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Earth Planets Space, Vol. 64 (No. 12), pp.1149-1156, 2012
doi:10.5047/eps.2012.06.006

P-wave velocity structure in the southernmost source region of the 2011 Tohoku earthquakes, off the Boso Peninsula, deduced by an ocean bottom seismographic survey

Kazuo Nakahigashi1, Masanao Shinohara1, Kimihiro Mochizuki1, Tomoaki Yamada1, Ryota Hino2, Toshinori Sato3, Kenji Uehira4, Yoshihiro Ito2, Yoshio Murai5, and Toshihiko Kanazawa1

1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
2Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
3Graduate School of Science, Chiba University, Chiba 263-8522, Japan
4Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan
5Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan

(Received February 5, 2012; Revised May 9, 2012; Accepted June 17, 2012; Online published January 28, 2013)

Abstract: We present the result of a seismic experiment conducted using ocean bottom seismometers and controlled sources in the region off Ibaraki and the Boso Peninsula. This region is the southern edge of the rupture zone of the 2011 off the Pacific coast of Tohoku Earthquake. We estimated the P-wave seismic velocity structure beneath the profile using a 2-D ray-tracing method. The crustal structure in the southern area is more heterogeneous than that of the northern area. This heterogeneity is thought to be related with subducting the Philippine Sea plate (PHS). The plate boundary between the landward plate and the Pacific plate (PAC) is positioned at depths of 20 km at a distance of 170 km from the southern end of the profile. The subducting PHS is imaged on the southern part of the profile. However, we could not obtain a distinct image of the contact zone of PHS and PAC. The contact zone of PHS and PAC is estimated to have a large heterogeneity resulting from strong deformation due to the collision of the two plates. We infer that the termination of the rupture, and the large afterslip in the collision region, are caused by this strong heterogeneity.
Key words: Crustal structure, Philippine Sea plate, Pacific plate, The 2011 off the Pacific coast of Tohoku Earthquake, ocean bottom seismometer (OBS), plate boundary.


Corresponding author E-mail: kazuo@eri.u-tokyo.ac.jp


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