Stable during mid- to late August 2004

Awu's eruption on 6 June 2004 and its elevated seismicity in early August 2004 was previously reported (BGVN 29:10). This report covers the last half of August 2004, which had not been reported on previously. Since the 6 June eruption, observation of the summit failed to reveal any significant changes (table 3). The hazard status of Awu during this August report remained at Level 2, having been elevated to 4 (the highest on a scale of 1 to 4) at the time of the 6 June eruption and then lowered on 14 June.

Table 3. Seismicity at Awu during August 2004 as reported by DVGHM.

Information Contacts: Dali Ahmad, Directorate of Volcanology and Geological Hazard Mitigation (DVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

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

August 1968 (BVE 8)

Earthquakes cause inner crater avalanches

The following information, from the Bulletin of Volcanic Eruptions for 1968, is for an unknown starting date in August, supposedly ending in September. The earthquakes may have been tectonic and related to the large Northern Celebes Earthquake on 10 August 1968.

"Rumblings. Continuous quakes recorded, including 10 felt ones. Those quakes caused avalanches from the inner crater wall filling the lake. Besides, a landslide in the inner crater wall the size of a house blocked the path to the crater bottom."

Information Contacts: Djajadi Hadikusumo, Geological Survey of Indonesia..

October 1968 (CSLP 33-68)

Reported to be active

Card 0114 (09 October 1968) Reported to be active

"The names of volcanoes in activity are Karangetang and Awu. Both are in Sangir Talaud Islands, North Sulawesi." [This report was received during the course of a Banua Wuhu eruption, and may simply be a listing of other recently active volcanoes in the area.]

Information Contacts: Governor of North Sulawesi, Manado, Celebes, Indonesia.

February 1992 (BGVN 17:02) Cite this Report

Lake pH drops; vapor plume

During 4 March fieldwork, a thin white vapor plume continued to emerge from the crater. The volume of the crater lake seemed unchanged from the previous month at about 600,000 m³, but its pH had dropped to 3, from 5 in February. Lake-water temperature ranged from 31 to 36°C. Solfataras N of the crater had temperatures of 78-101°C, while those S of the crater were at 55-100°C. Deep volcanic earthquakes occurred at a rate of ~1/week.

Information Contacts: W. Modjo and W. Tjetjep, VSI.

April 1992 (BGVN 17:04) Cite this Report

Rapid drop in crater lake level; lithic ejecta surround small new crater

The crater lake was visited on 11 May, following a sudden drainage of ~80% of the lake (from ~3.5 x 10⁶ m³ to 0.7 x 10⁶ m³) on 1 February. Water temperature was 31.1°C and pH was 2-3, similar to 4 March values (17:02), cooler but more acid than the 36°C and pH 5 measured in February. Fumaroles along the inner N wall of the crater emitted steam that rose 25-40 m and had temperatures of 70-92°C. Active solfataras, with temperatures of 70.6-97.4°C, had left substantial sulfur along the S and E walls. In the SE section of the crater, a deep vent 20 m in diameter produced a thick 50-m-high steam cloud that smelled of sulfur and was accompanied by an audible boiling sound. The presence of lithic ejecta around the vent suggested that it had been formed by a phreatic explosion.

Tectonic and volcanic A-type earthquakes were recorded at the volcano every month during January 1991-January 1992; volcanic A-type events ranged from 2 to 18/month.

Information Contacts: W. Modjo, VSI; UPI

July 2003 (BGVN 28:07) Cite this Report

Elevated seismicity during last half of 2000

The Volcanological Survey of Indonesia (VSI) issued reports of activity at Awu during June-July 2000, November-December 2002, and more recently during January-early March 2003, all of which are summarized here.

During June 2000, VSI reported an increase in seismicity, especially deep volcanic earthquakes (table 1). Satellite-relayed monitoring (by ARGOS) showed an increase in seismic energy beginning on 18 May 2000; deformation data showed inflation of ~800 µrad since 23 May.

