Spatial variation in seismotectonics and stress conditions across the Kurile and Japan trenches inferred from the analysis of focal mechanism data in Hokkaido, northern Japan

We analyzed seismotectonic characteristics of focal mechanism data across the Kurile and Japan trenches in the Hokkaido region, north Japan, to check for spatial variation in seismotectonics. For this purpose we selected highly constrained focal mechanism data from the Harvard CMT catalog and Japanese National Research Institute for Earth Science and Disaster Prevention (NIED) for the period 1976 to July 2007. Further, we selected data with high double couple components within a depth range of ±7.5 km from the inferred boundary of the subducting Pacific slab. We found a systematic variation in seismotectonic affinity of the data exhibiting eight distinct zones in the region. Among them, the outer rise region exhibits a normal fault regime with biaxial extension along and across the trenches. In the regions hosting the Pacific slab at shallow depths, most of the focal mechanism data lie in the thrust regime and the odd-event regime. This is due to a complicated stress condition with a major component of seismic deformation being accommodated by extensional and strike-slip movements in a thrust regime. Dominantly uniaxial compression acting downdip of the subducted Pacific slab is found in the regions hosting the rupture zones of large subduction earthquakes such that during an interseismic period tectonic stress is loaded by subduction. In the proximity of the Hidaka collision zone the stress conditions suggest that the impact of collision on ongoing seismotectonics is inevitable. In such regions we found thrust regimes with biaxial compression along the collision and subduction directions that favor bulging and/or contortion of the subducted slab.