Submarine landslide and tsunami hazards offshore southern Alaska: Seismic strengthening versus rapid sedimentation

The southern Alaskan offshore margin is prone to submarine landslides and tsunami hazards due to seismically active plate boundaries and extreme sedimentation rates from glacially enhanced mountain erosion. We examine the submarine landslide potential with new shear strength measurements acquired by Integrated Ocean Drilling Program Expedition 341 on the continental slope and Surveyor Fan. These data reveal lower than expected sediment strength. Contrary to other active margins where seismic strengthening enhances slope stability, the high-sedimentation margin offshore southern Alaska behaves like a passive margin from a shear strength perspective. We interpret that seismic strengthening occurs but is offset by high sedimentation rates and overpressure. This conclusion is supported by shear strength outside of the fan that follow an active margin trend. More broadly, seismically active margins with wet-based glaciers are susceptible to submarine landslide hazards because of the combination of high sedimentation rates and earthquake shaking. Shear strength is lower than expected offshore southern Alaska Low shear strength enhances susceptibility to submarine slope failure and tsunami hazards Seismic strengthening is counteracted by the high sedimentation rates from glacially enhanced erosion of coastal mountains Plain Language Summary The southern Alaskan offshore margin is prone to submarine landslides and tsunami hazards due to seismically active plate boundaries and extreme sedimentation rates from glacially enhanced mountain erosion. We examine the submarine landslide potential with new shear strength measurements. We interpret the lower strength is due primarily to the high sedimentation rates and resulting sediment overpressure. This conclusion is supported by comparing shear strength measured outside of the high-sedimentation area, that exhibit expected typical active margin shear strength. This study reveals lower-than-expected sediment strength offshore southern Alaska and is thus more susceptible to submarine landslides and associated tsunami hazards than previously thought. Tsunamis generated off southern Alaska can impact other Pacific coastal locations. More broadly, seismically active margins with wet-based glaciers are susceptible to submarine landslide hazards because of the combination of high sedimentation rates and earthquake shaking.