Interplate channel layer controls of slip during and after the 2011 Tohoku-Oki earthquake

There are remarkable differences between the middle and southern segments of the Japan Trench in terms of the seismic and aseismic slips on the plate interface and seismic velocity structures. The large coseismic slip of the 2011 Tohoku-Oki earthquake was limited to the middle segment, yet the observed negative residual gravity anomaly area in the southern segment corresponds to the postseismic slip area of the Tohoku-Oki earthquake. A model can explain the different slip behaviors of the two segments by considering their structural differences. The model indicated that the plate interface in the south was covered with a thick channel layer, as noted by seismic survey imaging, and this layer resulted in the residual gravity anomaly. Numerical simulations that assumed obvious frictional heterogeneity caused by the layer existing only in the south successfully reproduced M9 earthquakes recurring only in the middle, followed by evident postseismic slip in the south. We suggest that, while the layer makes the megathrust less compliant to seismic slip, it promotes aseismic slip following the growth of seismic slip on the fault in an adjacent region.