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.

Geologic Background. 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 and was constructed over the remnants of three dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous historical eruptions. More than 40 parasitic 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. Historical activity dates back to the 7th century CE. A major eruption in 1986 produced spectacular lava fountains up to 1600 m height and a 16-km-high subplinian eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: JMA.