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Report on Izu-Oshima (Japan) — January 1987


Scientific Event Alert Network Bulletin, vol. 12, no. 1 (January 1987)
Managing Editor: Lindsay McClelland.

Izu-Oshima (Japan) Small explosions one day after tremor resumes

Please cite this report as:

Global Volcanism Program, 1987. Report on Izu-Oshima (Japan) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198701-284010



34.724°N, 139.394°E; summit elev. 746 m

All times are local (unless otherwise noted)

"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 (see Caption) 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."

Geological Summary. Izu-Oshima volcano in Sagami Bay, east of the Izu Peninsula, is the northernmost of the Izu Islands. The broad, low stratovolcano forms an 11 x 13 km island constructed over the remnants of three older dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous recorded eruptions datining back to the 7th century CE. More than 40 cones are located within the caldera and along two parallel rift zones trending NNW-SSE. Although it is a dominantly basaltic volcano, strong explosive activity has occurred at intervals of 100-150 years throughout the past few thousand years. A major eruption in 1986 produced spectacular lava fountains up to 1,600 m high and a 16-km-high eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: JMA.