To investigate variations in physical properties accompanying the development of the d¿collement, we used a neural network analysis of seismic attributes derived from 3D seismic reflection data gathered in the Nankai Trough convergent margin off the Muroto peninsula of southwest Japan. This analysis resulted in a detailed map of the geological features along the d¿collement and revealed a drastic change in physical properties at the sole of the proto-thrust. Ocean Drilling Program (ODP) data suggest that the d¿collement physical property change coincides with the initiation of slip. Furthermore, changes in the structure of the accretionary prism and reorganization of the d¿collement geometry also attend the initiation of plate boundary slip.