Spatial distribution of microearthquakes beneath the Japan Trench from ocean bottom seismographic observations

The area of the Japan Trench, about 40°N off the Tôhoku District of north-eastern Honshu, Japan, was for the first time extensively covered by an ocean bottom seismograph (OBS) array. Seven OBSs were available for analyses of natural earthquakes and the observation period lasted two weeks during 1980 July and August. During this period, each OBS registered between 300 and 600 earthquakes. About 100 events were simultaneously recorded by at least three OBSs. Hypocentres were calculated by using a generalized inversion technique. Sixty-five events, mostly with magnitudes (M) from 1.5 to 3.0, which were not located by a land network, were determined to be within or near the area covered by the OBS array. The OBS array had nearly uniform detectability for earthquakes with M over 2.0 within the latitude range of 39.5° — 41.5°N and the longitude range of 143° — 146.5°E. Within this area there is a highly active region of microseismicity beneath the seaward wall of the Japan Trench. The seaward activity is concentrated within 100 km of the trench axis and is low beneath the Northwest Pacific Basin. A seismicity gap was found just westward of the trench axis, beneath the landward wall of the trench at a water depth of between 6000 and 4000 m. Seismic activity continues landward from the region west of the gap. The OBS array enabled us to determine the focal depth of microearthquake activity with better resolution than was previously possible. The results show that the microearthquake activity is concentrated within the upper part of the oceanic lithosphere. On the seaward side, most events occur at a depth of less than 30 km beneath the trench wall, while the activity on the landward side starts to deepen westward exactly at the trench axis. This paper reports the first direct observations to clarify the spatial distribution of microearthquakes associated with the bending and/or subducting of the oceanic plate just beneath the Japan Trench axis.