We propose a new mechanism of widespread surficial co-seismic sediment entrainment by seismic motions in subduction earthquakes. Our physical experiments show that shear from sediment-water relative velocities from long-period earthquake motions can mobilize synthetic fine marine sediment. High frequency vertical shaking can enhance this mobilization. According to our results, the largest tsunamigenic earthquakes that rupture to the trench may be distinguishable in the sedimentary record.

We discovered marine sediments on the Black Sea coast in eastern Romania at an altitude of 10 m and reveal that a tectonic block quickly descended 4 m below sea level, allowed for the deposition of sediments, and was then suddenly raised by 10 m sometimes during the last 200–300 years. This type of displacement needs to be taken into account when calculating past sea levels, while their recent and strong character should be seen as hazardous for coastal settlements.