2:45 PM - 3:00 PM
[SCG50-05] Three-dimensional seismic velocity structure along the Sea of Japan with large events derived from seismic tomography for whole Japanese Islands including Reflection seismology data and NIED MOWLAS Hi-net and S-net data
Keywords:seismic tomography, Reflection seismology, Sea of Japan, 2007 Chuetsu-oki Earthquake, 2007 Noto Peninsula Earthquake
1. Introduction
The seismic velocity structure beneath the Japanese Islands is extremely complex since the Japanese Islands consists of the Eurasian (EUR), the North American (NA), and the Philippine Sea (PHS) plates where the PHS and the Pacific plates subduct beneath the EUR and the NA plates. The National Research Institute for Earth Science and Disaster Resilience (NIED), operates the high-sensitivity seismograph network (Hi-net) and the Japan Meteorological Agency (JMA), the National Universities, and other institutes operate seismic networks for the detection of microseismicity. Online ocean bottom seismic networks are operated in some area by NIED, JMA, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Earthquake Research Institute, the University of Tokyo. JAMSTEC started the operation of the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) near the Nankai Trough and NIED started the operation of the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net). DONET was transferred to NIED from April 2016. NIED started operation of Monitoring of Waves on Land and Seafloor (MOWLAS) consisting of Hi-net, the full range seismograph network (F-net), S-net, DONET, Strong-motion Seismograph Networks (K-NET and KiK-net), and Volcano Observation Network (V-net). JAMSTEC conducted the reflection seismology at the Sea of Japan and those signals are observed at the NIED Hi-net stations. We accumulate those signals from 5-9 shots and use for seismic tomography to improve the shallow zone off the coast of Sea of Japan.
2. Data and method
In this study the target region is 20-48°N and 120-148°E. It covers the whole Japanese Islands from Kuril Islands to Ryukyu Islands and Pacific Ocean where the seismic stations of inland Hi-net and offshore the S-net and the DONET are deployed. We used 14,850,442 P- and 12,278,029 S-wave arrival time data from 294,865 natural sources and 11,089 P-wave arrival time data from 482 air gun shots and conducted the seismic tomography.
We applied these data to the grid-type tomographic method (Zhao et al., 1992) with spatial velocity correlation and station corrections to the original code (Matsubara et al., 2004, 2005).
We adopted the one-dimensional (1D) velocity structures for P- and S-waves used in the NIED Hi-net routine as the initial seismic velocity models (Ukawa et al., 1982).
3. Results
We can quite better resolution at depths of 0, 5 and 10 km between Yamagata and Noto Peninsula than that with only natural sources since we added of reflection seismology with an interval of approximately 1 km. We obtain quite low-V zone beneath the Sea of Japan between Noto Peninsula and Sado Island. In case of no air gun data, we cannot image that extremely low-V zone there. This shallow low-V zone is clarified first in this study because of the use of air gun data. The velocity structures with air gun data below the depth of 20 km are almost same as those without the air gun data.
We relocated the all events determined by NIED Hi-net and NIED S-net from October 2000 to December 2019 with magnitudes equal to or larger than 1.5.
The mainshock of the 2019 Off Yamagata earthquake is located at the boundary of northwestern high- and southeastern low-Vp zone between the upper and lower low-Vs zones with low-Vp/Vs zone. Aftershocks are located at the west side of the mainshock with eastward dipping.
The mainshock of the 2007 Chuetsu-oki earthquake is located at southern edge of the high Vp zone with 1.83 Vp/Vs zone. The aftershocks are distributed at the top of the high-V zone with Vp/Vs as 1.80-1.85.
The mainshock of the 2007 Noto Peninsula earthquake is located at the boundary of shallow high-Vp and deeper low-Vp zone and at the boundary of northern low-Vs and southern high-Vs zone within high Vs and low-Vp/Vs zone. Aftershocks are located within the high-Vp and high-Vs and surrounding the low-Vp/Vs zone.
The seismic velocity structure beneath the Japanese Islands is extremely complex since the Japanese Islands consists of the Eurasian (EUR), the North American (NA), and the Philippine Sea (PHS) plates where the PHS and the Pacific plates subduct beneath the EUR and the NA plates. The National Research Institute for Earth Science and Disaster Resilience (NIED), operates the high-sensitivity seismograph network (Hi-net) and the Japan Meteorological Agency (JMA), the National Universities, and other institutes operate seismic networks for the detection of microseismicity. Online ocean bottom seismic networks are operated in some area by NIED, JMA, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Earthquake Research Institute, the University of Tokyo. JAMSTEC started the operation of the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) near the Nankai Trough and NIED started the operation of the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net). DONET was transferred to NIED from April 2016. NIED started operation of Monitoring of Waves on Land and Seafloor (MOWLAS) consisting of Hi-net, the full range seismograph network (F-net), S-net, DONET, Strong-motion Seismograph Networks (K-NET and KiK-net), and Volcano Observation Network (V-net). JAMSTEC conducted the reflection seismology at the Sea of Japan and those signals are observed at the NIED Hi-net stations. We accumulate those signals from 5-9 shots and use for seismic tomography to improve the shallow zone off the coast of Sea of Japan.
2. Data and method
In this study the target region is 20-48°N and 120-148°E. It covers the whole Japanese Islands from Kuril Islands to Ryukyu Islands and Pacific Ocean where the seismic stations of inland Hi-net and offshore the S-net and the DONET are deployed. We used 14,850,442 P- and 12,278,029 S-wave arrival time data from 294,865 natural sources and 11,089 P-wave arrival time data from 482 air gun shots and conducted the seismic tomography.
We applied these data to the grid-type tomographic method (Zhao et al., 1992) with spatial velocity correlation and station corrections to the original code (Matsubara et al., 2004, 2005).
We adopted the one-dimensional (1D) velocity structures for P- and S-waves used in the NIED Hi-net routine as the initial seismic velocity models (Ukawa et al., 1982).
3. Results
We can quite better resolution at depths of 0, 5 and 10 km between Yamagata and Noto Peninsula than that with only natural sources since we added of reflection seismology with an interval of approximately 1 km. We obtain quite low-V zone beneath the Sea of Japan between Noto Peninsula and Sado Island. In case of no air gun data, we cannot image that extremely low-V zone there. This shallow low-V zone is clarified first in this study because of the use of air gun data. The velocity structures with air gun data below the depth of 20 km are almost same as those without the air gun data.
We relocated the all events determined by NIED Hi-net and NIED S-net from October 2000 to December 2019 with magnitudes equal to or larger than 1.5.
The mainshock of the 2019 Off Yamagata earthquake is located at the boundary of northwestern high- and southeastern low-Vp zone between the upper and lower low-Vs zones with low-Vp/Vs zone. Aftershocks are located at the west side of the mainshock with eastward dipping.
The mainshock of the 2007 Chuetsu-oki earthquake is located at southern edge of the high Vp zone with 1.83 Vp/Vs zone. The aftershocks are distributed at the top of the high-V zone with Vp/Vs as 1.80-1.85.
The mainshock of the 2007 Noto Peninsula earthquake is located at the boundary of shallow high-Vp and deeper low-Vp zone and at the boundary of northern low-Vs and southern high-Vs zone within high Vs and low-Vp/Vs zone. Aftershocks are located within the high-Vp and high-Vs and surrounding the low-Vp/Vs zone.