Table 1. Seismicity reported at Awu during 13 June 2000-2 March 2003. Courtesy VSI.

During 14-16 October 2002, tremor was recorded and was followed by a felt tectonic earthquake with an amplitude of I-II MMI on 10 October. Soon after the tremor activity decreased, volcanic earthquakes began to be recorded (table 1). VSI reported a significant increase in seismicity during mid-November 2002; volcanic earthquakes that normally occurred less than five times per day occurred 81 times on 15 November. Activity decreased to normal levels by late 2002. Visual observations of the summit did not reveal significant changes. Volcanic earthquakes continued during January-early March 2003 (table 1). Awu remained at Alert Level 2 (on a scale of 1-4).

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

May 2004 (BGVN 29:05) Cite this Report

Great Sangihe Island stratovolcano erupts dome in June; ~27,000 evacuated

A dome-extruding eruption occurred in the previouly lake-bearing summit crater of Mount (Gunung) Awu, a stratovolcano in Northern Indonesia off the N end of the island of Sulawesi (Celebes). Details of the eruption are still emerging, but an early dome had clearly extruded by 2 June 2004 (figure 1).

Figure 1. A close up of Awu's new dome as it appeared amid loose tephra and through a steam-laden atmosphere on 2 June 2004, soon after emplacement began. Courtesy Penduduk.

Prior to the eruption the crater contained a green lake. Before 1992, water volume was 35,000 x 10³ m ³, but it decreased continuously, and in 2003, only 50 x 10³ m ³ remained. Research carried out in 1993-1995 attributed the water loss to active faulting beneath the crater. Water inside the crater was of great concern because of its potential to produce lahars that could threaten settlements around Awu. Prior to the eruption, thick vegetation covered the crater's inner and outer rims.

Awu's previous eruption took place 12 August 1966. It took 39 lives, injured more than 1,000, and forced ~11,000 evacuations.

Signs of Awu threatening to erupt became clear mid-May 2004. They included a 15 May (felt, I MMI) tectonic earthquake, followed by two volcanic earthquakes. On 16 May, there were 12 volcanic earthquakes recorded, events interpeted as a signs of fluid moving up, and supported by the appearence of tremor with peak-to-peak amplitude of 8 mm. A gas plume rose 75 m above the crater's rim. On 17 May there were 4 volcanic earthquakes; peak-to-peak tremor amplitudes had dropped to 5 mm. This pattern continued through 18 May, with the number of volcanic earthquakes typically standing at ~6 and tremor amplitudes at 5 mm. The S minus P (S - P) times dropped from 2.0-1.75 sec to 0.5 sec, suggesting a shallower earthquake source, a possible indication of stress moving towards the surface.

In addition to the above observations, VSI scientists regarded the shortest historical repose time at Awu volcano as 25 years, an interval that had passed since the last eruption, and this became an additional reason for raising the alert level on 18 May.

Figure 1, a photo from the VSI website shows a close-up of a dome on 2 June 2004 with intense steam escaping, indicating that at least portions of a dome had emerged by that time. In figure 1, the dome and surrounding tephra predominanly appear as gray, darker-colored spines and angular blocks and fragments, but occasional clasts of large white fragments, presumably pumice, lie sprinkled across the surface.

Seismicity increased on 4-5 June during 2330-0130 when more than 30 shallow volcanic (Type A) earthquakes occurred. In contrast, typical May seismicity only included one earthquake per day. On 5 June during 1000-1300, instruments recorded 85 earthquakes. On 6 June during 0200-0430, they recorded 50; and during 0900-1010, they recorded 2-3 earthquakes per minute. Tremor followed, with maximum peak-to-peak amplitude of 24 mm. The hazards status quickly increased to its highest level ('IV,' WITA).

At 1230 on 6 June, explosion earthquakes of small size occurred, followed by a rain of thin ash, which fell to the N. Visible white ash reached 500-750 m above the summit. An explosion sent ash 1 km above the crater rim, and the ash fell around the summit. Tremor prevailed until 2000, with maximum amplitude of 5 mm. At this point, 20,000 residents had already been evacuated.

