Abstract
Abstract 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.
Highlights
The 2004 mid Niigata prefecture earthquake with M6.8 occurred in the north of central part of Japan on October 23, 2004
We attempt to estimate a velocity structure of the hypocentral area of the 2005 mid Niigata prefecture earthquake by means of the Double Difference (DD) tomography algorithm developed by Zhang and Thurber (2003)
To check the resolution of the inversion, we carried out a checker board resolution test (CRT) and a restoring resolution check (RRT), both of which had been used in the many tomographic studies
Summary
The 2004 mid Niigata prefecture earthquake with M6.8 occurred in the north of central part of Japan on October 23, 2004. We attempt to estimate a velocity structure of the hypocentral area of the 2005 mid Niigata prefecture earthquake by means of the DD tomography algorithm developed by Zhang and Thurber (2003). 2. Data and Analysis After the occurrence of the main shock, in order to determine the precise aftershock location, we started with a temporal seismic observation in the aftershock area of the earthquake.
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