We performed 3-D seismic tomography in the forearc region of the northeastern Japan subduction zone using both onshore and offshore seismic station data. We obtained the Vp, Vs, and Vp/Vs structures around the plate boundary with high spatial resolution. The position of the plate boundary as defined by relocated hypocenters coincides with the sharp velocity boundary between the oceanic crust and the mantle wedge. The mantle wedge above the coseismic slip area of the 1978 and 2005 off-Miyagi interplate earthquakes (M > 7) is characterized by high Vp and Vs, but low Vp/Vs. Off Fukushima, however, where large earthquakes rarely occur, we found a high Vp/Vs anomaly at the tip of the mantle wedge. The spatial distribution of serpentinized mantle wedge limits the spatial extent of the strongly coupled area on the plate boundary, and thus can explain the difference in seismic activity between the off-Miyagi and off-Fukushima regions.