Seismicity declined rapidly after . . . 4 October (figure 22). No additional eruptions had occurred as of early December. Steam emission continued steadily through November, with the plume reaching 1,300 m above the crater. A series of 10 microearthquakes, centered in the E part of Oshima Island 3 km E of the summit (Mihara-yama) cone occurred 7-10 November, the first seismicity there since 21 November 1987. Seismicity at the summit continued unchanged through November at relatively low levels. A seismometer near the summit recorded 160 earthquakes during November, down from 633 in October. Seismicity and steam emission remained similar in early December. No tremor has been recorded since late April.

Figure 22. Daily number of earthquakes at Oshima, April-November 1990. The small 4 October eruption (arrow) was accompanied by high seismicity. Most of the earthquakes were centered on the summit (Mihara-yama) cone. Courtesy of JMA.

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.