INTERPLATE SEISMIC ACTIVITY NEAR THE NORTHERN JAPAN TRENCH DEDUCED FROM OCEAN-BOTTOM AND LAND-BASED SEISMIC OBSERVATIONS

The off-Sanriku area, the landward slope area of the northern Japan Tr ench, is characterized by its active seismicity which is directly rela ted to the subduction process of the Pacific plate. In July-August 199 2, a seismic swarm activity was observed near the trench. Five earthqu akes 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 di stribution of these events and to study the characteristics of the swa rm activity in detail. We relocated 357 events from 23 July to 26 Augu st with focal depth errors less than 5 km by using both the OBS and th e land network data. The swarm events are not distributed homogeneousl y over the plate boundary but form several clusters. A remarkable conc entration of the earthquakes was found in the vicinity of the hypocent er of the largest event during the OBS observation (M 6.2, 29 July); t his formed the most active cluster. Almost all the earthquakes, includ ing the M 6.2 event, occurred at depths of about 15 km and were concen trated along a thin plane dipping to the west at 5 degrees. The locati on of the plane coincides well with that of the plate boundary derived from previous seismic explorations. We also relocated epicenters of t he off-Sanriku events by introducing station corrections to the land n etwork data during the period when the OBS array was absent. The corre ctions 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 ex tremely low seismic activity beneath the inner trench wall, where low seismic velocity, mechanically weak, unconsolidated sediments overly t he subducting Pacific plate.