours later. As a result, PHIVOLCS raised the Alert Level from 1 to 2 early the next day.

Two more phreatic explosions occurred the following morning (14 January) at 0849 and 1143 that each produced ash plumes, but they were largely obscured by summit clouds. Minor amounts of ash were reported in Camalig. By the evening, PHIVOLCS had raised the Alert Level again to 3 after three explosions, 158 rockfall events, and the observation of bright incandescence at the summit crater. By 2000 on 14 January they noted the growth of a new lava dome and the beginnings of a lava flow towards the southern flank.

Two lava collapse events on the morning on 15 January each lasted 5-10 minutes. They originated from the lava flow front and produced rockfall and small-volume pyroclastic density currents. Ash plumes drifted SW and rained ash on Travesia, Muladbucad Grande, Maninila, Masarawag, Poblacion, Iraya, Ilawod, Calzada, Inamnan Grande, Inamnan Pequeno, Maguiron, Quitago and Mauraro in the municipality of Guinobatan and on the Baranguays of Cabangan, Anoling, Sua, Tumpa, Quirangay, Gapo, and Sumlang, and Baranguays 1 to 7 in the municipality of Camalig. A degassing event at 1107 produced a grayish to dirty white ash column that rose to a maximum of height of approximately 1,000 m above the summit before drifting WSW.

Lava effusion continued from the summit during 16-21 January 2018 with flows down the Mi-isi and Bonga gullies and occasional short-duration lava fountaining. Tens of daily lava collapse events accompanied the growth of the flow in the Mi-isi gully which had reached about 3 km from the summit by 18 January. Debris from the growing summit dome also descended the Matanag and Buyuan Gullies. Pyroclastic density currents descended the Mi-isi, Matanag, and Buyuan Gullies. Ash plumes rose up to 2 km and drifted SW from the summit crater and caused ashfall in Camalig, Guinobatan, and Polangui (figures 26-28).

Figure 26. Mayon emitted ash and steam along with pyroclastic density currents that flowed down the SW flank on 16 January 2018. View is looking N from S of the airport in Lagazpi City, Philippines, about 12 km S. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 27. Pyroclastic density currents (PDC's) descended the W flank of Mayon on 16 January 2018. Incandescence at the base of the PDC was also visible. Lava was fountaining at the summit and incandescent blocks were rolling down the Mi-isi drainage on the S flank. Image taken near Legazpi city, 12 km S. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 28. Lava flows at Mayon descended the Mi-isi drainage on the S flank and were visible from Legazpi city on 17 January 2018. Courtesy of The Express, photo from European Pressphoto Agency.

Activity increased on 22 January 2018 with lava fountains at the summit reaching 200-500 m high, the lava flow into the Mi-isi drainage extending beyond 3 km, and two new flows in the Bonga gully and upper Buyuan watershed. A dense 5-km-tall ash plume erupted at 1243 during a phreatomagmatic event that lasted for 8 minutes (figure 29). It generated pyroclastic density currents in several drainages within 4 km of the summit vent including Mi-isi, Bonga, Buyuan, Basud, San Andres, Buang, Anoling and other minor drainages. Ash was blown W and fell on the municipalities of Guinobatan, Camalig, Oas, Polangui and Iriga City. Five additional episodes of lava fountaining to 700 m occurred overnight that fed the Mi-isi and Bonga gully flows, and generated ash plumes to 2.5 and 3 km above the summit. This increase in activity led PHIVOLCS to raise the Alert Level to 4. By the following day, more than 50,000 people had evacuated to emergency shelters and civil aviation authorities temporarily closed airports in the cities of Legazpi and Naga.

Figure 29. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite acquired this image of the area around Mayon in the Philippines on 22 January 2018. The image combines natural-color data with thermal infrared bands (7-2-1). The substantial ash plume from the explosion that day rose to 10.9 km altitude and drifted NW and W, and the emerging lava dome appeared as a thermal hotspot at the summit. Courtesy of NASA Earth Observatory.

