A seismicity gap beneath the inner wall of the japan trench as derived by ocean bottom seismograph measurement

Abstract

An ocean bottom seismograph (OBS) array covered the area of the Japan Trench about 40°N off the Tohoku District of northeastern Honshu, Japan. The OBS array was operated for two weeks in June 1981. Arrival time data of 8 OBSs were used to study a spatial distribution of microearthquakes around the trench. During the observation period 400–500 events with a signal-to-noise ratio greater than 2 were detected by each OBS. The arrival times of 223 of these events could be picked. 38 earthquakes were located in the area of the Japan Trench within the latitude range of 38.5°–41.0°N and the longitude range of 142.0°–146.0°E, and about a hundred events were located beneath or near Honshu Island.S-P time distribution recorded by each OBS showed seismic activities to exist on both sides of the trench axis, while the seismic level was low beneath the inner wall of the trench. Epicentral distribution showed a seismicity gap beneath the inner wall of the trench: the area of the landward activity continued eastward up to the position where the water depth is about 4000 m. Seismic activity existed beneath the trench axis and the seaward wall of the trench. The seismicity gap suggested from the 1980 OBS observations in the vicinity of the present study area was clearly confirmed by the present observations.The OBS array enabled us to determine the focal depth of microearthquake activity with better resolution than was possible from land observations. The OBS data showed that the landward activity beneath the continental slope was concentrated in the very shallow part of the lithosphere at a depth less than 30 km. Foci started to deepen westward with a very low dip angle of less than 10°, which was the most shallow part of the upper plane of the double seismic zone.The focal depth distribution showed that events just beneath the trench axis occurred to depths slightly greater than 50 km, while the foci of the seaward activity were concentrated in the uppermost lithosphere at depths less than 30 km. Seismic cross-sections clearly show that the activity beneath the trench axis and seaward wall of the trench is separated from the landward activity, which is a shallow extension of the double seismic zone, by the seismicity gap.