Estimation of the stress field in the region of the 2000 Western Tottori Earthquake: Using numerous aftershock focal mechanisms

Abstract

We estimated the regional stress field before and after the 2000 Western Tottori Earthquake (Mw = 6.6) from numerous focal mechanisms. To constrain the stress field prior to the main shock, we compared the spatial distribution of the static stress changes generated by the main shock with the stress field following the main shock. In the northern and central parts of the aftershock region, it is inferred that the deviatoric stress magnitude prior to the main shock was too large to be affected by the static stress changes. The direction of the maximum principal stress axis prior to the main shock is consistent with the tectonic stress field. On the other hand, in the southern part of the aftershock area, in and around the region where the main shock slip was large, it is found that the stress field is spatially heterogeneous after the main shock because of the large magnitude of the static stress changes. Around the southern edge of the fault, the spatial distribution of the static stress changes is consistent with that of the P axis azimuths of the aftershocks, and it is inferred that the deviatoric stress magnitude prior to the main shock was small enough to be affected by the static stress changes (5 MPa). The strength of the preexisting aftershock fault planes in the southern edge might be exceptionally weak. It is found that the stress field prior to the main shock was inhomogeneous on scales smaller than the length of the main shock fault.