An earthquake swarm [W] of Oshima Island, accompanied by subterranean rumbling, began in late October and was continuing in mid-November (table 1). Similar earthquake swarms occurred in January 1972 and November 1973. Ten minor explosive eruptions have occurred at Oshima since 1962, the latest [in 1974].

Table 1. Number of felt and recorded earthquakes at Oshima, 30 October-17 November 1977. [JMA replaced the data in the original table.]

Further Reference. Yamashina, K., and Nakamura, K., 1978, Correlations between tectonic earthquakes and volcanic activity of Izu-Oshima volcano, Japan: JVGR, v. 4, p. 233-250.

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 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 eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: T. Tiba, National Science Museum, Tokyo.