Earth Planets Space, Vol. 63 (No. 7), pp. 835-840, 2011
Masanao Shinohara1, Tomoaki Yamada1, Kazuo Nakahigashi1, Shin'ichi Sakai1, Kimihiro Mochizuki1, Kenji Uehira2, Yoshihiro Ito3, Ryusuke Azuma4, Yuka Kaiho5, Tetsuo No5, Hajime Shiobara1, Ryota Hino3, Yoshio Murai4, Hiroshi Yakiwara6, Toshinori Sato7, Yuya Machida1, Takashi Shinbo1, Takehi Isse1, Hiroki Miyamachi6, Koichiro Obana5, Narumi Takahashi5, Shuichi Kodaira5, Yoshiyuki Kaneda5, Kenji Hirata8, Sumio Yoshikawa8, Kazushige Obara1, Takaya Iwasaki1, and Naoshi Hirata1
1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
2Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan
3Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
4Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
5Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
6Faculty of Science, Kagoshima University, Kagoshima 892-0871, Japan
7Graduate School of Science, Chiba University, Chiba 263-8522, Japan
8Meteorological Research Institute, Japan Meteorological Agency, Tsukuba 305-0052, Japan
(Received April 14, 2011; Revised May 13, 2011; Accepted May 19, 2011; Online published September 27, 2011)
The 2011 off the Pacific coast of Tohoku Earthquake occurred offshore of northeast Japan region on March 11th, 2011. In order to study the aftershock activity of this event, we started deployment of seventy-two ocean bottom seismometers (OBSs) four days after the mainshock. In the south of the source region, thirty-four long-term OBSs (LT-OBSs) had been deployed before the occurrence of the mainshock, and we recovered three LT-OBSs to clarify the depth distribution of aftershocks. Using the data of OBSs, ninety-nine aftershocks were located. Most of the aftershocks were located in a depth range of 5-30 km and concentrate in the plate boundary region. In addition, aftershocks occurred within the subducting oceanic crust and the 6.2-km/s layer of the landward plate. No aftershocks were found in the mantle of the subducting plate. From the results of a previous seismic survey using OBSs and controlled sources, the subducting Philippine Sea plate is estimated to be in contact with the subducting Pacific plate. The southern end of the seismic activity region of the aftershocks corresponds to the contact region of two subducting plates. We infer that the rupture of the mainshock sequence was terminated at the oceanic plate contact region.
Key words: The 2011 off the Pacific coast of Tohoku Earthquake, aftershock, subduction, ocean bottom seismometer (OBS).