The 2004 mid Niigata prefecture earthquake with M6.8 occurred in the north of central part of Japan. It was a reverse fault by the regional compression stress field in NW-SE direction. Several aftershocks with M≥6 were occurred. The large aftershocks occurred on plural fault planes. The plane was either parallel or normal to the main shock one. We estimated three dimensional velocity structures in and around the focal area of the earthquake by using a Double Difference tomography method. The arrival time data were picked from seismograms at the deployed seismic stations settled by Kyoto and Kyushu universities in collaboration, NIED, ERI, and JMA. The velocity structure showed that a low velocity zone existed in the northwest part of the aftershock area. On the contrary, the velocity in the southeast became high. Moreover, the fault plane of the main shock inferred from the aftershock distribution was located at the velocity boundary.
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