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 H 2 O 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.