Time-lapse change in anisotropy in Japan’s near surface caused by the 2011 Tohoku-Oki earthquake

We apply seismic interferometry to strong-motion records to detect the change in anisotropy caused by the MW 9.0 Tohoku-Oki earthquake on 11 March 2011. It is known that large earthquakes change fluid condition in cracks, and thereby the polarization-anisotropy condition. The Tohoku-Oki earthquake increased the difference between fast and slow shear-wave velocities arising from shearwave splitting in most parts of northeastern Japan, but it did not significantly change polarization directions of the fast shear waves in the upper few hundred meters. The difference between fast and slow velocities increased gradually for more than one month after the main shock, whereas the decrease in velocity occurred suddenly after the event. Through monitoring of anisotropy and shearwave velocity, we find that the changes in anisotropy and velocity recover with time. The change in anisotropy are correlated with the change in velocity in the northeastern Honshu. Also, the change in the largest principal stress direction weakly correlates with that in anisotropy.