Non-eruptive May 2010 surface deformation from inferred deep instrusion

Oshima is an active volcano located on the northern tip of the Izu-Bonin volcanic arc. Our last report of activity at Oshima (BGVN 21:08) enumerated a flurry of shallow low-frequency earthquakes beneath the top and W flank of the volcano that started on 5 August 1996.

Since those relatively benign events, the Japan Meteorological Agency (JMA) had not observed any subsequent events worthy of note until May 2010 when land surface inflation was detected. The inflation was registered by a strainmeter, a Global Positioning System (GPS) network (run by the Geospatial Information Authority of Japan, GSI), and a tiltmeter network (run by the National Research Institute for Earth Science and Disaster Prevention, NIED).

In July 2010 seismicity in the shallow parts of and around Oshima began to increase. (High seismicity synchronous with inflation of the edifice was seen earlier, including in 2004 and 2007). These events were considered to be due to magma intrusion into the deeper part of the volcano. There were no remarkable changes in surface phenomenon. In September, the inflation that was detected in May began declining. Seismicity in the shallow parts of and around Oshima continued at a low level with some small earthquakes which temporally increased in the western offshore areas of Oshima on 22 December 2010.

The earthquakes increased in frequency again on 9 February 2011. GPS and strainmeter measurements indicated contraction since January, but the trend reversed to show inflation in October 2011. Seismicity remained at a low level. Very low level gas emissions were sometimes observed by a camera positioned on the NW summit. Based on a field survey on 28 October, no remarkable change in surface phenomena was observed.

No remarkable activity has been noted since October 2011. Throughout the noted activity, JMA held the Alert Level at 1.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/).

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

January 1972 (CSLP 02-72)

Earthquake swarm on 14-15 January

Card 1342 (24 January 1972) Earthquake swarm on 14-15 January

"Volcan Izu-Osima had been quiet since a small quantity of ashfall was observed around Miharayama summit crater in April 1971. A sudden earthquake swarm, however, occurred last week as follows: A swarm of volcanic earthquakes including 27 felt shocks (maximum intensity in JMA was IV) took place on 14 January 1972, at Osima Island, Izu Islands, Japan. This swarm started at about 1530 and faded away at about 0600 on 15 January (JST time). 183 earthquakes were recorded on the seismograph at the Osima Weather Station during this period, and intensities (JMA scale) of felt shocks were as follows: IV - three times; III - four times; II - seven times; I - 13 times.

"A small cliff collapse was reported, but no other damage. No abnormal volcanic phenomena appeared at Miharayama summit crater on this insular volcano and the state of the summit crater is now calm."

Information Contacts: Seismological Division, Japan Meteorological Agency.

November 1977 (NSEB 02:11) Cite this Report

Earthquake swarm in late October through mid-November

An earthquake swarm [W] of Oshima Island, accompanied by subterranean rumbling, began in late October and was continuing in mid-November (table 1). Similar earthquake swarms occurred in January 1972 and November 1973. Ten minor explosive eruptions have occurred at Oshima since 1962, the latest [in 1974].

Table 1. Number of felt and recorded earthquakes at Oshima, 30 October-17 November 1977. [JMA replaced the data in the original table.]

Further Reference. Yamashina, K., and Nakamura, K., 1978, Correlations between tectonic earthquakes and volcanic activity of Izu-Oshima volcano, Japan: JVGR, v. 4, p. 233-250.

Information Contacts: T. Tiba, National Science Museum, Tokyo.

January 1978 (GV 1975)

Fatal earthquake causes landslides and tsunami; volcanism unchanged

JMA submitted the following description of renewed seismic activity.

A large earthquake shook central Japan at 1224 on 14 January 1978. The epicenter was about 10 km W of Oshima island, focal depth was 0 km, and the magnitude was 7.0. A severe shock of intensity 5 (on the JMA scale of 0-7) (figure 1) was felt at JMA's Oshima Weather Station. The earthquake killed 25 persons, injured 139, and totally destroyed 94 houses and damaged 539 others in the Izu Peninsula and Oshima island. In the Izu Peninsula, numerous landslides and rockfalls blocked roads and railways, and destroyed many houses. On Oshima island, houses were destroyed or damaged mainly by strong earthquake motion. Weak tsunami waves were generated. The highest was 70 cm (peak to peak) at Okada port on Oshima island. Aftershocks occurred in a narrow belt elongated from Oshima island to the W (figure 2). Volcanism at Oshima volcano was not affected by the earthquake.]

Figure 1. Distribution of seismic intensities (JMA scale of 0-7) from the earthquake near Oshima at 1224 on 14 January 1978. Courtesy of JMA.

Figure 2. Distribution of foreshocks and aftershocks of the Oshima earthquake at 1224 on 14 January 1978. Courtesy of JMA.

Information Contacts: JMA.

August 1986 (SEAN 11:08) Cite this Report

Tremor and earthquake swarm but no change in thermal activity

On 20 July, JMA seismic stations on Oshima Island began to record volcanic tremor for the first time in 12 years. Tremor, recorded regularly every 2 hours, continued through early September. Event durations were about 10-30 minutes and amplitude was ~0.5 µm at station A, which at 1 km NNW of the active summit cone (Mihara) is the nearest of the five JMA seismic stations. No significant change in the volcano's weak fumarolic activity was observed by field surveys in the crater on 1, 4, and 18 August.

