2 July 2000
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Seismic activity in the sea near Niijima-Kozushima on 1 July 2000

Seismic activity began on the night of 26 June in association with the volcanic activity in western Miyakejima island and later moved to the sea west of Miyakejima, continues at an active level. In addition to this activity, there was also an M5.2 event occurred at 12:11 p.m. on 29 June in a separated place of the northern coastal region of Kozushima, and seismic intensity of 5 lower was observed near the epicenter.

Since the afternoon of 29 June, the seismic activity in the sea west of Miyakejima has moved farther to the northwest, and activity continued in an area in the sea about 10 km to the east of Kozushima. Scattered seismic activity in the vicinity of Kozushima had abated by the afternoon of the 30th.

At 4:02 p.m. on 1 July, there was an M6.4 event about 5km the east of Kozushima, with a seismic intensity of 6 lower recorded on Kozushima, and 5 lower on Niijima. The focal mechanism was a strike-slip fault type, with a northwest-southeast pressure axis, which is typical for this region. The east-west orientation of the aftershock distribution suggests that the involved is a right lateral strike-slip with an east-west strike.

GPS observation showed movement to the southeast in Shikinejima, and to the southwest in Kozushima. In addition, since the 28th, observation of Kozushima shows downward tilt to the northwest.

It is thought that this event, as with the M5.2 event on the 29th, may have been triggered by the effects of the volcanic activity on Miyakejima on the earth's crust. At the time of the volcanic activity on Miyakejima in 1962, high levels of seismic activity, including M5.9 and M5.8 events continued for a week in the sea to the west of the island, and abated after about one month. The recent seismic activity is similar to the 1962 activity. But the present area of activity neighbors Niijiam and Kozushima islands, and the potential for strong ground motion associated with M5 events makes it necessary to monitor the movement of activity in the sea near Niijima and Kozushima.