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.

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: Kyodo radio, Tokyo.