Interplate seismic activity near the northern Japan Trench deduced from ocean bottom and land-based seismic observations

Abstract

The off-Sanriku area, the landward slope area of the northern Japan Trench, is characterized by its active seismicity which is directly related to the subduction process of the Pacific plate. In July–August 1992, a seismic swarm activity was observed near the trench. Five earthquakes of M > 6 occurred successively within 2 weeks at the beginning of the activity. We deployed an ocean bottom seismograph (OBS) array in the epicentral region to make an in situ observation of the spatial distribution of these events and to study the characteristics of the swarm activity in detail. We relocated 357 events from 23 July to 26 August with focal depth errors less than 5 km by using both the OBS and the land network data. The swarm events are not distributed homogeneously over the plate boundary but form several clusters. A remarkable concentration of the earthquakes was found in the vicinity of the hypocenter of the largest event during the OBS observation ( M 6.2, 29 July); this formed the most active cluster. Almost all the earthquakes, including the M 6.2 event, occurred at depths of about 15 km and were concentrated along a thin plane dipping to the west at 5°. The location of the plane coincides well with that of the plate boundary derived from previous seismic explorations. We also relocated epicenters of the off-Sanriku events by introducing station corrections to the land network data during the period when the OBS array was absent. The corrections were estimated from the travel time residuals of the hypocenter relocation using the OBS and the land network data. The result of the epicenter relocation strongly indicates the existence of a zone of extremely low seismic activity beneath the inner trench wall, where low seismic velocity, mechanically weak, unconsolidated sediments overly the subducting Pacific plate.