Azimuthal anisotropy of Rayleigh waves in the crust in southern Tohoku area, Japan

To reveal seismic anisotropy within the crust in the southern Tohoku area, Japan, we analyze the azimuthal anisotropy of Rayleigh waves in the frequency range of 40-65mHz. Rayleigh wave anisotropy is estimated by array analysis of 102 teleseismic events that occurred between 2007 and 2014. The frequency variation of the estimated azimuthal anisotropy indicates depth variation of the seismic anisotropy within the crust: (1) north-south fast velocity direction around the upper crust and (2) trench-normal fast velocity direction in the deeper crust. We also conduct a time-lapse analysis of Rayleigh wave anisotropy to demonstrate the stability of the estimated anisotropy in time and to estimate possible changes in seismic anisotropy associated with the 2011 Tohoku-oki earthquake. We demonstrate that the azimuthal location of teleseismic events influences the accuracy of time-lapse analysis. Moreover, a decrease of azimuthal anisotropic ratio in time-lapse results might be related to changes in the velocity structure associated with release of crustal stress during the Tohoku-oki earthquake.