Seismicity increased on 7 June; during the period 0000 to 0800 hours seismometers recorded 165 deep volcanic earthquakes, 18 shallow volcanic earthquakes, and continuous volcanic tremor-amplitude maxima exceeded 46 mm.

At 1117 on 7 June, an eruption began at 1800 hours, with ash plumes rising 1 km above the summit. After the eruption on 7 June, seismic signals similar to tremor occured (at 1807), with continuous, peak-to-peak amplitudes of about 12-45 mm (maximum).

During 7-8 June from 2000 to 0600, visual observers noted that 500- to 700-m-high ash clouds still hung over the summit. For the interval 0600-0600 8-9 June, VSI reported, "All day long there were many explosions." In additon, five major explosions were noted, at 1510, 1630, and 1730 on 8 June, and at 0606 and 0910 on 9 June. Presumably due to each of those larger outbursts, dark gray ash plumes rose up 1-2 km above the summit.

Ash thickness at Tahuna was about 0.5-1.5 mm. Beginning on 8 June 2004 at 0800, Tahuna airport was closed. VSI noted that the ash rain could have reached Tabukan Utara and part of Kendahe, caused by the wind to the SW.

At 0529 on 10 June, Awu began a sustained eruption, described as the climax, lasting 34 minutes (figure 2). That event sent a column of gray to black ash to 3 km above the summit. The outburst was accompanied by low rumbling sounds and tephra.

Figure 2. Ash plume at Mount Awu at 0529 on 10 June 2004. Courtesy of Wittiri, VSI, Directorate of Volcanology and Geological Hazard Mitigation.

By 11 June, explosions and seismicity decreased drastically, with tremor amplitudes of only 2-3 mm. Until 13 June VSI recorded no deep volcanic earthquakes. At 0600 on 13 June authorities reduced the hazard status and some W- and ESE-flank residents returned home.

Figure 3 documents fresh deposits, the presumably new dome, and denuded vegetation. Ash generally fell to the ESE. During the first eruption, ash fell on Tahuna city and its vicinity with a thickness of 0.5-1 mm. Surrounding villages received ash deposits as follows: Lenganeng, 2 mm; Naha, 2 mm; Bahang, 1.5 mm; Kalakuhe, 1.5 mm; and Mala, 1.5 mm.

Figure 3. Recent tephra deposited below the Mount Awu lava dome, 12 June 2004. For scale, note the backpack-clad person standing on fresh tephra and amid stripped vegetation in the right-central foregound. Courtesy of A. Solihin, VSI, Directorate of Volcanology and Geological Hazard Mitigation (DVMBG).

Inspection of the crater at an undisclosed time revealed a lava dome 300 x 250 m in plan view and 40 m in height. It is uncertain whether these values represent an early dome (figure 1) or larger, later dome (figure 3).

On 14 June, observers saw a thin white plume rising 50-100 m above the crater. Beginning 17 June, the hazard status dropped to level II (Waspada). Following 18 June, seismicity declined, and instruments no longer recorded tremor. The latest Awu report, which discussed the interval 28 June-4 July, noted level II hazard status, plumes 50-200 m tall, and the observation of incandescent material, suggesting continued dome growth.

UN Reports. According to an 8 June report from the UN Office for the Coordination of Humanitarian Affairs, the evacuation process triggered by Awu's eruption started on the evening of 6 June and continued through at least 8 June. The total number of people expected to be evacuated was ~27,000 (12,065 from Tahuna, 5,690 from Kendahe, and 9,248 from Tabukan Utara). As of 8 June, 17,326 people had been evacuated. These displaced people were accommodated in government buildings, schools, and houses of prayer. The Directorate of Vulcanology strongly advised the temporary halting of flights from Manado (at the N end of Sulawesi Island) to Sangihe Island.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Directorate of Volcanology and Geological Hazard Mitigation, Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Office for the Coordination of Humanitarian Affairs (OCHA), United Nations, New York, NY 10017 USA.

