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

Abstract

[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.

Anisotropic tomography of Hokkaido reveals delamination‐induced flow above a subducting slab

“…As in our model, Kita et al . [] identified an elongated high‐velocity anomaly at shallow depths along the Hidaka Belt. Starting at a depth of ~30 km, this anomaly grades downward into a large isometric anomaly that extends eastward, as in our results.…”

“…Previous tomography results by Kita et al . [] demonstrated very good vertical and horizontal resolutions when resolving velocity anomalies, which can be explained by the large amount of data available in Hokkaido. In this paper, we mainly focus on the anisotropy.…”

“…We note nearly perfect correlation between our model and the P and S velocity models of shallow layers below southern Hokkaido developed by Kita et al . [], which were based on much denser data coverage than was our model. As in our model, Kita et al .…”

“…Based on detailed analysis of the tomographic results in the area of the Hidaka Belt, Kita et al . [] identified an elongated high‐velocity body below the suture zone, which was interpreted as a result of lithospheric thrusting due to collision between the Japan and Kuril fore‐arc segments.…”

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

“…4). Kita et al (2010aKita et al ( , 2012 suggested the region with low seismic velocity corresponds to a part of the fore-arc that is subducted in this region. Local geology supports the suggestion for crustal thickening.…”