Spatially heterogeneous stress field in the source area of the 2011 Mw 6.6 Fukushima-Hamadori earthquake, NE Japan, probably caused by static stress change

Abstract

In order to know whether principal stress orientations in the source area rotated after the 2011 April 11 Mw 6.6 Fukushima-Hamadori earthquake in NE Japan, we investigated detailed spatial distributions of stress orientations for both the pre- and post-main shock periods using a large amount of focal mechanism data. We applied stress tensor inversions to focal mechanism data from Japan's National Research Institute for Earth Science and Disaster Prevention's F-net broadband seismic network and the Japan Meteorological Agency (JMA). The σ3-axes estimated for the pre-main shock period are predominantly oriented WSW–ENE, and are relatively homogeneously in space. In contrast, the orientations of the σ3-axes show a significantly heterogeneous distribution in space for the post-main shock period. In the northern subarea of the focal region, the σ3-axes are oriented NW–SE. In the east and west portions of the central subarea, they are oriented NNW–SSE and WNW–ESE, respectively, almost perpendicular to each other. In the southern subarea, the σ3-axes are oriented WSW–ENE. On the whole, the σ3-axis orientations show concentric circle-like distribution surrounding the large slip area of the MwMw 6.6 main shock rupture. The change of principal stress axis orientations after the earthquake is not significant because of the sparse data set for the pre-main shock period. We calculated static stress changes from the Mw 6.6 main shock and three Mw > 5.5 earthquakes to compare with the observed stress axis orientations in the post-main shock period. The σ3-axis orientations of the calculated total static stress change show a concentric circle-like distribution surrounding the large slip area of the main shock, similar to that noted above. This observation strongly suggests that the spatially heterogeneous stress orientations in the post-main shock period were caused by the static stress change from the Mw 6.6 main shock and other large earthquakes. In order to estimate the differential stress magnitude in the focal area, we calculated deviatoric stress tensors in the post-main shock period by assuming that they are the sum of the deviatoric stress tensors in the pre-main shock period and the static stress changes. Comparison of the calculated and observed stress tensors revealed differential stress magnitudes of 2–30 MPa that explain the observed stress orientations, considering the probable range of estimated stress ratios in the pre-main shock period.