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 35–90 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.