Seismic activity in the subducting Philippine Sea plate along the Suruga trough revealed by OBS observation.

Abstract

A seismicity observation with 10 pop-up-type ocean bottom seismometers were conducted along the Suruga trough, south off central Japan, for about one month in 1984. This observation detected more than 300 earthquakes, among which about 60 earthquakes were located around the Suruga trough. In this observational period, seismically the most active area is the middle part of the Suruga trough. Applying station corrections, we precisely located the earthquakes around the Suruga trough. The hypocenters show a thin seismic zone dipping westward from the trough axis. The focal depths at the trough axis, 0-10 km, are shallower by 5 to 10 km than the principal focal depths of hypocenters obtained by the land-based seismic network. The hypocentral distribution by the present survey is interpreted as the seismic activity in the uppermost part of the Philippine Sea plate subducting from the Suruga trough axis. In contrast to the high seismicity at the middle part of the trough, only a small number of earthquakes were located in the northern and southern parts. The microearthquakes along the Suruga trough observed in the present survey tended to occur simultaneously in the segment over about 50 km length, synchronizing with the activation of the small swarm around the northeastern edge of the Nankai trough.