A 10-hour earthquake swarm occurred during the morning of 21 August. Three of the events were felt at the weather station ~5 km NNW of the summit. As in previous episodes of seismicity, epicenters were on the N part of the island, 4-5 km NNE of the summit crater.

Information Contacts: JMA.

September 1986 (SEAN 11:09) Cite this Report

More earthquakes; continued tremor; rumbling

A seismic station ~5 km N of the crater recorded 354 earthquakes in September, a marked increase from 41 in August (table 2). Nine events were felt at the Oshima weather station in September, the largest a M 4.0 shock accompanied by rumbling on 11 September at 2146. Earthquake swarms were recorded 11-13 and 26-27 September, with epicenters on the W and N coasts of the island respectively. Volcanic tremor with maximum amplitude of 0.7-0.8 µm continued throughout the month.

Table 2. Number of local earthquakes recorded by a seismic station about 5 km N of the crater at Oshima, April-September 1986. Courtesy of JMA.

The previous earthquake swarms were redorded in 1877, 1983, and most recently 16-24 August 1985, when almost 300 events with a maximum magnitude of 2.9 occurred just beyond the NW part of the somma.

Information Contacts: JMA.

October 1986 (SEAN 11:10) Cite this Report

Eruption column to 3,200 m after four months of seismicity

[Preliminary press information in this report is superseded by data from geologists in 11:11]. An eruption from the central crater (Mihara-yama) began the evening of 15 November. An eruption column rose 3200 m above the summit. Small eruptions were continuing the next day, sending ejecta to 500 m. Ash fell over an extensive area. By the morning of 19 November a 5-lobed lava flow had destroyed an observatory tower and Shinto shrine near the crater rim, and was moving down the NW flank at ~1 m/hour. By midday the flow was ~50 m long. "Lava" was ejected to 500 m above the summit and ash rose more than 1.5 km. In the mid-afternoon of 21 November, a 600-m-long fissure opened, extruded lava that flowed toward populated areas, and sent ejecta to about 1,100 m height. Thousands of residents and tourists in 5 communities N and W of the crater were forced to evacuate. Forty forest fires were reported and domestic flights were rerouted to avoid the volcano. Damage to crops was estimated at $1.2 million, but no casualties were reported.

UPI reported that JMA recorded 19 earthquakes with epicenters in the ocean, 10 km from Oshima, on the second day of the eruption. The strongest (4 on the JMA intensity scale of 7) occurred that evening at 1846. AP reported that by 19 November more than 150 submarine events had been recorded. Seventy more recorded earthquakes that began at 1417 on 21 November preceded the fissure opening. One event registered 5 and at least two registered 4 on the JMA intensity scale.

Information Contacts: UPI; AP.

November 1986 (SEAN 11:11) Cite this Report

Lava from summit vent and two fissures; epicenters plot along NW/SE axis; 12,000 evacuated; summit deflated before eruption

Phase 1 (15-19 November). A vigorous eruption began at 1718 on 15 November. The first phase, lasting 4 days, produced almost continuous lava fountains up to 500 m high (200-300 m average) and a lava lake within the crater of the 1777 cinder cone (Mihara-yama; vent A in figure 3). About 14 x 10⁴ metric tons of scoria fell SE of vent A on 16 November and 1 x 10⁴ metric tons SW of the vent the next day. The lava lake overflowed the crater rim at 0955 on 19 November, sending a flow 0.6 km W before Phase 1 ended at 2320. A total of 1.3 x 10⁶ m³ (33 x 10⁶ metric tons) of lava were erupted at a rate of 1.3 x 10⁴ m³/hour during the first phase.

Figure 3. Sketch map showing Oshima's new cinder cone, vents, lava flows, and scoria fall. Contour interval is 200 m. Courtesy of the Earthquake Research Institute.

Phase 2 (21-24 November). After less than 2 days of quiet, the second phase began at 1615 on 21 November. A fissure opened 1.2 km NNW of the crater, on the caldera floor (B on figure 3) and produced lava fountains, flows, and a 9-10-km-high eruption cloud. Flows moved mostly NE and were ~1 km long by 2000. An en-echelon fissure opened a few tens of meters from B at 1627. Scoria fall from vent B extended E and had a maximum thickness of >20 cm, 3.5 km from the vent (figure 3). The first eruptive activity outside the 3 x 4 km caldera began at 1746, from a 1.5-km-long fissure (C) NW of the caldera rim. Lava fountains and flows emerged, feeding a 1.5-km-long flow that soon advanced to within 1 km of Motomachi, the island's main town, on the W coast. At 2100 lava fountaining declined; two hours later ashfall reached the Boso Peninsula, 50 km NE. Eruptive activity weakened by dawn, and by 0600 ashfall had declined. Total Phase 2 ashfall was 500,000 m³. The mass evacuation of Oshima's 10,200 residents and 2,000 tourists the night of 21-22 November was the largest operation of its kind during a natural disaster in Japan; it was completed in less than 12 hours using 39 vessels.

At 0700 on 22 November, lava production began from vent A' on the NE flank of the 1777 cinder cone; lava flowed <1 km NE. On 23 November at 1330, lava began to emerge from the S part of fissure B, advancing about 0.5 km to the NE. By 1800, steam from vents A and A' was the only evidence of eruptive activity. The volcano was quiet by 25 November.

