This research letter presents a conceptual model and data from physical experiments for a new mechanism of sediment entrainment on the seafloor during the huge co-seismic motion imposed by the large subduction earthquakes. This new mechanism introduces the concept of sediment entrainment being due to the motion of the sediment bed instead caused my movement of the water above. Furthermore, by identifying the sedimentary fingerprint of megathrust ruptures with high tsunamigenic potential, the authors propose a new approach to constraining the seismic and tsunami hazard in subduction zones.