Report on Kozushima (Japan) — October 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 10 (October 1991)
Managing Editor: Lindsay McClelland.
Kozushima (Japan) Earthquake swarm N of island
Please cite this report as:
Global Volcanism Program, 1991. Report on Kozushima (Japan) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:10. Smithsonian Institution.
34.219°N, 139.153°E; summit elev. 572 m
All times are local (unless otherwise noted)
An earthquake swarm occurred 24-25 October in the sea 7 km N of Kozu-shima (figure 1). The largest shocks (M 4.9) were recorded at 1746 and 1754 on the 24th. Changes in surface activity were not observed on the island or in the epicentral area. The seismicity was the first for the area since 23 April . . . .
Geologic Background. A cluster of rhyolitic lava domes and associated pyroclastic deposits form the small 4 x 6 km island of Kozushima in the northern Izu Islands. Kozushima lies along the Zenisu Ridge, one of several en-echelon ridges oriented NE-SW, transverse to the trend of the northern Izu arc. The youngest and largest of the 18 lava domes, 574-m-high Tenjoyama, occupies the central portion of the island. Most of the older domes, some of which are Holocene in age, flank Tenjoyama to the north, although late-Pleistocene domes are also found at the southern end of the island. Only two possible historical eruptions, from the 9th century, are known. A lava flow may have reached the sea during an eruption in 832 CE. Tenjosan lava dome was formed during a major eruption in 838 CE that also produced pyroclastic flows and surges. Earthquake swarms took place during the 20th century.
Information Contacts: JMA.