Shear Wave Splitting Observed in the Southwestern Part of Fukushima Prefecture, Northeastern Japan

We have investigated shear wave polarization anisotropy in the southwestern part of Fukushima Prefecture, northeastern Japan. Waveform data we analyzed are obtained by a local seismic networks of Tohoku University and Utsunomiya University. We used a cross correlation method to detect the shear wave splitting. For the shallow events beneath the southwestern part of Fukushima Prefecture, observed that fast shear wave oscillation directions (FSODs) are oriented to the NW-SE. The observed delay times are less than 0.1s and tend to have large values in high seismicity regions. For the intermediate-depth events, observed FSODs are oriented parallel to the dip direction of the subducted slab and most of the observed delay times are less than 1 s. We estimated the location and the cause of anisotropy based on these observations. In the upper crust beneath the southwestern part of Fukushima Prefecture anisotropy is estimated to be caused mainly by preferredly oriented cracks controlled by the tectonic stress.