Strong anticorrelation between intensity of plate boundary PP reflection and seismicity had been revealed by a seismic reflection-refraction survey conducted in 1996 in a seismic-aseismic boundary region on the forearc slope of the Japan Trench. Amplitude of the strong reflection was explained by the presence of a thin layer (∼200 m) of low P wave velocity (3–4 km/s) at the top of the plate boundary. We conducted another seismic survey in 2001 in the same region as that of 1996 and verified the strong anticorrelation with planer extension along the plate boundary. Therefore existence of the thin layer of low P wave velocity along the plate boundary is expected within the aseismic regions. Extremely low P wave velocities along the plate interface at depths of around 10–20 km suggest that the layer may include fluid, clay minerals, and/or serpentine-chlorite. Because these materials have low mechanical strength or cause low friction between the overriding and subducting plates, large strain may not be accumulated and aseismic slip may be dominant. As a result of the above speculation, the intense reflection observed within the aseismic regions may imply aseismic slip between the plates. Amplitude of the plate boundary PP reflection phase was found maximum about reflections from deeper plate interface, although the reflected waves propagate for longer distances. One possible explanation to this phenomenon is the presence of serpentinized wedge mantle material that is buoyancy-driven to move along the top of the slab.