Three-dimensional seismic velocity structure beneath Japanese Islands from Sea of Japan to Pacific Ocean including NIED S-net, Hi-net (MOWLAS) data

Abstract:

1. Introduction

We study the three-dimensional (3D) seismic velocity structure of the Japanese Islands including beneath the Sea of Japan and Pacific Ocean. National Research Institute for Earth Science and Disaster Resilience (NIED) operates the high-sensitivity seismograph network (Hi-net) after the Kobe Earthquake since 2000, the seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net) after the 2011 Tohoku-oki Earthquake and Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) originally constructed by JAMSTEC since 2016 part of MOWLAS. We conduct the seismic tomography for arrival time data detected by these networks operated by NIED as well as other organizations.

2. Data and method

The target region, 20-48°N and 120-148°E, covers the whole Japanese Islands. We used P- and S-wave arrival times for 32952 earthquakes recorded at about 1500 stations including the NIED S-net and DONET from April 2016 to June 2018 and those for events near the coast of the Sea of Japan detected at the stations operated by universities from 2000 to 2004 picked by Integrated Research Project on Seismic and Tsunami Hazards Around the Sea of Japan supported by MEXT as well as those used by Matsubara et al. (2017). Totally 6356481 P- and 3534482 S-arrival times from 112631 events are available.

3. Result and discussion

We constructed the seismic velocity model beneath the Sea of Japan at depths of 10-20 km in the northern and central coastal area. P-wave velocity beneath the Sea of Japan is high at depths of 10-35 km, however, that along the coast of Sea of Japan in western Japan is moderate. The mid-Tertiary break up and formation of the Sea of Japan affected the lithospheric velocity structure in this region. The reactivation of the younger compression has developed the tsunamigenic source faults. The information of the lithospheric structure provides the essential information to the structure of faults.

The S-net data improved the velocity structure of the overlying plate beneath Pacific Ocean. The probable Mesozoic rift structure trending NS from the coast of Tohoku to the west of Hidaka collision zone is an important feature. The 2018 Eastern Iburi earthquake (M6.7) at the high-Vp upper mantle may be related to the reactivation of the rift related structure in the upper mantle to the lower crust.