During subduction, the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths. Thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth. To characterize this transition, we studied 44 large to moderate-sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the Kuril--Kamchatka arc. Precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances. At depths of 30--50 km, our results delineate a transitional thrust zone, characterized by earthquakes showing thrust faulting on moderately dipping nodal planes.

The northwesterly dipping nodal plane dips approximately 40¿, some 10¿--15¿ steeper than the dip of low-angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km. For earthquakes along the transitional thrust zone, secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds. In the former case, the transitional thrust zone is interpreted as the deep-seated portion of the plate interface whose dip increases with depth. The alternative implies that the transitional thrust zone occurs below the plate interface within subducted crust of the Pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km. The inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region. However, a locked interface does not seem to affect the state of strain in the double seismic zone. The upper, compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km. At this depth, the two layers of seismicity in the double seismic zone are separated by as much as 30--40 km, too large a distance to be accommodated by models of seismogenesis invoking either the gabbro-eclogite transition or dehydration of subducted oceanic crust. ¿ American Geophysical Union 1995