Earth Planets Space, Vol. 63 (No. 7), pp. 749-754, 2011
Tomomi Okada1, Keisuke Yoshida1, Sadato Ueki1, Junichi Nakajima1, Naoki Uchida1, Toru Matsuzawa1, Norihito Umino1, Akira Hasegawa1, and Group for the aftershock observations of the 2011 off the Pacific coast of Tohoku Earthquake2
1Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate
School of Science, Tohoku University, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
2The members of the group are listed at the end of this paper.
(Received April 26, 2011; Revised June 16, 2011; Accepted June 17, 2011; Online published September 27, 2011)
Shallow seismic activity in the crust of the overriding plate west of the source area increased significantly after the 2011 Mw 9.0 Tohoku earthquake which ruptured the plate boundary to the east off northern Japan beneath the Pacific Ocean. In order to understand the cause of this distinctive change in seismicity, we have precisely relocated earthquake hypocenters for several earthquake sequences that occurred during the period March 11-April 6, 2011, following the Tohoku earthquake by the double-difference method. Hypocenter distributions were used to discriminate the fault plane from the auxiliary plane of the focal mechanisms for those earthquake sequences. We then calculated the Coulomb stress change on those fault planes caused by the 2011 Mw 9 earthquake. In all cases, the estimated Coulomb stress changes at the plausible fault planes for those post-mainshock sequences are positive. The positive Coulomb stress change is mainly due to the reduction of normal stress on the fault plane of the earthquake sequences by the large, low-angle thrust fault responsible for the 2011 Mw 9 earthquake. The present observations suggest that static stress transfer possibly triggered those post-mainshock earthquake sequences.
Key words: Triggered earthquake, focal mechanism, hypocenter distribution, Coulomb stress change.