Improving Risk Assessment of Subaqueous Landslides and Tsunami Generation by Understanding the Roles of Pore Pressure and Sediment Cohesion

Subaqueous landslides present a hazard to communities through their ability to damage infrastructure and generate tsunamis. Efforts to assess tsunami risk often depend on observations from prior events, such as volume of sediment remobilized, velocity of remobilized sediments, slope angle, and/or recurrence intervals. However, geology and hydrology are important factors in slope failure likelihood and tsunamigenic potential. Here physical experiments and natural observations demonstrate the importance of excess pore pressures and sedimentology (i.e., clay content) for assessing failure likelihood and tsunamigenic potential. The implications of excess pore pressure and clay concentration in future risk assessments are characterized. Excess pore pressure is identified as capable of lowering slope resistance to failure, triggering non-tsunamigenic slope failure, and preconditioning a slope for tsunamigenic failure. Excess pore pressures within slopes reduce slope stability and thus areas with excess pore pressures should be considered at increased risk. Bathymetric features such as pockmarks/craters and surface eruptions/mud volcanism are identified as past or present indicators of excess pore pressures. Clay concentration is identified as determining slope failure’s potential for generation of intact blocks, a requisite for tsunamigenic land sliding. Measurement of slope clay concentrations are shown to provide a first-order estimation of failure behaviour to be tsunamigenic. Overall, this work notes that understanding how a slope might be preconditioned for failure is a necessary input for a more holistic risk assessment.