The San-in shear zone in southwest Japan revealed by the GEONET data

Abstract:

The most remarkable lineament of microseismicity in the Japanese Islands exists along the northern coastline in the San-in region that is a northern half of the westernmost part of Honshu in southwest Japan. Several M~7 earthquakes including the 1943 Tottori, and the 2000 Western Tottori earthquakes occurred along the lineament. However, no well-developed active fault has been identified in and around the lineament. Though several previous studies proposed a large dextral shear zone corresponding to seismicity lineament in the San-in region, none of them dealt with geodetic data. In this study, we report that a high strain rate is concentrated along the seismicity lineament from cGNSS observations. We propose to call this deformed zone “the San-in shear zone”.

The GNSS data used in this study are from the GEONET data operated by the Geospatial Information Authority of Japan. We estimate interseismic velocities by fitting a linear function to daily GNSS coordinates estimated with both Bernese and GIPSY software. We recognize the following characteristics in the velocity distribution in the San-in region and its vicinity (Fig. 1). First, a northern limit of the observed northwestward velocity due to interplate coupling on the subducting Philippine Sea plate locates in and around the Seto Inland Sea. Second, the inland San-in region does not significantly deform though the northern coast of the San-in regions moves eastward with a velocity of 4 mm/yr relative to the inland region. The observed right-lateral shear deformation is generally localized along the lineament of seismicity and continues more than 200 km long along the coast of the Japan Sea. Velocity profile across the shear zone is roughly reproduced by a 2-dimensional dislocation model for a vertical strike-slip fault with a locking depth of 5~15 km.

The San-in shear zone can be interpreted as strain portioning to accommodate strike slip component of oblique subduction along the Nankai Trough. The Median Tectonic Line (MTL) is a famous example of such a strain portioning. The San-in shear zone may be the second and parallel fault system 150 km north of the MTL, which accommodates a half of dextral component of the MTL.