Existence of a seismic belt in the upper plane of the double seismic zone extending in the along-arc direction at depths of 70-100 km beneath NE Japan

We relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the Pacific slab beneath NE Japan. We found a seismic "belt" parallel to the iso-depth contour of the plate interface that is beneath the forearc area at depths of 70-100 km, indicating that the distribution of the upper plane earthquakes is non-uniform. The location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where H₂O contents change in the slab crust. Events in the upper seismic plane have mainly down-dip compression-type focal mechanisms but several events have normal fault-type (NF-type) versions, whose spatial distribution appears to correspond to these boundaries. These NF events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions.