Earth Planets Space, Vol. 63 (No. 7), pp. 841-845, 2011
Tomoaki Yamada1, Kazuo Nakahigashi1, Asako Kuwano1, Kimihiro Mochizuki1, Shin'ichi Sakai1, Masanao Shinohara1, Ryota Hino2, Yoshio Murai3, Tetsuo Takanami3, and Toshihiko Kanazawa1
1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
2Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
3Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
(Received April 22, 2011; Revised June 22, 2011; Accepted June 23, 2011; Online published September 27, 2011)
The 2011 Tohoku earthquake hit the forearc region of the Japan Trench on March 11, 2011. The rupture zone seemed to reach off the coast of the Kanto region. We had conducted ocean bottom seismographic observations off the coast of the Kanto in 2008 and estimated 851 hypocenter locations around the south part of the 2011 off the Pacific coast of Tohoku Earthquake by using over 50 ocean bottom seismometers (OBSs) and routine data jointly. The hypocenters distributed some clusters, and we found a few seismic gaps at the boundary zones of the clusters. The most remarkable seismic gap was positioned at the edge of intervened Philippine Sea plate (PHS) between the North American plate (NA) and Pacific plate (PAC). We compare the epicenter distributions with the 2011 aftershock distribution determined by routine data. The aftershocks are segmented spatially and there are some seismic gaps among the segments. The remarkable low places are consistent with the boundary zones of each cluster we estimated from the 2008 data. We infer that those regions have strong heterogeneity resulting from strong deformations caused by various subduction processes, such as intervening PHS between NA and PAC, seamount chains and changes in physical properties.
Key words: Seismic gap, Philippine Sea plate, Pacific plate, subduction, Japan Trench, the 2011 off the Pacific coast of Tohoku Earthquake.