Japan Geoscience Union Meeting 2018

Presentation information

[EJ] Evening Poster

S (Solid Earth Sciences) » S-CG Complex & General

[S-CG59] Structure and evolution of Japanese islands - Formation of island arc systems and earthquake cycles

Tue. May 22, 2018 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Hiroshi Sato(Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo), Masanao Shinohara(Earthquake Research Institute, University of Tokyo), Masahiro Ishikawa(横浜国立大学大学院環境情報研究院, 共同), Makoto MATSUBARA(National Research Institute for Earth Science and Disaster Resilience)

[SCG59-P02] Spatial distribution of reflection intensity on 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 Shimbo4, Yuya Machida4, Ryota Hino5, Ryosuke Azuma5 (1.Graduate School of Science, Chiba University, 2.Earthquake Research Institute of Tokyo University, 3.NIED, 4.JAMSTEC, 5.Graduate School of Science, Tohoku University)

Keywords:Philippine Sea Plate, Traveltime mapping, Ocean Bottom Seismometers , 3D P-wave velocity structure, Boso Slow Slip Events, Izu-Bonin-Mariana arc

In the region off the Boso Peninsula, Japan, is known to have complex structure its underneath. The Pacific plate (PAC) is subducting westward beneath the landward plate from the Japan trench, while the Philippine Sea plate (PHS) is subducting northwestward under the landward plate from the Sagami trough. These complex tectonic interactions have caused various seismic events such as the Boso Slow Slip Events (SSEs). To better understand these seismic events, it is important to determine the structure under this region.

In May 2017, we published a 2D P-wave velocity structure under the survey area, showing geometry of the upper surface of PHS (UPHS) and reflection intensity variation along the UPHS. From our result and previous studies, relatively strong reflection from the UPHS can be observed near the main slip area of Boso SSEs. The reflective area may be related to the Boso SSEs, however, it is still insufficient to link both only from the 2D models. Therefore, further work is needed in order to reveal spatial distribution of the relatively strong reflection area.

From July to August 2009, we conducted a marine seismic experiment using airguns as sources off the southeast coast of the Boso Peninsula. Airguns were shot along the 4 survey lines and 27 Ocean Bottom Seismometers (OBSs) were deployed in the survey area. We used 18 OBSs to determine 3D P-wave velocity structure, which has 150(x)×90(y)×30 (z) km size.

We estimated 3D velocity structure from airgun data recorded in the OBSs by using the FAST (First Arrival Seismic Tomography; Zelt and Barton, 1998). We also picked the reflection traveltimes likely reflected from the UPHS and applied them to the Traveltime mapping method (Fujie et al. 2006) to estimate spatial locations of the reflectors.

As a result, the reflections from the UPHS seem to be concentrated in the western and the eastern part of the survey area. The western reflective area is close to the main slip area of the Boso SSEs, which may suggest the presence of a thin low velocity layer along the UPHS in the area. The eastern reflective area seems to overlap with the local high velocity structure seen in our 3D velocity structure. The high velocity structure partially overlaps with the northern extent of relatively high velocity structure seen in the forearc region of the Izu-Bonin subduction zone.