The assumption that the source of magma for volcanoes in subduction zones is located in the mantle immediately above the top of the subducting slab, at approximately 100 km depth and straight beneath the volcanoes is incorrect in northeastern Japan. The combination of evidence from velocity tomography in the mantle wedge above the slab and mapping of earthquake size distribution within it strongly points to a source of fluids at the top of the slab at 140 to 150 km depth, from where material rises along an inclined path to the volcanoes.