Microearthquakes and tectonics around an outer rise: the Zenisu ridge, Japan

Abstract

To investigate the activity and tectonic implications of intraplate microearthquakes occurring within the deflecting oceanic plate along the Nankai-Suruga trough, Japan, we deployed eight ocean bottom seismometers (OBS's) on the Zenisu ridge, for a period of 25 days in 1985. Hypocenters of approximately 40 microearthquakes were identified around the Zenisu ridge, using the OBS data. The OBS-located hypocenters were distributed mainly on the southern half flank of the ridge. There are no earthquakes located by the OBS data within the upper layer of the Philippine Sea plate. The focal depths range from 18 to 35 km, indicating that the earthquakes occurred within the lower layer of the mechanically strong part of the lithosphere. The composite focal plane solution of the microearthquakes that occurred on the Zenisu ridge was a strike-slip or reverse fault type, having P-axis in the WNW. The tectonic origin of the earthquakes located on the ridge was demonstrated to be found in the bending, with a regional compression, of the Philippine Sea plate along the eastern Nankai-Suruga through.