We determined, for the first time, a 3-D seismic structure of P and S wave velocity (Vp, Vs) in the forearc region of northeast (NE) Japan from the Pacific coast to the Japan Trench using a large number of high-quality arrival times from sub-oceanic earthquakes that are well located with sP depth phase data. Strong lateral heterogeneities in Vp and Vs are revealed. A majority (95%) of great interplate earthquakes are located outside the low-V zones while a few (5%) of them are located inside the low-V zones. This may suggest a different degree of interplate seismic coupling at the plate interface parallel to Japan Trench. The low-V zones probably contain fluids and may represent decoupled areas. We propose that this new approach of investigating 3-D seismic structure can be applied to other forearcs of the world for a better understanding of interplate coupling and its influence on big earthquakes.