High-resolution seismic velocity structure beneath the Hokkaido corner, northern Japan: Arc-arc collision and origins of the 1970 M 6.7 Hidaka and 1982 M 7.1 Urakawa-oki earthquakes

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 35-90km. The images also indicate that several smaller-scale high-velocity zones are located at depths of 0-35km, 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 30km 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.