By 24 November a 3-km-long fissure had formed on the SE part of the island (near Habu port), oriented in the same NW-SE direction as the other newly formed fissures, but no eruptve activity was observed. The lava flow that threatened Motomachi had stopped 200 m from the town as of 25 November. An occasional column of steam to ~30 m above the summit was reported on 28 November.

Eruptive activity resumed at 0547 on 18 December with a 2-hour episode that produced 800-m-high ash columns, lava fountains, and at least one series of explosions. Fifty of the 800 residents that had returned to the island gathered at a local high school. The remainder of the residents, who were still at 34 school gyms and other facilities in central Tokyo, had been scheduled to return 19 December.

The total volume of November lava was ~18 x 10⁶ m³, and scoria and ash volume was 1.9 x 10⁶ m³. Preliminary petrologic data indicated that scoria from Phase 1 contained ~53% silica. Samples of scoria, lava flows, and bombs from Phase 2 contain ~54-57% silica. One sample of pumice collected from the E side of the island contains ~61% silica.

Seismicity. From January to the beginning of October, seismic activity was mainly distributed in the northern and western parts of the island, generally at levels of <40 recorded events (M <3) per day with occasional peaks of more than 100 events/day. Seismicity dropped to very low levels in October, and continued at low levels until 15 November.

Seismicity rapidly increased just after noon on 16 November, 19 hours after the eruption began, and reached a peak at 1846 the same day. The number of recorded events/hour fluctuated between 0 and 13, totalling 48 on 17 November, 40 on the l8th, and 9 on the l9th, dropping to near 0 after 0900. The largest events in Phase 1 had magnitudes of 3.9 (at 1846 on 16 November) and 4.1 (at 1455 on 18 November). Epicenters of Phase 1 events were in two broad zones roughly 4-7 km wide that extended N and W of the summit caldera (figure 4). Most events were located closer to the coast than to the summit caldera.

Figure 4. Plots of earthquake epicenters at Oshima, courtesy of the Earthquake Research Institute: a) 1725, 15 November-1200, 21 November; b) 0000, 22 November-1415, 28 November; c) 0000, 1 December-0320, 7 December.

Seismicity increased just after 1400 on 21 November, about two hours before the start of Phase 2. The number of recorded events peaked at the same time as the onset of Phase 2, when ~80/hour, including seven with M > 4, were recorded. Seismicity dropped sharply to near zero within a few days after the conclusion of the eruption on 23 November.

Phase 2 epicenters on 22 November (following the most active part of the eruption) were primarily in a NW-SE-trending zone (figure 4), roughly 3 km wide, centered ~1.5 km NE of the rift zone where Phase 2 eruptions occurred. Early Phase 2 epicenters have not yet been located because of crowded seismic records. Shallow events (<5 km) were concentrated near the vents at the summit of the island, while deeper events (>5 km) were concentrated near the NW and SE coasts and offshore.

Epicenters on 1-9 December were concentrated along the same zone as those of Phase 2 (figure 4), close to the coastline, with shallow events occurring close to the SE coast.

Episodic tremor began on 7 July, more than 18 weeks before the eruption began. Pulses lasted 10-20 minutes at regular intervals of approximately 3 hours until 20 July. Tremor amplitude increased several times during the months before the eruption. The pattern became more irregular at about the beginning of October, then tremor suddenly stopped on 27 October. Continuous tremor started several days later.

A higher amplitude episodic tremor was superimposed on the continuous tremor every 1-2 hours beginning on the morning of 15 November. At the onset of Phase 1 that afternoon, tremor amplitude increased significantly and the characteristic period became significantly longer. Tremor stopped as Phase 1 ended on 19 November at 2320.

Episodic tremor resumed the evening of 20 November. At about noon on 21 November, tremor became continuous, then stopped about 1-2 hours before the onset of Phase 2 at 1615. During Phase 2, tremor was not distinguished; it may have been masked by the high seismicity.

Ground deformation. Prior to the eruption, levelling surveys had been conducted at 3-10-year intervals along the two roads leading to the summit (the 1777 scoria cone). The points near the caldera were remeasured on an annual basis. Results indicate that the summit was deflating before the eruption, with ~40 mm of displacement measured from 1983 to 1986 near the center of the summit cone. Between April 1984 and August 1986, electronic distance measurements across the caldera showed contraction, consistent with deflation.

Before the eruption, a continuously recording tiltmeter near the SE end of the island showed a tilt direction consistent with deflation of the summit region. Tilt accelerated at the start of Phase 1. Between phases 1 and 2, tilt was approximately constant. At the beginning of Phase 2, however, tilt quickly reversed, gradually became flat, then returned to the original direction of movement by about noon on 22 November.

Data from a continuously recording volumetric strainmeter on the NW part of the island indicated that contraction started about July 1981 and continued until the eruption. Contraction accelerated with Phase 1 (consistent with the direction and trend of the tiltmeter on the SE part of the island) then became flat. When Phase 1 ended, expansion began then sharply accelerated 2 hours before the beginning of Phase 2.

The data indicated that expansion continued for nearly all of Phase 2 and ended about 2000 on 21 November, after which a gradual contraction was recorded. Expansion similar to that seen on the Oshima strainmeter, but two orders of magnitude smaller, was seen on strainmeters on the Izu Peninsula; this expansion continued for a few days after the eruption at a lesser rate.

