The Kuril arc collides with the northeast Japan arc in the southern part of Hokkaido, Japan. 3-D tomographic inversion of data from a dense network of sensitive ocean-bottom seismographs and land stations has allowed imaging of previously unseen details of the arc-arc collision structure. A low velocity body dips gently southwestward, at depths of 35 to 45 km, from east of the Hidaka Mountains to the source area of the 1982 Urakawa-oki destructive earthquake (Ms 6.8). The low velocity body is the lower half of the lower crust of the Kuril arc, which must have been delaminated by the collision. We believe that the continuing collision of the delaminated lower crust with the northeast Japan arc resulted as an episode of aseismic slow slip prior to the 1982 Urakawa-oki earthquake as well being the reason for the high seismic activity in this region.