Numerous episodes of intense lava fountains during the nights of 23-26 January each lasted from a few minutes to more than an hour. They generated 150-600 m high fountains and continued to feed the flows in the Mi-isa and Bonga gullies. Ash plumes also rose from 0.5-5 km above the crater. The Mi-isa gully flow remained at 3 km from the summit, and the Buyuan flow had reached 1 km by 24 January. Pyroclastic density currents in the Mi-isi, Lidong/Basud, and Buyuan drainages were also observed. The PDCs in the Buyuan drainage traveled more than 5 km from the summit crater (figures 30-33).

Figure 30. Ash and steam plumes rose from the summit crater of Mayon while lava flows descended drainages on the S flank as seen from the town of Daraga, 10 km S, on 23 January 2018. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 31. An ash plume rises, likely from a pyroclastic density current, in a drainage on the SE flank of Mayon, a few kilometers N of the town of Daraga on 23 January 2018. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 32. Ash and pyroclastic density currents emerged from the summit of Mayon on 24 January 2018, sending ashfall to nearby communities and filling drainages with pyroclastic debris. Image taken from Daraga, 10 km S. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 33. Lava flows were very active on the S flank of Mayon, visible from about 12 km SSE in Legazpi on 25 January 2018. Courtesy of The Express, AFP/Getty Images.

By the evening of 26 January 2018, the lava fountaining episodes had transitioned into aseismic lava effusion, feeding incandescent flows into the Bonga and Mi-isi gullies on the S flank, and advancing the flow in the Bonga significantly downslope to 1.8 km. Fewer fountaining episodes continued during 27-28 January. Heavy rainfall during 28-29 January remobilized deposits from pyroclastic density currents and generated sediment-laden stream flows in several channels (figure 34) and channel-confined lahars on the Binaan Channel.

Figure 34. Sediment-laden streams posed hazards to residents of Camalig (11 km SW) at Mayon on 28 January 2018 after heavy rains and numerous PDC's had filled the drainages with debris. Courtesy of The Express, photo from European Pressphoto Agency.

A significant increase in lava effusion and fountaining at the summit during the evening of 29 January 2018 fed PDCs into the Mi-isi and Bonga Gullies, and resulted in significant ashfall in Camalig and Guinobatan to the SW. Intermittent lava fountaining to 200 m, flow-front collapses that generated PDC events, low-level ash emissions, and slow lava effusion from the summit crater continued during 30 January-4 February (figures 35 and 36). The Mi-isi and Basud lava flows had advanced to 3.2 and 3.6 km, respectively, from the summit crater by 1 February, and the Bonga-Buyuan flow had advanced 4.3 km by 3 February.

Figure 35. Steam-and-ash plumes rose steadily above Mayon on 31 January 2018. Image taken at the port in Legazpi City, about 15 km S. Courtesy of The Express, Getty Images.

Figure 36. Lava effusion at the summit of Mayon had decreased from a week earlier (see figure 33) by 31 January 2018. Courtesy of The Express, Getty Images.

Activity during February-March 2018. Lava fountaining reached 550 m above the summit crater on 5 February and increased to near-continuous activity the next day. Lava flows and incandescent rockfalls were observed throughout the night in the Mi-isi and Bonga-Buyuan channels. High volumes of incandescent lava flows advanced to 3.2, 4.5, and approximately 3.0 km down the Mi-isi, Bonga-Buyuan and Basud channels. Pyroclastic density currents from the collapsing flow fronts reached 4.6, 4.4, and 4.2 km from the summit crater in the same drainages during 7 February. Near-continuous fountaining accompanied by steam plumes that rose up to 800 m continued through 10 February.

Lava fountaining became sporadic and weak beginning on 11 February. Heavy rainfall during 13 February generated channel-confined lahars in the Anoling channel. By 14 February, lava flows remained at 3.3 km, 4.5 km, and 900 m down the Mi-isi, Bonga and Basud gullies, and PDCs had deposited material to distances of 4.6, 4.5, and 4.2 km in the same drainages. Intermittent lava fountaining continued through 22 February. The fountains generally rose 100-600 m above the summit and were often audible more than 10 km from the summit.

