Regional Coseismic and Postseismic Upper-crustal Deformation Deduced from VP/VS Changes after the 2011 Tohoku-Oki Megathrust Earthquake

The 2011 M_W 9.0 Tohoku-Oki megathrust earthquake perturbed medium properties in Japanese islands by lithospheric displacements. We investigated the medium perturbation in the upper crust of Japanese islands after the megathrust earthquake from temporal changes in V_P/V_S ratios that are measured by azimuth. We could infer the physical state of the upper crust after the megathrust earthquake. We observed V_P/V_S changes between -0.0458 (±0.0012) and 0.0422 (±0.0033) in the Japanese islands. The medium-property changes appeared to recover to the original state in a couple of years to a decade. The upper crust presents complicated deformation in short distances from the epicenter. The peak deformation is observed in the radial or tangential directions to the megathrust earthquake in long distances. The V_P/V_S changes display a 2θ azimuthal variation, suggesting azimuthal-anisotropic deformation. This anisotropic feature is attributed to the crack alignment caused by lithospheric displacement. The observed V_P/V_S changes present the discriminative crustal responses and medium perturbation for the megathrust earthquake.