Shear-wave splitting in a region with newly-activated seismicity after the 2011 Tohoku earthquake

The 2011 Tohoku earthquake off the Pacific coast of Japan was the largest earthquake since the setup of a high-density seismic network in Japan. This network has recorded a change in seismicity patterns after the Tohoku earthquake, with seismicity increasing in a number of areas, including the southeast of the Tohoku region. Here, we present new research into shear-wave splitting during crustal earthquakes within this area of newly-activated seismicity by comparing shear-wave splitting before and after the 2011 event. In the pre-2011 Tohoku earthquake dataset, polarization azimuths within the area of newly-activated seismicity and the region immediately surrounding this area were almost NNE-SSW and ESE-WNW, respectively. Post-2011 Tohoku earthquake data in the same area also records a NNE-SSW polarization azimuth direction. Data with an ESE-WNW polarization azimuth most probably relate to the regional stress field in the area associated with the westward subduction of the Pacific Plate; however, data from the area of seismicity-activated with NNE-SSW polarization azimuths are inconsistent with the expected regional stress field. Our shear-wave splitting analysis suggests that the orientation of the maximum stress axis in the area that underwent activation in seismicity did not change significantly after the 2011 Tohoku earthquake.