[J08-3] Imaging and interpreting lithospheric structures using seismic and geodetic approaches III

[J08-3-02] Geometry and spatial variations of seismic reflection intensity of the upper surface of the Philippine Sea plate off the Boso Peninsula, Japan

^○Akihiro Kono¹, Toshinori Sato¹, Masanao Shinohara², Kimihiro Mochizuki², Tomoaki Yamada², Kenji Uehira³, Takashi Shimbo³, Yuya Machida⁴, Ryota Hino⁵, Ryosuke Azuma⁵ (1.Graduate School of Science, Chiba University, Chiba, Japan, 2.Earthquake Research Institute of Tokyo University, Tokyo, Japan, 3.National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan, 4.Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan, 5.Graduate School of Science, Tohoku University, Sendai, Japan)

In the region off the Boso Peninsula (BP), Japan, the Pacific plate is subducting westward beneath both the Honshu island arc (HIA) and Philippine Sea plate (PHS), while the PHS is subducting northwestward under the HIA. These complex tectonic interactions have caused numerous seismic events such as the Boso Slow Slip Events (SSEs). To better understand these seismic events, it is important to determine the structure in this region.
Although some seismic surveys have been conducted off the BP, still further work is needed for farther offshore. We conducted a marine seismic experiment off the BP, from July to August 2009. Airgun shooting was conducted along 4 survey lines, and 27 Ocean Bottom Seismometers (OBSs) in total were deployed.
We estimated 2-D and 3-D P-wave velocity structure from the airgun data using the PMDM (Progressive Model Development Method; Sato and Kenett, 2000) and the FAST (Zelt and Barton, 1998). Within our survey data, we recognized numerous reflections and estimated its depth by conducting the Traveltime mapping method (Fujie et al. 2006) (TTMAP) in both the on-line area and off-line area.
From the 2-D velocity model, we found that the dip of the upper surface of the PHS is shallow far off the BP. The TTMAP results in the 2-D models indicate that an area of high reflection intensity corresponds to the main slip area of the Boso SSEs. The TTMAP in the 3-D model is still under analysis.
Acknowledgement
The marine seismic experiment was conducted by R/V Hakuhou-maru of Japan Agency for Marine-Earth Science and Technology, and the OBSs were retrieved by Shincho-maru of Fukada salvage co. Ltd..We would like to thank captains and the crew of Hakuho-maru and Shincho-maru. This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions, and from the Grants in Aid for Scientific Research (25287109).