TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 64 (No. 12), pp. 1137-1148, 2012

Precise aftershock distribution of the 2011 off the Pacific coast of Tohoku Earthquake revealed by an ocean-bottom seismometer network

Masanao Shinohara1, Yuya Machida1, Tomoaki Yamada1, Kazuo Nakahigashi1, Takashi Shinbo1, Kimihiro Mochizuki1, Yoshio Murai2, Ryota Hino3, Yoshihiro Ito3, Toshinori Sato4, Hajime Shiobara1, Kenji Uehira5, Hiroshi Yakiwara6, Koichiro Obana7, Narumi Takahashi7, Shuichi Kodaira7, Kenji Hirata8, Hiroaki Tsushima8, and Takaya Iwasaki1

1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
2Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
3Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
4Graduate School of Science, Chiba University, Chiba 263-8522, Japan
5Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan
6Faculty of Science, Kagoshima University, Kagoshima 892-0871, Japan
7Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
8Meteorological Research Institute, Japan Meteorological Agency, Tsukuba 305-0052, Japan

(Received February 3, 2012; Revised August 25, 2012; Accepted September 5, 2012; Online published January 28, 2013)

Abstract: The 2011 off the Pacific coast of Tohoku Earthquake occurred at the plate boundary between the Pacific plate and the landward plate on March 11, 2011, and had a magnitude of 9. Many aftershocks occurred following the mainshock. Obtaining a precise aftershock distribution is important for understanding the mechanism of earthquake generation. In order to study the aftershock activity of this event, we carried out extensive sea-floor aftershock observations using more than 100 ocean-bottom seismometers just after the mainshock. A precise aftershock distribution for approximately three months over the whole source area was obtained from the observations. The aftershocks form a plane dipping landward over the whole area, nevertheless the epicenter distribution is not uniform. Comparing seismic velocity structures, there is no aftershock along the plate boundary where a large slip during the mainshock is estimated. Activity of aftershocks in the landward plate in the source region was high and normal fault-type, and strike-slip-type, mechanisms are dominant. Within the subducting oceanic plate, most earthquakes have also a normal fault-type, or strike-slip-type, mechanism. The stress fields in and around the source region change as a result of the mainshock.
Key words: The 2011 off the Pacific coast of Tohoku Earthquake, aftershock, focal mechanism, seismicity, ocean-bottom seismometer (OBS).

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

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