Quieter lava effusion with fewer fountaining events was more typical behavior beginning on 23 February. Numerous episodes of lava-collapse pyroclastic density currents were visually observed on the Mi-isi, Basud, and Bonga-Buyuan Gullies within 2-4 kilometers of the summit crater during the second half of February. Deflation of the lower slopes that began on 20 February was recorded by electronic tiltmeter, consistent with the transition to seismically quieter lava effusion at the summit crater. However, the overall electronic tiltmeter and the continuous GPS data indicated that the volcano was still inflated relative to October and November 2017 levels.

Weak fountaining, lava effusion, and degassing were noted during 25-28 February. The sporadic fountains generated plumes that rose 800 m, and weak effusion continued to feed the flows in the drainages. Gravity-driven lava flow movement and degassing with ash plumes rising 600 m above the summit were the primary activity at Mayon on 1 March, although occasional lava fountaining events were still observed. Based on the decrease in activity at the summit, the decrease in seismicity, continued deflation, and significantly lower SO₂ emissions, PHIVOLCS lowered the Alert Level to 3 on 6 March 2018.

Brief periods of weak fountaining and lava flows were observed during 7-24 March. The fountaining generated dark gray ash plumes that rose 100-300 m above the summit crater before drifting SW, and were sometimes audible more than 10 km from the summit crater. At night, lava flows continued moving downslope within 3.3, 4.5, and 1.9 km of the crater in the Mi-isi, Bonga, and Basud gullies. Steam plumes rose as high as 2.5 km above the summit before drifting SW on 7 March. Intermittent bluish steam-laden plumes rose to 700 m before drifting SW on 14 March. A slight inflation of the lower flanks beginning on 11 March 2018 was recorded by electronic tiltmeters through at least 22 March. Overall deformation data indicated that the edifice was still inflated relative to pre-eruption baselines.

Beginning around 24 March 2018, the primary activity consisted of intermittent lava collapse events in the Mi-isi gully located between 4-5 km from the summit and steam-laden plumes that drifted SW from the summit. Lava flow effusion at the crater was last detected on 18 March. Ground deformation since 20 February 2018 recorded deflation despite short-term episodes of inflation of its lower and middle slopes, and incandescence at the summit had diminished from intense to faint. Lava flows had begun to stabilize, producing fewer rockfalls and infrequent pyroclastic density currents, the last of which was observed on 27 March 2018. This continued decrease in activity led PHIVOLCS to lower the Alert Level to 2 on 29 March 2018.

VAAC, SO₂, and MIROVA information. The Tokyo VAAC reported the first ash emission from Mayon on 13 January 2018 as a plume that rose to 5.2 km altitude and drifted SW. Many subsequent ash emissions were obscured by meteoric clouds and were only occasionally observed in satellite imagery. The ash plume from the large explosion on 22 January was observed in satellite imagery at 10.9 km altitude drifting NW. Numerous daily VAAC reports were issued through February; they were intermittent in March, ending on 23 March 2018. Plumes generally were reported at 5.2-7.6 km altitudes. Small sulfur dioxide plumes were captured by the OMI and OMPS satellite instruments on several days between 22 and 31 January 2018 (figure 37).

Figure 37. SO2 anomalies from Mayon were captured by the OMPS and OMI instruments on the SUOMI and AURA satellites during January 2018. Upper left: 22 January 2018; upper right: 23 January 2018; lower left: 26 January 2018; lower right: 31 January 2018. Courtesy of NASA Goddard Space Flight Center.

The MIROVA project thermal anomaly graph of log radiative power clearly captured the onset of activity at Mayon in mid-January 2018 (figure 38). Thermal activity increased through early February and then slowly decreased through mid-March 2018 when lava effusion ended.

Figure 38. The sudden onset of thermal activity at Mayon is apparent in this MIROVA project graph of log radiative power for the year ending on 11 May 2018. The data is based on the satellite-based MODIS infrared thermal imagery. Thermal activity peaked at the end of January and dropped off gradually through mid-March 2018; it then decreased abruptly after that. Courtesy of MIROVA.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); The Express (URL: https://www.express.co.uk); European Pressphoto Agency (EPA) (URL: http://www.epa.eu/); Getty Images (URL: https://www.gettyimages.com/); Agence France Presse (AFP) (URL: https://www.afp.com/).

