Earth Planets Space, Vol. 63 (No. 7), pp. 669-673, 2011
Youichi Asano1, Tatsuhiko Saito1, Yoshihiro Ito2, Katsuhiko Shiomi1, Hitoshi Hirose1, Takumi Matsumoto1, Shin Aoi1, Sadaki Hori1, and Shoji Sekiguchi1
1National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba 305-0006, Japan
2Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science,Tohoku University, 6-6 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan
(Received April 15, 2011; Revised June 9, 2011; Accepted June 10, 2011; Online published September 27, 2011)
We estimated centroid moment tensors of earthquakes that occurred from 2003 to 2011 in and around the focal area of the 2011 Mw 9.0 megathrust earthquake in eastern Japan. The result indicates that earthquakes occurring before the mainshock, which included foreshocks off Miyagi, were basically interplate earthquakes with thrust-type focal mechanisms. On the other hand, the aftershocks exhibited a variety of focal mechanisms. Interplate aftershocks with thrust focal mechanisms did not occur within the large coseismic slip area estimated from GPS data but instead occurred in the surrounding regions. This implies that slip could no longer occur in the coseismic slip area due to the large amount of stress release during the mainshock rupture, whereas the aftershocks in the surrounding regions were caused by a stress concentration in these regions due to the large co-seismic slip associated with the mainshock asperity. Normal-fault-type aftershocks were widely distributed in the overriding plate and the outer-rise of the Pacific Plate. These aftershocks may have been due to a tensional stress change caused by the coseismic slip. Thrust-fault-type aftershocks in the subducting Pacific Plate were also interpreted as being due to compressional stress change as a result of the coseismic slip.
Key words: Foreshock, aftershock, centroid moment tensor, focal mechanism.