Observations of microseismicity in the southern Kuril Trench area by arrays of ocean bottom seismometers

Abstract

Summary Two ocean bottom seismometer (OBS) arrays were deployed to study microearthquake activity in the southernmost Kuril Trench area; at a junction of the Kuril and Japan Trenches. The first OBS observation was carried out from 1982 June to 1982 July and the second was from 1983 July to 1983 August. Each array consisted of 10 OBSs covering adjacent areas with an overlap. During the observation periods each OBS recorded about 50 earthquakes a day in average, a tenth of which were located in and near the arrays. A detailed picture of seismic activity associated with subduction and/or bending of the Pacific plate was obtained at the very point where it started to subduct. Spatial distribution of microseismicity in the southern Kuril Trench area is, in general, similar to that found in the Japan Trench area off Sanriku. A prominent microseismicity was found both landward and seaward of the axis of the Kuril Trench. The seaward activity had a sharp seaward boundary; along the edge of the seaward trench wall, in the Pacific Basin beyond which no detectable event with magnitude of greater than 2 was observed. Although seismicity was high beneath a continental slope between the Kuril Trench and Hokkaido, there was a seismicity gap beneath the inner trench wall. The gap coincided with an area of between 3 and 5 km in sea depth. The depth of the events beneath the seaward trench wall ranged from 0 to 30 km and just beneath the trench axis it was slightly deeper (0–50 km). A detailed comparison of seismicity between the Kuril and Japan Trenches showed a variation in spatial distribution along the trench axis: microseismicity near the trench axis is not uniform. The activity beneath the continental slope formed a seismic zone dipping landward with a low dip angle of less than 10°, which seemed to be a shallower extension of the lower plane of the double structured seismic zone in the Kuril Trench area. It shows a contrast to the Japan Trench area where the seismicity corresponds to the upper plane.