October 2004 (BGVN 29:10) Cite this Report

Volcanic seismicity ends in early August; weak plumes

Awu extruded a new dome in its crater by 2 June 2004 (BGVN 29:05). Several photos received from the Directorate of Volcanology and Geological Hazard Mitigation (DVGHM) taken from the crater's upper S side illustrate the crater prior to and just after the 2004 dome emplacement (figures 4-6). Elevated seismicity continued into the week ending on 8 August 2004 (table 2). During 12-25 July, observers saw white thin-medium plumes gently rising to 50 m above the summit. A report covering 9-15 August, noted that the Awu observation post documented a weak plume 200 m tall. They also reported nine type-B earthquakes. A brief message from DVGHM on 7 December noted that Awu was then quiet.

Figure 4. A N-looking photo of the Awu's crater taken in September 1995. Note the large ephemeral pond on the crater floor. Courtesy of DVGHM; photo by Kristianto.

Figure 5. A N-looking photo from 25 May 2003 showing the active crater at Awu. Compared to the photo from 1995 (figure 7, above), the pond on the crater floor had shrunken. A photo from 8 December 2002 (not included in this report) showed that at that time the pond was largely gone. Courtesy of DVGHM; photo by Endi T. Bina.

Figure 6. A N-looking photo of Awu's crater on 14 June 2004 showing the newly emplaced intra-crater dome and associated deposits. Disruption in the crater is also apparent, for example, the burial and heavy damage to vegetation . Thick steam made it difficult to see the distinctive rim on the crater's far side. Courtesy of DVGHM; photo by Agus Solihin.

Table 2. Summary of volcanic type-A earthquakes and tectonic earthquakes at Awu during 22 June through 15 August 2004. Volcanic type-B volcanic earthquakes also occurred occasionally, perhaps once a week, except in the 9-15 August interval, when they occurred nine times. Data for several days and time intervals (eg., 6 and 11 July, and 26 July-1 August) was not available. Courtesy of DVGHM.

Aviation reports. The Volcanic Ash Advisory Centre at Darwin, Australia, issued 15 reports (Volcanic Ash Advisories) regarding Awu during June 2004. These were the first and only Awu reports available in their archive of reports going back to 1998. The first message (on 8 June) was "Major eruption possible, but no eruption yet." Similar terminology accompanied Advisories until 12 June. The 9 June report noted "continuous small eruptions" and "four larger explosions in past two days." A plume also seen on satellite imagery was estimated by pilots to be at ~ 4.5-6 km. Later it became difficult to see the plume with satellite imagery. On 10 June two Advisories noted thin plumes directed NE extending ~ 37 km. The plumes were seen on imagery at 2325 and 0220 UTC (in aerospace shorthand, the imagery came from DVGHM, DMSP, GOES, and NOAA 17 satellites). The final Advisory, on 14 June, noted "Eruption details: Nil obs[erved] ash." That notice also commented that the alert status had dropped and no significant activity had been recorded, but a white plume rose ~ 100 m above the summit in the last 24 hours.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Directorate of Volcanology and Geological Hazard Mitigation, Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Office for the Coordination of Humanitarian Affairs (OCHA), United Nations, New York, NY 10017, USA; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).

April 2005 (BGVN 30:04) Cite this Report

Stable during mid- to late August 2004

Awu's eruption on 6 June 2004 and its elevated seismicity in early August 2004 was previously reported (BGVN 29:10). This report covers the last half of August 2004, which had not been reported on previously. Since the 6 June eruption, observation of the summit failed to reveal any significant changes (table 3). The hazard status of Awu during this August report remained at Level 2, having been elevated to 4 (the highest on a scale of 1 to 4) at the time of the 6 June eruption and then lowered on 14 June.

Table 3. Seismicity at Awu during August 2004 as reported by DVGHM.

Information Contacts: Dali Ahmad, Directorate of Volcanology and Geological Hazard Mitigation (DVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).