October 2018 (BGVN 43:10) Cite this Report

Low activity during April-September with some ash plumes and ongoing crater incandescence

Mayon is a frequently active volcano in the Philippines that produces ash plumes, lava flows, pyroclastic flows, and lahars. In early 2018, eruptive activity included lava fountaining that reached 700 m above the summit, and lava flows that traveled down the flanks and collapsed to produce pyroclastic flows (figure 39). Lava fountaining and lava flows decreased then ceased towards late March. Lava effusion was last detected on 18 March 2018, and the last pyroclastic flow for this eruptive episode occurred on 27 March 2018 (see BVGN 43:04). The hazard status for was lowered from alert level 4 to 3 (on a scale of 0 to 5) on 6 March 2018 due to decreased seismicity and degassing; the level was lowered again to 2 on 29 March. This report summarizes the activity during April through September 2018 and is based on daily bulletins issued by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and satellite data.

Figure 39. Sentinel-2 thermal satellite images showing the lava flow activity at Mayon during January through March 2018. Three lava flow lobes flowed down the Mi-isi, Bonga-Buyuan, and Basud channels, and are shown in bright orange/red in these images. False color images created using bands 12, 11, 4. Courtesy of Sentinel Hub Playground.

The hazard status remained on Alert level 2 (increasing unrest) throughout the reporting period. Activity was minimal with low seismicity (zero to four per day) and a total of 19 rockfall events throughout the entire period. White to light-brown plumes that reached a maximum of 1 km above the crater were observed almost every day from April through September (figure 40). Two short-lived light brown plumes were noted on 27 and 28 August and both reached 200 m above the crater.

Figure 40. An emission of white steam-and-gas at Mayon and a dilute brown plume that reached 200 m above the crater was seen on 24 May 2018. Courtesy of PHIVOLCS.

On the days that sulfur dioxide was measured, the amount ranged from 436 to 2,800 tons per day (figure 41). Mayon remains inflated relative to 2010 baselines but the edifice has experienced deflation since 20 February, a period of inflation from 2-14 April, and slight inflation of the mid-slopes beginning 5 May, which then became more pronounced beginning 25 June. No other notable inflation or deflation was described throughout the reporting period.

Figure 41. Measurements of sulfur dioxide output at Mayon during 1 April-30 September 2018. Data courtesy of PHIVOLCS.

Incandescence at the summit was observed almost every night (when weather permitted) from April through to the end of September 2018, and this elevated crater temperature is also seen in satellite thermal imagery (figure 42). Thermal satellite data showed a slight increase in output during April through to June, although not as high as the earlier 2018 activity, with a decline in thermal output starting in July (figure 43).

Figure 42. Sentinel-2 thermal satellite image showing an elevated thermal signature in the crater of Mayon and a steam-and-gas plume on 15 May 2018. Similar indications of activity in the crater were frequently imaged on cloud-free days from April through September. False color image created using bands 12, 11, 4. Courtesy of Sentinel Hub Playground.

Figure 43. Log radiative power MIROVA plot of MODIS thermal data for the year ending 11 October 2018 at Mayon. An elevated period of activity reflecting the lava flows in January through March is notable, followed by a second period of lower intensity activity during May into June, then a prolonged period of reduced activity through to the end of the reporting period; the August anomaly was not at the volcano. Courtesy of MIROVA.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

May 2019 (BGVN 44:05) Cite this Report

Intermittent ash emissions; persistent summit incandescence, October 2018-April 2019

Steep-sloped and symmetrical Mayon has recorded historical eruptions back to 1616 that range from Strombolian fountaining to basaltic and andesitic flows, as well as large ash plumes, and devastating pyroclastic flows and lahars. A phreatic explosion with an ash plume in mid-January 2018 began the latest eruptive episode which included the growth of a lava dome with pyroclastic flows down the flanks and lava fountaining (BGVN 43:04). Activity tapered off during March; occasional ash emissions continued through August 2018. Minor ash emissions and summit incandescence were intermittent from October 2018-April 2019, the period covered in this report. Information is provided primarily by the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Pyroclastic density currents were reported in early November 2018; ash plumes were produced from phreatic events a few times during both November and December 2018. Emissions produced SO₂ anomalies during January-March 2019; a series of events in early March generated several small ash plumes. Satellite images showing a thermal anomaly at the summit were recorded multiple times each month from October 2018-April 2019 (figure 44).

