Earth Planets Space, Vol. 63 (No. 7), pp. 559-564, 2011
Guangfu Shao1,2, Xiangyu Li1,2, Chen Ji1,2, and Takahiro Maeda3
1Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
2Earth Research Institute, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
3National Research Institute for Earth Science and Disaster Prevention, Tsukuba 305-0006, Japan
(Received April 12, 2011; Revised June 16, 2011; Accepted June 17, 2011; Online published September 27, 2011)
Focal mechanism and slip history of the 2011 Mw 9.1 off the Pacific coast of Tohoku, Japan earthquake were derived rapidly from teleseismic body and surface waves. Multiple double couples (MDC) analysis was first conducted using 1-hour long period seismic waves, yielding a single double couple with a seismic moment of 5.06 × 1022 N m, whose low angle nodal plane orients 199° and dips 10° west. Spatiotemporal rupture history was then constrained using both broadband body waves and long period seismic waves. The solution was updated twice in three days. Our preferred model, which based on the MDC fault plane, local JMA hypocenter and calibrated alignments of body and surface waves, revealed a complex rupture process. The rupture initiated slowly at a depth of 23 km. It first propagated in downdip and bilaterally directions along the subduction interface for 45 s and then broke a 80 km by 250 km near trench asperity in the up-dip direction and produced up to 60 m slip. The fault plane below the hypocenter had two more subevents, occurring in 70-95 s and 110-160 s, respectively. The total seismic moment was 5.8 × 1022 N m. The abnormal high slip near the trench suggests a recurrence interval larger than 500 yr.
Key words: Multiple double couples analysis, finite fault rupture, near trench asperity.