Azimuthal Seismic Anisotropy in the Upper Crust of the Japanese Islands Induced by the 2011 Tohoku-Oki Megathrust Earthquake

We investigated V _P /V _S changes in the upper crust of the Japanese islands after the 2011 M _W 9.0 Tohoku-Oki megathrust earthquake. Abrupt V _P /V _S changes with azimuth-dependent variations were observed after the megathrust earthquake. The V _P /V _S changes ranged between −0.0458 (±0.0012) and 0.0422 (±0.0033). Large localized V _P /V _S changes over regional distances suggested medium-dependent deformation. Peak V _P /V _S changes were observed along paths subparallel or subperpendicular to the directions toward the megathrust earthquake. The V _P /V _S changes displayed characteristic 2θ variations as a function of the azimuth difference, suggesting azimuthal seismic anisotropy. The V _P /V _S ratio recovered gradually over time. Some regions presented permanent V _P /V _S changes. The azimuthal seismic anisotropy may have developed from preferential crack orientation as a consequence of the combined effects of lithospheric displacements and a depth-dependent ambient stress field. It appears that recovery of the medium properties may take decades.