Logo link to homepage

Report on Izu-Oshima (Japan) — August 1986


Scientific Event Alert Network Bulletin, vol. 11, no. 8 (August 1986)
Managing Editor: Lindsay McClelland.

Izu-Oshima (Japan) Tremor and earthquake swarm but no change in thermal activity

Please cite this report as:

Global Volcanism Program, 1986. Report on Izu-Oshima (Japan) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 11:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198608-284010



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

All times are local (unless otherwise noted)

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.

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.