Spatial Heterogeneity of the Mantle Wedge Structure Corresponding to Interplate Coupling in NE Japan Forearc Region

The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the NE Japan arc. Interplate coupling along the plate interface is estimated to be strong by a backslip modeling of the land GPS observation in the middle to southern part of the arc [Suwa et al., 2006]. However, the off-Miyagi region and the off-Fukushima region show different characteristics of the interplate seismic activity. In the off Miyagi region, the large earthquakes with thrust mechanisms have occurred at an interval of about 40 years, and an interplate earthquake of M 7.2 occurred in this region on 16 August 2005. In the off-Fukushima region, few large interplate earthquakes have occurred while the background microseismicity is very high. In order to clarify differences in the seismic velocity structures, corresponding to the differences in the seismic activity between these regions, we estimated a 3D seismic velocity by the double-difference tomography method using both land station data and offshore station data. In our results, most of the relocated hypocenters are along the plate boundary. The subducting oceanic crust and the mantle wedge of the overriding plate were imaged as the landward dipping low velocity layer and the high velocity layer above it, respectively. In the mantle wedge, there are some velocity variations. Comparing spatial extents of the rupture areas of the 1978 and 2005 earthquakes [Yamanaka and Kikuchi, 2004; Yaginuma, 2006] and the velocity structure, we found that the location of high Vp, high Vs and low Vp/Vs anomaly corresponds to the rupture areas of the large interplate earthquakes. The high velocities and low Vp/Vs feature is interpreted as the non-serpentinized mantle wedge, and this may be the reason why M7 earthquakes repeatedly occurred in off-Miyagi region. In the off-Fukushima forearc region, high Vp/Vs area is found in the tip of this low velocity mantle wedge. This low Vp/Vs area is corresponding to the low backslip area revealed by GPS study [Iinuma et al. 2007]. We think that serpentinized mantle wedge delimited the extent of the seismogenic zone. The size of the seismogenic part is smaller in the off-Fukushima region with lower seismic coupling than in the off-Miyagi region.