Multi-fault Rupture and Dynamic Triggering During the 2012 Mw 8.6 Sumatra Earthquake

Abstract

On April 11, 2012, Mw 8.6 strike-slip earthquake struck the northern Sumatra offshore region and became the largest intraplate event ever recorded there. Preliminary study results show a remarkably complex rupture pattern, on a fault system manifested by north–south trending paleotransform faults and east–west striking abyssal hill fabric. To image the rupture pattern of the earthquake, we back-projected teleseismic P wave recordings observed at three regional seismic networks (EU, AU, and F-net). Our results indicate that the earthquake ruptured on a conjugate fault system, composed of two subparallel WNW-ESE trending faults, and a NNE-SSW striking fault in between, for duration of at least 120 s. The rupture started at the WNW end of the first WNW-ESE trending fault located at the east of the conjugate fault system and propagated toward ESE. It then jumped to the second NNE-SSW trending fault located west of the first fault, and first extended SSWward and 15 s later NNEward. After its end on the second fault, the rupture jumped onto the third WNW-ESE trending fault and continued WNWward. The static Coulomb stress changes calculated from the rupture of the first and then the second faults, based on simplified uniform slip models, suggest that initiation of the rupture on the second fault could be statically and dynamically triggered by the first fault. However, the third fault was primarily dynamically triggered by S or even Love waves radiated from the southern branch of the second fault.