Information Contacts: Y. Hayakawa, S. Aramaki, Y. Ida, H. Watanabe, K. Yamaoka, and H. Glicken, Earthquake Research Institute, Univ of Tokyo; Y. Sawada, JMA.

January 1987 (SEAN 12:01) Cite this Report

Small explosions one day after tremor resumes

"A brief eruption on 18 December was followed by relative quiet through 13 February . . . . Volcanic tremor was recorded on 17 December from 0915 to 0921 for the first time since the late November eruption. Tremor recurred at 1519 and 2139, lasting for 5 minutes each time; amplitudes were about equal to those of the first episode. The next day, six tremor episodes, lasting 4-6 minutes each, occurred at ~2-hour intervals prior to 1645, when continuous pre-eruption tremor began. Steady tremor continued until 1723 when amplitude increased abruptly, saturating the signal at most seismographs on the island.

"The summit was obscured by bad weather but the eruption probably began around 1725 when an observer heard a weak noise on the N slope. Frequent explosions (up to 10/minute) were heard between 1731 and 2000, and air shocks were felt at Motomachi, 4 km away. Incandescent fragments were seen being ejected from the summit crater during a brief break in the clouds. The eruption probably ended around 1930, based on the amplitude of large tremors, but an isolated explosion occurred at 2121.

"The eruption scattered a small number of bombs on the S slope of the summit cinder cone (Mihara-yama) but no lava flows were observed during air and ground surveys. Ejecta volume is estimated at 2 x 10³ tons, compared with 5 x 10⁷ tons during the November eruption.

"Tremor activity resumed on 1 January, occurring at 1-3-hour intervals 1-8 January [but see 12:04] and 1-hour intervals 9-22 January. Periods of tremor lasted 5-10 minutes; amplitudes were the same as on 17 and 18 December (figure 5). From 22 to 25 January, the interval between periods of tremor increased to 2 hours and amplitude doubled. Continuous tremor began the night of 25 January, showing a much smaller amplitude than the periodic tremor. On 4 February periodic tremor emerged again, and as of 13 February was continuing with an amplitude similar to that of mid-January. The source of periodic tremor appeared to be below Mihara-yama cone. The periodic tremor was accompanied by a change in rock strain of 0.4-2.0 x 10-8 detected by a borehole-type strainmeter 5 km N of Mihara-yama. Step-like changes in ground tilt were detected by a tiltmeter 2 km N of Mihara-yama simultaneous with the beginning of periodic tremor in February [see also 12:4]. Tilt indicated uplift of the summit during tremor.

Figure 5. Amplitude of volcanic tremor vs. time at Oshima, 1 January-8 February 1987. Courtesy of JMA.

"Other activity was quiet or steady. Every day a weak steam cloud rose 5-10 m above the northern slope craters (formed during the fissure eruption in November). Steam emission from Mihara-yama was steady and weak. Seismicity declined steadily from December to February. Microearthquakes occurred in a belt extending NNW-SSE past the summit area and into the sea. No shocks were felt in January and February although two events were felt in Motomachi in December (one was a M 3.0 on 27 December). Numerous tiltmeters and a strainmeter showed no anomalous change of trend except for the short-duration changes caused by periodic tremor mentioned above."

Information Contacts: JMA.

April 1987 (SEAN 12:04) Cite this Report

Steam emission; tremor and strain changes; earthquake swarms

Since the 18 December explosion, no additional eruptive activity has been reported. Steam jets rose daily to 10 m above some of the November fissures and white clouds usually extended 50 m above the summit.

Seismicity declined steadily after the November eruptive event and continued to decline through March. There was no change in the distribution of epicenters, which were concentrated in a zone trending NNW-SSE on the island. Volcanic tremor resumed on 1 January, after 10 days of quiet, occurring at almost regular 1-hour intervals (figure 6) [but see 12:1]. Periodic tremor had declined by February but continued at [January] amplitudes. Durations of tremor episodes ranged from 10 to 30 minutes. The tremor source (estimated from amplitude at each seismometer) appeared to be below the central cone (Mihara-yama).

Figure 6. Daily frequency of intermittent volcanic tremor at Oshima. Courtesy of JMA.

Periodic tremor was accompanied by step-like strain changes, recorded by a borehole strainmeter 108 m below sea level, 5 km N of Mihara-yama. Step-like expansion began on 1 January, reversing to contraction on 22 January [see also 12:1]. Other strainmeters had shown gradual contraction since the 21 November eruptive activity, before strain virtually ceased.

UPI reported that earthquake swarms began 6 May off the NW coast of the island, 10-20 km below the ocean floor. Of the 1,571 total recorded earthquakes 6-12 May [but see 12:6], 895 occurred on the 11th, the day of the largest event (M 5.1).

Information Contacts: JMA; UPI.

May 1987 (SEAN 12:05) Cite this Report

Earthquake swarm; no new eruptive activity

Minor earthquakes began on 6 May, centered 15 km NW of Izu-Oshima Island at ~10-20 km below sea level. More than 900 events had been recorded by 11 May, when seismicity forced Japan Railways to suspend operations in part of Chiba Prefecture E of Tokyo. The earthquakes were felt on Oshima and in Ajiro on the SE part of the Izu Peninsula. 73 events were recorded on 15 May; 140 minor shocks were recorded between 0100 and 0900 on 16 May. The earthquakes were continuing on 17 May. The largest events of the swarm had maximum intensities of 3 on the JMA scale and occurred at 1039 on 10 May, 0635 on 11 May, and 0420 on 17 May [but see 12:6]. No new eruptive activity was reported at the volcano.

