Estimation of seismic velocity structure beneath the ocean (including Japan Sea) as well as beneath the Japanese Islands using offshore events with NIED Hi-net pick data and NIED F-net focal depth

1. Introduction We investigate the 3D seismic velocity structure beneath the ocean using offshore events with seismic tomographic method. We used the combination of the hypocenter catalog by NIED F-net and pick data by NIED Hi-net for offshore events as well as the hypocenter catalog and pick data by NIED Hi-net for events within the seismic network. We also investigate the effect of 125 seismic stations of seafloor observation network for earthquake and tsunamis along the Japan Trench (S-net) operated by NIED since 2016. We conduct the checkerboard test with the assumption of data at S-net stations. 2. Data and method The target region, 20-48°N and 120-148°E, covers the Japanese Islands. A total of manually picked 4693781 P-wave and 2342621 S-wave arrival times for 796779 earthquakes recorded at approximately 1300 stations from October 2000 to December 2014 is available for use in the tomographic method. The inversion reduces the RMS of the P-wave traveltime residual from 0.507 s to 0.193 s and that of the S-wave data from 0.609 s to 0.238 s after eight iterations. We assume the body waves from the earthquake beneath the Pacific Ocean with magnitude larger than 3.5 are detected at 125 S-net stations. We conducted the checkerboard resolution test with the synthetic data. 3. Result and discussion Our new analysis revealed the failed rift structures, marked by higher Vp at lower crust and lower Vp at the upper crust, along the Japan Sea cost of central Honshu. Judging the age of the rift fill, they were produced during the formation of the Japan Sea. Similar, rifting structure also detected along the Pacific coast of Northern Honhsu, It is marked by uplifted Moho surface, trending NS to NNW-SSE direction. Geologically, it is interpreted as a rift structure formed in the late Mesozoic. However, the tectonic meaning of this structure is still poorly understood. The new tomographic result using S-net data makes us possible to understand the tectonic framework of the Mesozoic rifting off Tohoku. The checkerboard resolution test with the assumed S-net data has a well-resolved at depths of 10-20 km beneath the Pacific Ocean between Honshu and Japan Trench. Ray paths from offshore events and the S-net stations pass through the upper crust beneath the ocean. We may obtain the fine 3D structure after the accumulation of the data at S-net stations.