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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.

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[1] Using travel time data from both a nationwide dense seismic network and a dense temporary seismic network, we obtain a high-resolution three-dimensional seismic velocity structure beneath the Hokkaido corner. Considerable inhomogeneity in the seismic velocity structure is clearly imaged above the subducting Pacific slab. Our results indicate that a broad low-velocity zone of P and S waves, with velocities consistent for crustal rocks, is observed west of the Hidaka main thrust at depths of 35-90 km. The images also indicate that several smaller-scale high-velocity zones are located at depths of 0-35 km, striking approximately north-south and inclined to the east-northeastward at 40 -60 . All of these anomalous high-velocity zones are located at the deeper extension of Neogene thrust faults. The clearest high-velocity zone is located beneath the Hidaka metamorphic belt and is in contact with the eastern edge of the broad low-velocity zone. Moreover, the boundary between the clearest high-velocity and the broad low-velocity zones corresponds to the fault plane of the 1970 Mj (magnitude determined by the Japan Meteorological Agency) 6.7 Hidaka earthquake. The western boundary of another small high-velocity zone, at depths of 20 to 30 km within the broad low-velocity zone, corresponds to the fault plane of the 1982 Mj 7.1 Urakawa-oki earthquake. These observations suggest that these two large and anomalously deep inland earthquakes occurred at sharp material boundaries under a northeast-southwest compressional stress field caused by ongoing arc-arc collision process.