Figure 44. Small but distinct persistent thermal anomalies were recorded in satellite imagery from the summit of Mayon during October 2018-April 2019. Top left: 12 October 2018. Top right: 26 November 2018. Middle left: 11 December 2018. Middle right: 30 January 2019. Bottom left: 14 February 2019. Bottom right: 25 April 2019. All images are using the "Atmospheric penetration" filter (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Very little activity was reported at Mayon during October 2018. Steam plumes rose daily from 250-750 m above the summit before drifting with the prevailing winds and dissipating. Incandescence was observed at the summit most nights during the month, and seismicity remained low with only a few earthquakes reported. Leveling data obtained during 30 August-3 September indicated significant short-term deflation of the volcano relative to 17-24 July 2018. New leveling data obtained on 22-31 October indicated inflation of the SE quadrant and short-term deflation on the N flank relative to the 30 August-3 September data. The volcano remained inflated compared with 2010 baseline data. Electronic tilt data showed pronounced inflation of the mid-slopes beginning 25 June 2018.

Activity increased during November 2018. In addition to steam plumes rising to 750 m and an incandescent glow at the summit most nights, pyroclastic density currents and ash plumes were reported. The seismic monitoring network recorded pyroclastic density currents on 5 and 6 November. On 8 November around noontime, a small, short-lived brownish ash plume, associated with degassing, drifted WSW from the summit. A seismic event on the morning of 11 November was associated with a short-lived fountaining event that produced a brownish-gray ash plume that drifted SW. Another similar plume was reported on the morning of 12 November, also drifting SW before dissipating. Two phreatic events were observed on the morning of 26 November. They produced grayish to grayish-white ash plumes that rose 300-500 m above the summit before drifting SW. The following morning, another event produced a grayish ash plume 500 m above the summit that drifted SW. On 30 November a 1-minute-long ash emission event produced a grayish white plume that also drifted SW.

Steam plume emissions and incandescence at night continued at Mayon during December 2018. The seismic network recorded a four-minute-long event shortly after noon on 9 December that produced a grayish-brown ash plume which drifted W. Precise leveling data obtained on 8-13 December 2018 indicated a slight inflation of the volcano relative to 22-31 October 2018. A 30-second-long ash emission event in the afternoon on 18 December produced a brownish ash plume. Two phreatic events were observed on the morning of 27 December. They produced grayish to grayish-white ash plumes that rose 600 and 200 m above the summit, before drifting SW (figure 45).

Figure 45. Ash plumes rose a few hundred m from the summit of Mayon on 27 December 2018. Courtesy of Twitter users "k i t" (left) and "georgianne" (right).

Very little surface activity except for white steam-laden plumes that crept downslope and drifted NW or SW was noted during January 2019. Incandescence at the summit, visible with the naked eye, became more frequent during February 2019, along with continued steam plumes. Precise leveling data obtained on 25 January-3 February 2019 indicated a slight deflation relative to 8-13 December 2018. However, continuous GPS and electronic tilt data showed inflation of the mid-slopes since June 2018. Small SO₂ plumes were detected by the TROPOMI satellite instrument a few times during January-March 2019 (figure 46).

Figure 46. Emissions of SO2 that exceeded 2 DU (Dobson Units) occurred a few times at Mayon during January-March 2019. Top left: 25 January. Top right: 16 February. Lower left: 4 March. Lower right: 15 March. Courtesy of NASA Goddard Space Flight Center.

