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.
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The analyses conducted in this study are based on the earthquake source information and phase arrival times that are available from the Japan Meteorological Agency earthquake catalog (http://www.data.jma.go.jp/ svd/eqev/data/bulletin/deck_e.html). We thank Professor Keisuke Yoshida for the data of principal stress field. We are grateful to Lucy Flesch (Editor) and two anonymous reviewers for valuable review comments that improved the presentation of this manuscript. This work was supported by the Korea Meteorological Administration Research and Development Program under grant KMI2018-02910. Additionally, this research was partly supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), which is funded by the Ministry of Education (NRF-2017R1A6A1A07015374).
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)