Information Contacts: Kyodo radio, Tokyo.

June 1987 (SEAN 12:06) Cite this Report

Three earthquake swarms; intermittent tremor continues

Three earthquake swarms occurred in May. The first, 15 km NW of the island, lasted from 6 to about 20 May. It was followed by two weaker swarms; in the SE part of the volcano 22-26 May, and near the summit from 22 May to mid-June (figure 7). About 89 shocks were recorded at the Oshima weather station during the 6-20 May swarm (figure 8), the largest a M 5.1 event at 0635 on 11 May. Swarms have occurred in that area one or two times a year since 1978, most recently in October 1986.

Figure 7. Earthquake swarms in Oshima area, May 1987. Epicentral areas are shaded. Courtesy of JMA.

Figure 8. Daily seismicity of the earthquake swarm NW of Oshima, 5-31 May 1987. Courtesy of JMA.

The 22-26 May swarm was the first increase in seismicity in the SE part of the island since the eruption. The largest event, at 0626 on 23 May, had a magnitude of 3.0 and was felt at the weather station. Seismicity in the SE part of the island had decreased steadily and exponentially since the 1986 eruption (figure 9).

Figure 9. Daily seismicity in SE part of Oshima Island, 10 December 1986 to 10 June 1987. Courtesy of JMA.

An increase in summit-area microearthquakes also began on 22 May. Recorded events increased from about 20/day February-May, to ~40/day after 22 May, and to 100 daily after 29 May; the number of events decreased in mid-June. Steady microearthquake activity had continued near the summit since February, when observation by seismograph began in the area (figure 10). Tremor that began 1 January was intermittently recorded throughout May and June. Tremor episodes generally occurred at regular intervals of 1-2 hours and lasted for about 30 minutes. Beginning 14 May their regularity decreased, intervals between episodes grew from several hours to 20 hours, and episode durations increased to 60-300 minutes (figure 11). The regular tremor interval resumed on 7 June but toward the end of June became irregular again. Long-duration tremor episodes of more than 10 hours were often recorded. Weak steam emission continued at the summit of Miharayama, the central cone, producing about 50-m plumes. Steam rose steadily 0-5 m from craters that had formed on the N-flank fissure in the 1986 eruption.

Figure 10. Daily seismicity in the summit area of Oshima, 20 February-10 June 1987. Courtesy of JMA.

Figure 11. Duration of volcanic tremor at Oshima, January-June 1987. Courtesy of JMA.

Information Contacts: JMA.

July 1987 (SEAN 12:07) Cite this Report

Volcanic tremor; small earthquakes below summit

Intermittent volcanic tremor occurred during July. Durations of tremor episodes generally ranged from a few minutes to 2 hours but episodes lasting more than 10 hours were sometimes recorded. Small earthquakes with foci under the summit area were registered at a rate of ~40/day by the seismograph N of the summit caldera. A steam plume rose continuously to ~50 m above the summit crater.

Information Contacts: JMA.

September 1987 (SEAN 12:09) Cite this Report

Intermittent volcanic tremor; summit-area earthquakes

Small earthquakes beneath the summit cone increased from 20 September through the end of the month. The number of daily earthquakes registered by the seismograph in the summit crater reached a maximum of 136 on 2 October, about twice the number of the late September period. Two other increases, in May [12:6] and August, have been recorded since February.

Intermittent volcanic tremor occurred at a rate of 3-14 episodes/day during September. Episode duration generally ranged from 20 to 100 minutes, but often exceeded 2 hours. Tremor amplitude showed no change during the month. Steam rising from five summit-area vents was observed during a field survey 7-8 September. A steam plume rose continuously to ~50 m above the summit.

Information Contacts: JMA.

October 1987 (SEAN 12:10) Cite this Report

Renewed explosions; ash to 2,500 m; tourists evacuated

On 16 November at 1047 an eruption began with two explosions that sent a black ash and gas column to at least 2,500 m above the summit. Small eruptions continued every 5-10 minutes. Roads leading to the mountain were closed and about 150 tourists and others in the summit area were evacuated to a hotel on the N side of the crater at 1100. Heavy ashfall occurred on the S part of the island and more than 30 volcanic clasts fell near a shrine on the crater rim causing several small fires. A window was broken at a tea house near the crater but there were no reports of casualties or major damage. JMA recorded >1,000 small earthquakes before the eruption, and weak earthquakes continued afterwards. During an earthquake swarm in early October [12:9] the maximum daily number of recorded earthquakes reached twice the previous maximum for 1987.

The JMSA and the Japan Maritime Self-Defense Force (JMSDF) mobilized patrol boats and escort vessels off the island.

Information Contacts: A. Izumo, Yokohama Science Center; Japan Times, Tokyo; UPI.

November 1987 (SEAN 12:11) Cite this Report

Explosions after increased seismicity and new fumaroles

Small explosions began in the crater of the summit cinder cone (Mihara-yama) at 1047 on 16 November. Bombs were ejected to 1.6 km distance, instruments in the summit area were damaged, and nearby windows were cracked. The sounds were heard from as far as the Honshu mainland (at least 30 km away). The crater was obscured by clouds, but within the next hour aerial observers saw eruption columns rising to 5,000 m height. Tephra fell primarily to the E, and totaled a few thousand metric tons. Explosions were heard at irregular intervals until 1546. At about 1700, a lava fountain a few meters high was observed within the crater, but no flows formed. The next day, observations indicated that the crater floor had subsided 25 m since before the eruption.