Steam plumes rose 250-500 m above the summit and drifted generally W in early March 2019; incandescence continued daily at the summit. Phreatic events occurred on 7 and 8 March, producing ash plumes that rose 500 and 300 m from the summit before drifting SW (figure 47). Three more phreatic events occurred on the afternoon of 12 March; they produced light brown to grayish ash plumes that rose 500, 1,000, and 500 m, respectively, and drifted SW. Six phreatic events occurred throughout the day on 13 March, producing ash plumes that rose 200-700 m above the summit and drifted W. A single explosion the next day produced a 500-m-tall ash plume. The Tokyo VAAC reported an ash plume visible for several hours in satellite imagery drifting W at 3.7 km altitude on 13 March (UTC). An increase in the daily number of rockfall events from 1-2 per day to 5-10 per day was noted during the second half of March. Precise leveling data obtained on 20-26 March 2019 indicated a slight inflation relative to 25 January-3 February 2019.

Figure 47. A small ash emission at Mayon was reported by PHIVOLCS on 8 March 2019; the plume rose 300 m from the summit and drifted SW. Courtesy of PHIVOLCS.

Steam plumes drifted SW or NW throughout April, rising 200-400 m from the summit. Incandescence could be observed at night for the first half of the month. Leveling data obtained during 9-17 April 2019 indicated a slight inflation relative to 20-26 March 2019. Seismicity remained low during the month with only occasional volcanic earthquakes and rockfall events. Lenticular clouds around the summit were observed (figure 48), but these are an unusual meteorological occurrence caused by weather conditions not related to volcanic activity.

Figure 48. A double lenticular cloud surrounded the summit of Mayon early in the morning on 23 April 2019 and was captured by a local observer; it was not related to volcanic activity. Courtesy of Twitter user Ivan.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Twitter user "Ivan", Naga City, Philippines (URL: https://twitter.com/ivanxlcsn); Twitter user "k i t", Legazpi City, Philippines (URL: https://twitter.com/jddmgc); Twitter user "georgianne", Costa Leona, Philippines (URL: https://twitter.com/xolovesgia_).

November 2019 (BGVN 44:11) Cite this Report

Gas-and-steam plumes and summit incandescence during May-October 2019

Mayon, located in the Philippines, is a highly active stratovolcano with recorded historical eruptions dating back to 1616. The most recent eruptive episode began in early January 2018 that consisted of phreatic explosions, steam-and-ash plumes, lava fountaining, and pyroclastic flows (BGVN 43:04). The previous report noted small but distinct thermal anomalies, gas-and-steam plumes, and slight inflation (BGVN 44:05) that continued to occur from May into mid-October 2019. This report includes information based on daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and Sentinel-2 satellite imagery.

Between May and October 2019, white gas-and-steam plumes rose to a maximum altitude of 800 m on 17 May. PHIVOLCS reported that faint summit incandescence was frequently observed at night from May-July and Sentinel-2 thermal satellite imagery showed weaker thermal anomalies in September and October (figure 49); the last anomaly was identified on 12 October. Average SO₂ emissions as measured by PHIVOLCS generally varied between 469-774 tons/day; the high value of the period was on 25 July, with 1,171 tons/day. Small SO₂ plumes were detected by the TROPOMI satellite instrument a few times during May-September 2019 (figure 50).

Figure 49. Sentinel-2 thermal satellite imagery of Mayon between May-October 2019. Small thermal anomalies were recorded in satellite imagery from the summit and some white gas-and-steam plumes are visible. Top left: 30 May 2019. Top right: 9 June 2019. Bottom left: 22 September 2019. Bottom right: 12 October 2019. Sentinel-2 satellite images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 50. Small SO2 plumes rising from Mayon during May-September 2019 recorded in DU (Dobson Units). Top left: 28 May 2019. Top right: 26 July 2019. Bottom left: 16 August 2019. Bottom right: 23 September 2019. Courtesy of NASA Goddard Space Flight Center.

Continuous GPS data has shown slight inflation since June 2018, corroborated by precise leveling data taken on 9-17 April, 16-25 July, and 23-30 October 2019. Elevated seismicity and occasional rockfall events were detected by the seismic monitoring network from PHIVOLCS from May to July; recorded activity decreased in August. Activity reported by PHIVOLCS in September-October 2019 consisted of frequent gas-and-steam emissions, two volcanic earthquakes, and no summit incandescence.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://SO₂.gsfc.nasa.gov/).

