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

• Published in: Geophysical research letters Vol. 45; no. 23; pp. 12,793 - 12,803
• Main Authors: Kim, Ilgoo, Hong, Tae‐Kyung
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.12.2018
• Subjects: Anisotropy; Azimuth; azimuthal seismic anisotropy; Deformation; Deformation mechanisms; Earthquakes; Geological hazards; Hazard mitigation; Islands; Japanese islands; medium perturbation; megathrust earthquake; Mitigation; Orientation; P-waves; Ratios; Seismic activity; Seismic hazard; Seismic waves; Stress distribution; temporal change; upper crust
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL080742

We investigated VP/VS changes in the upper crust of the Japanese islands after the 2011 MW9.0 Tohoku‐Oki megathrust earthquake. Abrupt VP/VS changes with azimuth‐dependent variations were observed after the megathrust earthquake. The VP/VS changes ranged between −0.0458 (±0.0012) and 0.0422 (±0.0033). Large localized VP/VS changes over regional distances suggested medium‐dependent deformation. Peak VP/VS changes were observed along paths subparallel or subperpendicular to the directions toward the megathrust earthquake. The VP/VS changes displayed characteristic 2θ variations as a function of the azimuth difference, suggesting azimuthal seismic anisotropy. The VP/VS ratio recovered gradually over time. Some regions presented permanent VP/VS 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. Plain Language Summary We report a unique observation of azimuthal seismic anisotropy in the upper crust of Japanese islands, which was induced by the 2011 M9.0 Tohoku‐Oki megathrust earthquake. The azimuthal seismic anisotropy was caused by the direction‐dependent discriminative lithospheric displacement during the megathrust earthquake. We analyzed the traveltimes of local seismic waves and estimated the changes in Vp/Vs ratios after the megathrust earthquake. We observed the abrupt Vp/Vs changes after the megathrust earthquake. The Vp/Vs ratios recovered gradually with time for several years. This study may provide important information on the medium property changes after the megathrust earthquake and will be useful for mitigation of potential seismic hazards. Key Points Abrupt VP/VS changes occurred in the upper crust of the Japanese islands after the 2011 Tohoku‐Oki megathrust earthquake Peak VP/VS changes developed along the paths subparallel or subperpendicular to the megathrust Azimuthal seismic anisotropy developed due to the lithospheric displacements caused by the megathrust earthquake