Time-lapse change in anisotropy in Japan's near surface after the 2011 Tohoku-Oki earthquake

摘要

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 _W 9.0 Tohoku-Oki earthquake on 11 March 2011. 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. 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. 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). The change in the largest principal stress direction weakly correlates with the change in anisotropy.