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

Microearthquake observation using Ocean Bottom Seismometers (OBSs) was carried out to obtain a detailed distribution of microearthquakes beneath the area off Fukushima, in the middle section of the Japan Trench in the summer of 1997. The observation period spanned approximately one month. Almost all of the well-determined hypocenters occurred in the vicinity of the plate boundary in this region (approximately 100 km landward of the trench axis), while seismicity is markedly lower between this area and the trench itself. The seaward limit of the high seismicity region is close to the western end of the zone of direct contact between the oceanic crust and the overriding landward crust. Twenty-nine earthquakes were recorded within the overriding landward plate. Twelve earthquakes were recorded about 30 km below the plate boundary, and form a landward dipping plane that appears to be an up-dip continuation of the lower plane of the double seismic zone. The microseismicity characteristics of the plate boundary region are interpreted to be controlled by the geometry and physical properties of the plate interface. The seismic activity in the lower seismic plane near the trench is considered to relate to bending of the subducting plate and to dehydration of serpentinized Pacific Plate mantle.