Summit glow was observed at 0329 on 18 November, and ashfall to the SW soon followed. At about 0500, a gray ash column rose an estimated 1,600 m. Five hours later, at 1004, there were a few small explosions and a small amount of ash fell to the NW. Small eruption columns were observed until evening. The next day, helicopter observations indicated that the crater floor had subsided an additional 150 m [but see 13:1]. From 19 to 27 November, plumes a few hundred meters high occasionally rose from Mihara-yama. On 21 November, 5,200 of the 10,000 Oshima Island residents participated in evacuation drills that used six ships, 20 planes, and 145 motor vehicles. A white plume was the only activity at the volcano that day.

The groundmass of the ejecta contained abundant microphenocrysts of pyroxene and plagioclase. This suggested to volcanologists that the rock was not fresh magma, but was derived from the lava lake that has been cooling inside Mihara-yama crater since the November 1986 eruption.

Seismicity. Seismicity in the summit region had been gradually increasing since January 1987 (figure 12). A significant peak at the end of September 1987 was not accompanied by eruptive activity. Seismicity rapidly increased in the middle of November, reaching 543 events/day on 13 November and 586 on 14 November. Within a few days after the eruption on 16 November, seismicity rapidly declined to <50 events/day.

Figure 12. Number of earthquakes per day in the summit region of Izu-Oshima island, May-November 1987. Courtesy of Earthquake Research Institute, Univ of Tokyo.

Seismographs installed in the summit area two weeks before the eruption showed that the earthquakes were very shallow (less than ~400 m depth), and almost entirely confined to the area underneath the summit crater. Periods of both episodic and continuous tremor occurred in the months before the eruption. Tremor became continuous just before the eruption.

Ground deformation. Levelling and tilt data suggested that almost the entire summit region of Oshima has been subsiding since the November 1987 eruption. No clear ground deformation precursors were seen prior to the eruption. A stepwise change at all tiltmeters around the island occurred almost simultaneously with the start of the second series of eruptions at 0329 on 18 November (figure 13). The change is consistent with a center of inflation roughly 5 km NW of Mihara-yama. The volumetric strainmeter in the NW part of the island recorded an expansion consistent with the tilt (figure 13).

Figure 13. Map showing monitoring instrumentation on Izu-Oshima Island. Vectors show downward direction and magnitude of stepwise change in tilt, 18 November at 0326.

Fumarolic activity. A clear increase in fumarolic activity was noted before the eruption. A ring of fumaroles around the summit cone was observed at the end of July, and the activity of this ring increased with time. A second ring formed around the first ring in November. Fumarolic activity increased rapidly 5-10 days before the eruption.

Further References. Abe, K. and Takahashi, M., 1987, Description of the November 21, 1986 fissure eruption on the caldera floor of Izu-Oshima volcano, Japan: analysis of a series of photographs: Bulletin of the Earthquake Research Institute, Tokyo, v. 62, p. 149-162.

Aramaki, S., ed., 1988, The 1986-1987 eruption of Izu-Oshima volcano: Earthquake Research Institute, University of Tokyo, 61 p. (9 papers).

Endo, K., Chiba, T., Taniguchi, H., Sumita, M., Tachigawa, S., Miyahara, T., Uno, R., and Miyaji, N., 1988, Izu-Oshima 1986-1987 eruptions and the eruptive products: Proceedings, Kagoshima International Conference on Volcanoes, p. 119-122.

Special Issue — Oshima volcano, Izu: Journal of Geomagnetism and Geoelectricity (Terra Science Publishing Co.), v. 42, p. 139-363 (in English).

Special Issue — The 1986 eruption of Isu-Oshima volcano: Bulletin of the Geological Survey of Japan, v. 38 (1987), p. 601-753 (12 papers) (in Japanese with English abstracts and captions).

Special Issue — The 1986 eruption of Isu-Oshima volcano: Bulletin of the Volcanological Society of Japan, v. 33 (2nd series) (1988), (in Japanese with English abstracts and captions).

Watanabe, H., the 1986-1987 eruption of Izu-Oshima volcano: Proceedings, Kagoshima International Conference on Volcanoes, p. 37-40.

Information Contacts: Y. Ida, H. Watanabe, K. Yamaoka, S. Aramaki, and H. Glicken, Earthquake Research Institute, Univ of Tokyo; Kyodo radio, Tokyo.

January 1988 (SEAN 13:01) Cite this Report

Ash emission; more crater wall collapse

The number of earthquakes dropped to a low level after 24 November under both the summit crater (~10/day) and the E part of the island. Volcanic tremor below the summit crater ceased after an 18 November eruption, but resumed intermittently on 18 December and became continuous from 18 January. The amplitude of each tremor episode was small at first, but began increasing the evening of 5 January. The duration of each episode was several tens of minutes.

Crater floor subsidence of 175 m was associated with the November activity but the only change in summit area topography (noted during a helicopter survey on 5 January) was some further collapse of the crater wall.

Small eruptions on 25 January at 1530 and 27 January at 1425 ejected ash clouds to 300-400 m above the summit crater. Each lasted only a few minutes and they were not accompanied by air shocks or explosion sounds. Light ashfall occurred on the E side of the island. Tiltmeters detected no noticeable deformation associated with the eruptions. At other times, a white steam plume rose continuously to about 300-800 m above the crater. A 1,000-m white plume was noted on 27 January at 0900.

