P-wave tomography, anisotropy and seismotectonics in the eastern margin of Japan Sea

To understand the seismotectonics in the eastern margin of the Japan Sea, we determined the first high-resolution 3-D P-wave velocity structure and azimuthal anisotropy under the Japan Sea off Northeast Japan using 175,425 high-precision P-wave arrival times from 2833 local earthquakes recorded by 330 seismograph stations. P-wave arrival times from 145 suboceanic earthquakes relocated with sP depth phase are crucial to determine the structure of the crust and uppermost mantle under the Japan Sea. Our results show that strong velocity variations exist in the crust and uppermost mantle under the eastern margin of the Japan Sea. Many large crustal earthquakes occurred in or around low-velocity zones which may represent weak sections of the seismogenic crust. The P-wave azimuthal anisotropy is complex under the Japan Sea, which may also indicate the complex crustal structures there. In the eastern margin of the Japan Sea, the strong heterogeneities in the crust and upper mantle revealed by seismic tomography may reflect the complicated geologic structures such as the alternate rift zones, ridges, basins, horsts, grabens, volcanics, and continental fragments which were produced during the back-arc spreading, opening of the Japan Sea and the present compressional stage of the Honshu arc.