![]() Earlier studies indicated that both the polarization directions and the splitting time change after large earthquakes. Moreover, using a cluster of earthquakes, one can estimate the vertical variation of anisotropy. One can estimate the fast and slow shear-wave polarization directions and the delay time between the fast and slow shear waves, which are mean values along a ray path. Conventionally, shear-wave splitting is estimated by the cross-correlation method. In this study, we estimate the change in near-surface polarization anisotropy by applying seismic interferometry to seismograms recorded by KiK-net, a strong-motion recording network operated by the National Research Institute for Earth Science and Disaster Prevention (NIED). The earthquake, among the largest in recent history, resulted in a reduction in the near-surface velocity averaged over two months following the earthquake of about 5% throughout northeastern Japan (a region 1,200 km wide). The change in near-surface shear-wave velocity caused by the M W9.0 Tohoku-Oki earthquake on 11 March 2011 is documented by Nakata and Snieder. The change in the largest principal stress direction weakly correlates with the change in anisotropy. The comparison of the spatial distribution between changes in anisotropy and velocity indicates the changes in anisotropy and velocity are generally correlated, especially in the northeastern Honshu (the main island in Japan). Through monitoring of anisotropy and shear-wave velocity, we find that the changes in anisotropy and velocity partially recover with time they are, however, still different from the pre-event values after nine months. We show that the earthquake increased the difference between fast and slow shear-wave velocities arising from shear-wave splitting in most parts of northeastern Japan, but it did not significantly change fast shear-wave polarization directions in the near surface. We apply seismic interferometry to strong-motion records to detect the near-surface (i.e., an upper few hundred meters deep) change in anisotropy caused by the M W9.0 Tohoku-Oki earthquake on 11 March 2011. ![]()
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