The conventional local tomography can determine the 3D seismic velocity structure right beneath a seismic network, but it cannot determine the 3D structure outside a seismic network. In this study we show that such a limitation of tomography can be overcome if many earthquakes occur outside a seismic network. In the northeast Japan arc earthquakes occur actively from the Japan Trench to the Pacific coast. We detected and used sP-depth phase to relocate the suboceanic earthquakes accurately, and then used P- and S-wave arrival times from the relocated suboceanic events and the earthquakes under the northeast Japan land area to determine the 3D P- and S-wave velocity and Poisson’s ratio structures of the entire northeast Japan arc from the Japan Trench to the Japan Sea coast. Our results exhibit strong lateral heterogeneities under the forearc region. The mainshock hypocenters of large interplate earthquakes (M 7.0–8.2) mainly cluster near the boundaries of high and low velocity and Poisson’s ratio above the subducting Pacific slab. A few large earthquakes are located in areas with high velocity and low Poisson’s ratio. These results suggest that lateral heterogeneities on the slab boundary can affect the rupture nucleation of large thrust earthquakes and the degree of interplate seismic coupling.