Focal mechanism and slip history of the 2011 M w 9.1 off the Pacific coast of Tohoku Earthquake, constrained with teleseismic body and surface waves

Abstract

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.