ABSTRACT
We investigated S wave splitting in and around the focal area of the 1995 Hyogo-ken Nanbu earthquake (Ms=7.2), over a 3-year period after the mainshock. We discussed contributions of fractures of shear fault origin and cracks of tectonic stress origin with respect to fault healing. In our former study, we investigated the splitting using the data 9 - 12 months after the mainshock and found the direction of faster S wave polarization φ parallel to the fault strike within 500 m of the fault trace. Nevertheless such directions as above were not found near an adjacent active fault that activated in 1596, which suggests shear fault origin fractures have already healed. From the results, we proposed a hypothesis that fault healing process will be detected and monitored by continuous analyses of S wave splitting, i.e., temporal changes in φ. In the present study, we investigated S wave splitting before and after the above period in order to justify the hypothesis. Within 33 months after the mainshock, the fault parallel φ turned to the direction parallel to the regional maximum horizontal compressional stress. This strongly suggests that all the fractures of shear fault origin completed to heal, which is unexpectedly rapid process.
