Geologic Background:
A cluster of rhyolitic lava domes and associated
pyroclastic deposits form the small 4 x 6 km island of
Kozu-shima. The youngest and largest dome, 574-m-high
Tenjo-yama, occupies the central portion of the island.
Most of the older domes flank Tenjo-yama on the north.
Historic Activity:
- Only two historical eruptions, from the 9th century,
are known.
- The volcano last erupted in 838-840 AD, ejecting
pyroclastics and extruding a dome at the island's
center.
Recent Activity:
- Earthquake swarm occurred on April 23, 1991 but no
surface activity was reported. Another earthquake swarm
occurred on October 24-25, 1991.
- An earthquake swarm occurred on January 26-27, 1992,
3 weeks after another swarm at Nii-jima. A seismic swarm
occurred on April 21-25 and a second on May 8. Additional
swarms during May 14-16. A M 5.2 earthquake occurred 8 km
SW of volcano on June 15th 1992. Seismic swarm on August
18th. Weak earthquake swarm on Sept. 1, 1992. An
earthquake swarm occurred midway between Nii-jima and
Kozu-shima during Oct. 17-20, 1992.
- On March 25, 1993, a swarm occurred at Kozu-shima
island. Two earthquake swarms occurred near Kozu-shima
island in August 1993 (9-10 & 22).
- Strong earthquake swarm in early October 1995.
Data Sources:
- Global Volcanism Network (V. 16, No. 4 to V. 20, No.
10).
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