Seismic Activity in the Northeastern Japan Arc

The most recent data obtained from the high-gain seismograph network of Tohoku University are used to make an investigation on seismic activity and tectonics in the northeastern Japan arc. Seismic activity is closely related to the tectonic development of island arcs, and detailed analyses of the spatial distribution of microearthquakes in the northeastern Japan arc have led to conclusive evidence that almost all the shallow microearthquakes in the land area occur only in the layer with a P-wave velocity of 5.9 km/sec which was determined from explosion seismic observations. The configuration of the deep seismic zone in this region is discussed in relation to the plate subduction beneath the arc. The accurate determination of the hypocenters and focal mechanisms of microearthquakes has made it possible to investigate the subduction process beneath the arc.