Two faulting of the December 7, 2012 earthquake off Tohoku inferred from offshore tsunami records

A large earthquake (M7) that occurred off Tohoku, Japan, on 7 December 2012 generated a tsunami. The tsunami was observed at several offshore sites, and its amplitude was millimeters to ten centimeters. The observed records show clear dispersion in at least two directions of southeast and northeast from the source. Analyzing the observed records with considering the directional dependence of the dispersive wave, we estimated the initial tsunami height distribution as the tsunami source. The inverse modeling of the tsunami source suggests that the earthquake involves two faulting close to the Japan Trench: One is a reverse faulting at the outer rise, and the other is a normal faulting at the landward trench. The inferred geometry/location of the seafloor displacement due to these faulting has an error margin of several kilometers, which is significantly smaller than those obtained by seismogram analyses (a few tens of kilometers). Such precise location of the coseismic seafloor displacement by the tsunami waveform analysis would be helpful for a better estimation of stress contribution between neighboring multiple earthquakes.