Determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes. The peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate (underplating) at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging, but direct seismic evidence of this process is lacking. By comparing seismic reflection profiles with microearthquake distributions in central Japan, we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting Philippine Sea plate. This region coincides with the location of slow-slip events that may serve as signals for monitoring active underplating.