Two subevents across the Japan Trench during the 7 December 2012 off Tohoku earthquake (Mw 7.3) inferred from offshore tsunami records

Abstract

A tsunamigenic earthquake with a moment magnitude of 7.3 occurred near the Japan Trench, off Tohoku, northeast Japan, on 7 December 2012. Operational seismic monitoring inferred that the earthquake was composed of doublet sources of comparable magnitudes: the first event was reverse faulting and the second event, which occurred 10-20 s later, was normal faulting. An associated tsunami was observed at offshore stations, having an amplitude of 10-1 to 101 cm. Inverse modeling using the observed tsunami records reveals possible vertical seafloor deformations. We can then constrain, in terms of the source location, that the tsunamigenic earthquake involves two subfaults across the Japan Trench: one is the outer trench reverse faulting as the first event and the second is the inner trench normal faulting. The present study shows that tsunami analysis with seismograph analyses effectively constrains the epicenter locations, even in the case of complex earthquakes like doublets. Key Points Small but high S/N tsunami detected by widely distributed offshore observations Inverting tsunami source using the data and dispersive modeling Discussion on the doublet triggering based on the constrained source location © 2014. American Geophysical Union. All Rights Reserved.