S wave splitting in the aftershock region of the 1995 Hyogo-ken Nanbu earthquake

The 1995 Hyogc-ken Nanbu earthquake (Kobe earthquake; M7.2) occurred on January 17, 1995. A temporary aftershock observation was carried out by the Geophysical Research Group Organized by Universities for Prediction Seismology in 1995 from October 1995 to January 1996. A dense seismic network was deployed in and around the aftershock region, and some seismic stations were set directly on the active fault traces. We analyze the S wave splitting in and around the aftershock region of the Hyogo-ken Nanbu earthquake. We detect a spatial variation of S wave splitting which can be related to crack orientations in an area:where a large earthquake has occurred. At stations more than 1 km away from the Hyogo-ken Nanbu earthquake fault zone, the leading shear wave polarization directions (LSPD) are found to be parallel to the axis of the regional maximum horizontal compressional stress (E-W), suggesting that cracks caused by regional tectonic stress exist in the area. However, at stations within 500 m of the earthquake fault zones, the LSPDs are parallel to the fault strikes (N45 degrees-50 degrees E). This suggests that new fractures were produced parallel to the faults by shear faulting of the Hyogo-ken Nanbu earthquake and resulted in S wave splitting along the earthquake fault zone. Nevertheless, shear fault origin anisotropy was not evident at stations on the active faults which ruptured in 1596. This implies that healing processes have already closed the fractures produced by shear faulting in 1596. Therefore we can expect that S wave splitting can be a useful tool for monitoring healing processes of active faults.