2 August 2000
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Seismic activity in the area from Miyakejima to Niijima-Kozushima

*Since July 21st, the seismic activity in the area from Miyakejima to Niijima-Kozushima has continued to rise and fall. From July 30th, the zone of activity has expanded further seaward from the sea southwest of Miyakejima to the sea southwest of Mikurajima, and has continued in that state up to the present time (8:00 a.m. on August 2nd). As a result, at present there are three areas of seismic activity: the area from the sea west of Miyakejima to the sea west of Niijima-Kozushima (hereafter referred to as Sea Zone 1), the area from the sea east of Toshima to the vicinity of Kozushima (hereafter referred to as Sea Zone 2), and the area from the sea southwest of Miyakejima to the sea southwest of Mikurajima (hereafter referred to as Sea Zone 3).

There were nine events measuring maximum seismic intensities of 5 lower or greater from July 21st to 8:00 a.m. on August 2nd. The strongest event in that group was the M6.4 event with a maximum seismic intensity of 6 lower that occurred in Sea Zone 3 on July 30th. The focal mechanism for that event was a strike-slip fault type with a northwest-southeast pressure axis, which is typical of events occurring in the region. The results of GPS observation of the area indicated crustal movement associated with the event.

*The results of GPS observation of the area show that crustal movements in the area from Niijima-Kozushima, Toshima and the Izu Peninsula to the southern tip of the Boso Peninsula have continued since July 21st. The results of VLBI (very long baseline interferometry) observations in the southern tip of the Boso Peninsula, and strainmeter observations in the Izu Peninsula also showed crustal movement in those areas.

*The characteristics of the seismic activity and crustal movement, the distribution of the activity, and the focal mechanisms of the events suggest the present activity is associated with subterranean dike-formation magma activity in the sea to the east of Kozushima, as was concluded in the July 21st evaluation.

*As described above, the seismic activity and crustal movement in the area in the area from Miyakejima to Niijima-Kozushima is continuing at an unprecedented level of activity. Consequently, the further occurrence of M6.0 or greater events (on the order of seismic intensity 6 lower) cannot be ruled out. Also, in the event of an event on the order of M5 in the vicinity of Toshima, Niijima, Shikinejima, Kozushima, Miyakejima or Mikurajima, strong shocks (of around seismic intensity 5) can be expected.