December 2022 (BGVN 47:12) Cite this Report

Lava dome growth and gas-and-steam emissions during August-November 2022

Mayon, located in the Philippines, has steep upper slopes capped by a small summit crater. Recorded eruptions date back to 1616 CE and are characterized by Strombolian eruptions, lava flows, pyroclastic flows, and mudflows. Eruptions mostly originated from a central conduit. Pyroclastic flows and mudflows have commonly descended many of the approximately 40 drainages that surround the volcano. The most recent eruption occurred during January 2018 through October 2019 and included phreatic explosions, gas-and-ash plumes, lava fountaining, and pyroclastic flows (BGVN 43:04). Based on ground deformation parameters from EDM, Precise Leveling, electronic tilt and continuous GPS monitoring, slight inflation has been observed since 2020. More recent activity has been relatively low, consisting of small thermal anomalies, gas-and-steam plumes, and some inflation (BGVN 44:11). This report covers a new eruption during June through November 2022 using daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Low activity was reported during June; minor inflation, particularly on the NW and SE flanks have been recorded since April 2022. According to PHIVOLCS, changes in morphology of the summit lava dome emplaced in 2018 resulted in the increase in the Volcano Alert Level (VAL) from 0 to 1 (on a scale of 0-5). Minor extrusion was estimated at about 40,000 cubic meters during 6 June through 20 August, based on daily visual and camera monitoring data. Seismicity was at baseline levels for most of 2022, though short-lived spikes in the number of low-frequency volcanic earthquakes were recorded on 26 May and 20 June. PHIVOLCS reports that based on this data, dome growth was likely the result of gas pressurization at shallow depths.

Activity during August 2022 mostly consisted of moderate gas-and-steam emissions that rose as high as 900 m above the crater and drifted in different directions. Weather clouds often obscured the summit, thus crater incandescence could not be determined on a majority of days during early-to-mid-August. Sulfur dioxide emissions were measured on 12 August and were an average of 688 tons/day (t/d). During 25-31 August faint crater incandescence was reported based on telescope observations. Sulfur dioxide emissions decreased to 195 t/d based on measurements made on 30 August. Similar low activity continued during September; moderate gas-and-steam emissions rose 100-300 m and drifted in multiple directions. Faint crater incandescence was visible based on telescope observations on clear weather days. A single volcanic earthquake was detected on each day during 6-8, 13, 23-24, and 29 September. Sulfur dioxide emissions were measured on 6, 7, 13, and 24 September and ranged between 339 t/d and 599 t/d. Three volcanic earthquakes were detected on 26 September.

During October, moderate gas-and-steam emissions rose 200-800 m above the crater and drifted in different directions. Sulfur dioxide measurements were made on 1, 12, 13 October with values that ranged from 260 t/d to 391 t/d. Beginning on 2 October, re-mobilized, light-colored ash was deposited in Miisi Gully (S flank), which was likely derived from lava fragmentation as the lava dome continued to grow, according to PHIVOLCS. Three volcanic earthquakes were detected on 3 October. Daily visual and camera monitoring data showed that the lava dome grew an additional 48,000 cubic meters during 20 August through 4 October. One volcanic earthquake was detected each day on 5, 6, 17, and 23 October. Occasional faint crater incandescence was visible in clear weather and sometimes when using a telescope. On 7 October an aerial survey was conducted and reported that freshly extruded lava was visible at the base of the summit lava dome (figure 51). As a result, the VAL was raised to a 2 (on a 0-5 scale). There were two volcanic earthquakes detected each day on 19 and 22 October.

Figure 51. Aerial photos showing the summit lava dome at Mayon on 7 October 2022 with fresh lava effusions at the base (yellow arrows). Courtesy of PHIVOLCS.

Faint crater incandescence persisted on clear weather days during November. Gas-and-steam emissions also continued, rising 200-250 m above the crater and drifting NE, WNW, WSW, W, and SW. Three volcanic earthquakes were detected each day on 1 and 12 November. Sulfur dioxide emissions were measured on 3 and 23 November with values that ranged from 276 t/d to 788 t/d. A single volcanic earthquake was detected each day on 7, 10, 14, and 28 November.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).