Surficial sediment remobilization by shear between sediment and water above tsunamigenic megathrust ruptures: experimental study

Abstract. Large megathrust earthquakes that rupture the shallow part of the interface can cause unusually large co-seismic displacements and tsunamis. The long duration of the seismic source and high upper-plate compliance contribute to large and protracted long-period motions. The resulting shear stress at the sediment/water interface in, for example, the Mw9.0 2011 Tohoku-Oki earthquake, could account for the surficial sediment remobilization identified on the outer margin. Through physical tank experiments, we test this hypothesis by exploring shear between sediment and water, interactions between high and low frequency seismic waves, and sediment properties (chemistry, grain size, water content and salinity). Our results show that low-frequency motion during a 2011-like earthquake can entrain several centimeters of surficial sediment and that entrainment can be enhanced by high-frequency vertical oscillations. These experiments validate a new mechanism of co-seismic sediment entrainment in deep-water environments.