The M9.0 Tohoku-Oki earthquake took place off NE Japan, which is covered by a dense seismic network. In particular, KiK-net observed the strong motions of the main shock and numerous aftershocks both on the ground and at the bottom of boreholes at a depth of a few hundred meters. We applied cross-correlation analysis to the coda part of a seismogram observed by KiK-net and estimated not only the temporal change in shear velocity but also the change in polarization anisotropy between the bottom of the borehole and the ground. The shear velocity was suddenly reduced by 5 to 10% after the Tohoku-Oki earthquake over a wide area of NE Japan, which is consistent with the results of a previous study. Although some stations also reveal a change in polarization anisotropy, the polarization anisotropy change is smaller than the velocity change, which may suggest that the static stress change due to the coseismic slip on the Pacific plate is not large enough to change the direction of polarization anisotropy and/or deviatoric stress in the land area of NE Japan.
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