Probabilistic predictions of landslide tsunamis off Southern California

A review of tsunamis during the 1990s reveals about 30% of maximum runup peaks probably involved tsunamigenic mass failure. Submarine mass failure includes underwater slides, underwater slumps, and reef failures, most often triggered by a nearby earthquake. Earthquakes above magnitude 7 are believed to be accompanied by thousands of mass failure events, although most of these will not be tsunamigenic. A geological context derived from marine surveys is needed to identify prospective mass failures and to predict their size and location. Probabilistic, or Monte Carlo, calculations of underwater slides and slumps off Southern California yield probability distributions of mass failure sizes. Tsunami amplitude is estimated from accurate curve fits of numerical simulations of mass failure events. About 26–33% of all earthquakes trigger landslide tsunamis that locally surpass the earthquake tsunami in amplitude. A finite probability exists for mass failures to generate tsunamis with amplitudes in excess of 10 m. The probabilities of nearshore and offshore earthquakes can be converted directly into tsunami hazards from submarine mass failure. Indicators of prospective tsunamigenic landslides such as sedimentation rate or liquid limit improve our ability to predict future events and to assess their impact on coastal populations and development.