Information Contacts: JMA; Kyodo radio, Tokyo.

March 1988 (SEAN 13:03) Cite this Report

Tremor and discrete seismic events; steam emission

Continuous tremor resumed on 18 January, then became intermittent on 15 February, although its amplitude increased 27 February-6 March (figure 14). As of 5 April, tremor was continuous but of low amplitude. Earthquake activity increased sharply 3-4 March (figure 15), with most events apparently centered under the summit cone. The number of events counted by the instrument in the caldera exceeded 275 on 3 March and 502 on the 4th, but declined to a typical rate of 13/day on 6 March. A white steam plume continuously rose 300-800 m above the summit crater. On 18 March, an 1,800-m white plume was observed from the Oshima Weather Station, the highest plume seen since the 18 November 1987 eruption.

Figure 14. Amplitude of volcanic tremor at Oshima, 1 May 1987-31 March 1988. Courtesy of JMA.

Figure 15. Number of earthquakes/day at Oshima, 17 March 1987-31 March 1988. Courtesy of JMA.

Information Contacts: JMA.

August 1988 (SEAN 13:08) Cite this Report

Strong seismicity prompts volcanic activity warning

From late July to mid-August, numerous events clustering 10-20 km NW of Izu-Oshima Island . . . were detected at JMA's Oshima Weather Station, where 240 events were felt (figure 16a). Low-amplitude tremor was continuous. Episodes of larger amplitude tremor were superimposed at 1-hour intervals. On 21 August, the volcano emitted a white plume, its highest (1,900 m) since explosions resumed in November 1987. High levels of summit crater seismicity were recorded 19-23 August and 31 August-7 September, the last day of the report period (figure 16b). On 1 September, 423 events were recorded, the largest daily total since 3 March (502 events). The vigorous seismicity prompted the Oshima Weather Station to issue a volcanic activity warning.

Figure 16. Daily number of earthquakes centered (a) at Oshima volcano and (b) E of the Izu Peninsula. Courtesy of JMA.

Information Contacts: JMA.

June 1990 (BGVN 15:06) Cite this Report

Rapid decreases in steam cloud height, crater temperature, and tremor

Steam emission had gradually increased for more than 2 years and crater fumarole temperatures for at least 1 year, but fumarole temperatures began to drop in January (figure 17) and steam emission started a rapid decrease in mid-March. Steam emission had continued since the small explosions of 16-18 November 1987, when a crater, 300 m across and 150 m deep, was formed by collapse on the summit. Fumarole temperatures have been measured by infrared spot thermometry from the rim of the crater since December 1988.

Figure 17. Temperatures recorded at three points in the crater of Oshima, January 1989-June 1990. Courtesy of JMA.

Volcanic tremor, recorded almost continuously since January 1987, also began to decrease in March (figure 18). Since then, there have been few tremor events, with 10 weak tremor episodes recorded in April, and none in May or June. A M 6.5 earthquake on 20 February was centered at sea ~10 km W of the island. Aftershocks occurred along a N-S-trending line. On 1 March, a swarm of small summit-area earthquakes was recorded by a seismometer near the summit. The 1 March swarm was the most vigorous seismicity at the volcano since observations began in 1987 (figure 19). Following the swarm, seismicity on the island returned to normal levels.

Figure 18. Daily maximum amplitude of volcanic tremor at Oshima, May 1987-June 1990. Arrows at top of figure mark eruptions. Courtesy of JMA.

Figure 19. Daily number of recorded earthquakes at Oshima, January 1987-June 1990. Arrows at top of figure mark eruptions. Courtesy of JMA.

Information Contacts: JMA.

July 1990 (BGVN 15:07) Cite this Report

No volcanic tremor; magnitude 2.2 earthquake on N coast

As of 8 August, volcanic tremor has not been recorded since May. A magnitude 2.2 earthquake occurred 30 July on the N coast of the island. The shock was felt (intensity II, JMA scale) at the Oshima Weather Station, and was the first felt earthquake on the island since November 1987.

Information Contacts: JMA.

September 1990 (BGVN 15:09) Cite this Report

Small ash eruption; new collapse pit formed

A small eruption occurred at Mihara-yama cone on 4 October, the first since January 1988. A burst of summit earthquakes started at 0319 and two larger shocks were recorded at 0346 and 0347 (figure 20). Weak, lithic ashfall (no magmatic material was identified) occurred from around 0400 to 0445, thinly covering the N half of the island. Press sources reported that the column reached 1,200 m. The number of earthquakes declined toward evening, and a brief, weak emission of reddish ash was observed at 1400. Steam emission, vigorous on the 4th, decreased to normal by the following day.

Figure 20. Hourly number of earthquakes at Oshima, 1-10 October 1990. Arrows at top of figure mark eruptions. Courtesy of JMA.

A visit to the crater on 9 October revealed the presence of a new collapse pit in the N part of the crater floor (figure 21). The pit was 100 m across at the rim and 50 m in diameter more than 100 m down (the total depth is not known). Steam rose from the pit and several small collapses along the walls produced brownish clouds. The estimated total volume of the erupted ash (a few to ten thousand tons estimated weight) could not account for the total volume of the new pit. Geologists believed the collapse occurred at around 0346, coincident with the two recorded shocks. The main crater, 400 m across at the rim, 200 m at the floor, and 150 m deep, was formed by collapse during an eruption on 18 November 1987.

