Focal mechanisms of seismic events in the area of Niijima-Kozushima and the sea west of Miyakejima (for the current and 1962 seismic activity)

There were a number of seismic events in which the focal mechanism was a strike-slip fault type with a northwest-southeast compression axis, including an M3.8 event (a) at 11:44 a.m. on 27 June near the western coast of Miyakejima, and events in a zone of activity extending to the northwest, as well as an M6.4 event (b) at 4:01 p.m. on 1 July, and an M6.1 event (c) at 3:57 a.m. on 9 July.

Seismic events of M5.0 (d) at 11:56 a.m. on 29 June, and M5.2 (e) at 12:11 p.m., also on 29 June, both of which occurred near the northeast coast of Kozushima, had northeast-southwest tension axes, and demonstrated characteristics typical of earthquakes in that area. The M5.0 event (f) at 12:34 on 29 June had a northwest-southeast compressional axis and demonstrated characteristics typical of earthquakes occurring in the sea to the west of Miyakejima. Among earthquakes occurring on the island of Kozushima, the M3.5 event (g) at 12:31 p.m. and the M4.2 event (h) at 12:41 p.m., both of which occurred on 29 June, had northwest-southeast tension axes, which are not frequently seen in this area. Furthermore, in the highly active zone in the sea to the west of Miyakejima, normal fault type focal mechanisms (i-k) were observed.

The conjecture of a pure strike-slip fault type as the focal mechanism for the 1962 events has been accepted. The epicenters have also been plotted based on a recalculation of the hypocenters.

Japan Meteorological Agency