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Report on Kozushima (Japan) — September 1995


Bulletin of the Global Volcanism Network, vol. 20, no. 9 (September 1995)
Managing Editor: Richard Wunderman.

Kozushima (Japan) Strong earthquake swarm in early October

Please cite this report as:

Global Volcanism Program, 1995. Report on Kozushima (Japan) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 20:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199509-284030



34.219°N, 139.153°E; summit elev. 572 m

All times are local (unless otherwise noted)

At 2143 on 6 October, a M 5.6 earthquake occurred near Kozu-shima (figure 2). The earthquake, which had an intensity at Kozu-shima of V on JMA's scale, caused a few landslides there. A M 4.8 earthquake 14 minutes earlier had an intensity of IV. During the next several days, an earthquake swarm continued offshore to the SW of Kozu-shima (figure 2, bottom). The swarm's maximum depth shifted downward with time, reaching 20-25 km. None of this seismicity was thought to have been induced by volcanism.

Figure (see Caption) Figure 2. Kozu-shima epicenter map (top) and plot of focal depths versus time for September through 16 October 1995 (bottom). Courtesy of JMA.

Though obscured by epicenters on figure 2, Kozu-shima island has dimensions of 4 x 6 km and lies 20 km SSW of Nii-jima island and adjacent to the Izu Peninsula. Kozu-shima contains abundant rhyolitic surge deposits and lava domes. Its last eruption was in 838-840 AD. Seismicity near the volcano, and sometimes in vicinity of Nii-jima, has been episodically high in recent years.

Geological Summary. 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: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.