Figure 21. Sketch map and cross section of the crater at Oshima, 9 October 1990. Courtesy of JMA.

No deformation was detected before or after the eruption, and no tremor episodes were recorded. Seismicity decreased to pre-eruption levels by 13 October.

Information Contacts: JMA.

October 1990 (BGVN 15:10) Cite this Report

Seismicity and steam emission decline

Activity decreased, following ash emissions on 4 October . . . . Seismicity and steam emission declined rapidly following the 4 October activity, and no subsequent ash emissions had occurred as of 14 November. No tremor episodes were recorded during October.

Information Contacts: JMA.

November 1990 (BGVN 15:11) Cite this Report

Steam emission continues but seismicity declines

Seismicity declined rapidly after . . . 4 October (figure 22). No additional eruptions had occurred as of early December. Steam emission continued steadily through November, with the plume reaching 1,300 m above the crater. A series of 10 microearthquakes, centered in the E part of Oshima Island 3 km E of the summit (Mihara-yama) cone occurred 7-10 November, the first seismicity there since 21 November 1987. Seismicity at the summit continued unchanged through November at relatively low levels. A seismometer near the summit recorded 160 earthquakes during November, down from 633 in October. Seismicity and steam emission remained similar in early December. No tremor has been recorded since late April.

Figure 22. Daily number of earthquakes at Oshima, April-November 1990. The small 4 October eruption (arrow) was accompanied by high seismicity. Most of the earthquakes were centered on the summit (Mihara-yama) cone. Courtesy of JMA.

Information Contacts: JMA.

December 1990 (BGVN 15:12) Cite this Report

Decreased seismicity; continued steam emissions

Steady steam emission continued following the 4 October ash emission, reaching 1,000 m above the crater (figure 23). No additional eruptions from Mihara-yama cone had occurred as of 14 January 1991. A total of 57 earthquakes were recorded in December, down from 155 in November. No tremor episodes have been recorded since late April.

Figure 23. Monthly mean estimated plume heights from Oshima, 1987-90. Arrows mark eruptions. Courtesy of JMA.

Information Contacts: JMA.

May 1993 (BGVN 18:05) Cite this Report

Earthquake swarm and volcanic tremor

Continuous volcanic tremor began on the evening of 30 May at the active Mihara-yama cone, reaching peak amplitude at about 2300 that day. The next night, an increase in shallow earthquake activity began in the summit area. Both the earthquake swarm and tremor had stopped by 5 June. This was the first episode of volcanic tremor recorded since April 1990. Weak and steady steaming from the Mihara-yama crater continued without change throughout the period of increased seismicity.

Information Contacts: JMA.

October 1995 (BGVN 20:10) Cite this Report

Minor tremor and 48 earthquakes

On 4 October, local instruments recorded volcanic tremor of short duration and small amplitude. Throughout the month a significant but undisclosed number of earthquakes occurred in the adjacent N and W coastal areas. During October there were 48 earthquakes beneath the cone.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

August 1996 (BGVN 21:08) Cite this Report

Seismic activity increases

On 5 August a shallow low-frequency earthquake occurred beneath the top of the volcano. On the early morning of 22 August, seismicity increased on the W flank of Oshima and 8 earthquakes were felt. At the seismic station 4 km NW of the vent (Station C), 164 earthquakes were recorded on 22 August. The largest earthquake (M 2.5) was detected at 0136 on 22 August. On 24 August an earthquake was also felt. A total of 222 earthquakes was registered at Station C in August.

Information Contacts: Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

January 2013 (BGVN 38:01) Cite this Report

Non-eruptive May 2010 surface deformation from inferred deep instrusion

Oshima is an active volcano located on the northern tip of the Izu-Bonin volcanic arc. Our last report of activity at Oshima (BGVN 21:08) enumerated a flurry of shallow low-frequency earthquakes beneath the top and W flank of the volcano that started on 5 August 1996.

Since those relatively benign events, the Japan Meteorological Agency (JMA) had not observed any subsequent events worthy of note until May 2010 when land surface inflation was detected. The inflation was registered by a strainmeter, a Global Positioning System (GPS) network (run by the Geospatial Information Authority of Japan, GSI), and a tiltmeter network (run by the National Research Institute for Earth Science and Disaster Prevention, NIED).

In July 2010 seismicity in the shallow parts of and around Oshima began to increase. (High seismicity synchronous with inflation of the edifice was seen earlier, including in 2004 and 2007). These events were considered to be due to magma intrusion into the deeper part of the volcano. There were no remarkable changes in surface phenomenon. In September, the inflation that was detected in May began declining. Seismicity in the shallow parts of and around Oshima continued at a low level with some small earthquakes which temporally increased in the western offshore areas of Oshima on 22 December 2010.

The earthquakes increased in frequency again on 9 February 2011. GPS and strainmeter measurements indicated contraction since January, but the trend reversed to show inflation in October 2011. Seismicity remained at a low level. Very low level gas emissions were sometimes observed by a camera positioned on the NW summit. Based on a field survey on 28 October, no remarkable change in surface phenomena was observed.

No remarkable activity has been noted since October 2011. Throughout the noted activity, JMA held the Alert Level at 1.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/).