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.

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.