Mapping the Tohoku forearc: Implications for the mechanism of the 2011 East Japan earthquake (Mw 9.0)

To investigate the generation mechanism of the 2011 great East Japan (GEJ) earthquake (M 9.0), we determined a three-dimensional (3-D) seismic model under the NE Japan forearc region using a large number of P and S wave arrival times from local earthquakes. Our results show that the GEJ main-shock occurred in a high-velocity (Vp, Vs) zone with higher Poisson's ratio (σ). The aftershocks relocated with a master-event location (MEL) procedure indicate that most of them are located at the corner of the mantle wedge along the upper boundary of the subducting Pacific slab, and their focal depths are slightly shallower than the background seismicity. The tomographic images and spatial distribution of the aftershocks imply strong interplate coupling (asperity) in the main-shock area and weak coupling in the surrounding areas of the megathrust zone. We think that fluids in the crust and uppermost mantle in the subduction zone could have affected the earthquake generation through the physical role of fluid pressure and a variety of chemical effects. We conclude that the 2011 GEJ earthquake initiation and the rupture processes of the aftershock sequence were influenced by fluid extrusion into the rupture zone due to the dehydration of the subducting Pacific slab. Fluid-bearing structural heterogeneities with thermal-petrologic variations in the megathrust zone played a key role in the initiation of the 2011 GEJ earthquake and its aftershocks.