Hypocenter distribution of plate boundary zone off Fukushima, Japan, derived from ocean bottom seismometer data

The microearthquake observation using twenty-seven Ocean Bottom Seismometers (OBSs) was carried out to obtain the detailed distribution of microearthquakes beneath the area off Fukushima, the middle part of the Japan Trench in the summer of 1997. The hypocenters of 401 earthquakes were obtained during the observation period of about one month. The hypocenter distribution has high spatial resolution (an error of less than 3 km in the horizontal direction and less than 5km in depth), because the structure used for locating events was determined by airgun-OBS surveys in the study area and the hypocenters were finally located by a simultaneous inversion to determine hypocenters and one-dimension velocity structure. Almost all of the hypocenters determined with small spatial errors are in the vicinity of the plate boundary. The seismicity in the area within 100 km distance from the trench is low. The seaward limit of the high seismicity region is close to the western end of the direct contacting zone between the oceanic crust and the overriding landward crust. From focal mechanisms of earthquakes, we estimate that the coupling between the subducting plate and the overriding landward plate is not weak beneath the OBS network and there is intraplate-type earthquake activity in the subducting plate due to an abrupt increase of the dip angle of the subducting plate. Within the overriding landward plate, 29 earthquakes were located. Twelve earthquakes were located about 30km below the plate boundary, forming a landward dipping plane, which seems to be an up-dip continuation of the lower plane of the double seismic zone. The microseismicity characteristics around the plate boundary are interpreted to be controlled by geometry and a property of the plate boundary interface. We suggest that a moment stress due to abrupt bending of the subducting plate and dehydration embrittlement process of serpentinized mantle relate earthquake occurrence in the lower seismic zone beneath the forearc.