Structure sections across the Japan Trench from prestack depth migrated seismic data define the accretionary prism, a wedge-shaped seaward end of the continental framework, and a tectonized middle slope between them. The 10-km-wide accretionary prism is underthrust by sediment remaining with the subducted oceanic crust. Structure beneath the steep middle slope more nearly resembles the adjacent continental margin than the accreted mass. This deformed middle slope domain forms a buffer between the compliant accretionary wedge and a more rigid older continental framework. Along the plate boundary, subducted strata thicken beneath the middle and upper slopes and, where last imaged, are four times the input thickness beneath the accretionary prism. This interplate stratified layer appears to control interplate friction because the layer is imaged 45 km landward from the trench axis to 12-km depth and in that expanse, few upper plate earthquakes are recorded. Erosion of the base of the upper plate beneath the middle continental slope is indicated by margin subsidence and material flux. This erosion is concurrent with accretion at the front of the margin. ¿ American Geophysical Union 1994