Depth-Averaged Modelling of Earthquake-Triggered Submarine Landslides on Continental Margins and Canyon Terrains

Infrastructure located on the seabed is vulnerable to earthquake-triggered submarine landslides, although the propagation of failure and the post-seismic movement of slides on the continental margin or in a submarine canyon have not been well interpreted. A three-dimensional depth-averaged method was used to revisit the initiation, progressive failure, and runout of two typical scenarios: a back-analysis of the deep-seated Ashton Seamount landslide and a scenario prediction in a canyon system in the South China Sea. For the former, the shear band is propagated extensively through a weak layer prior to the slab failure of thick overlying sediment. The influence of soil sensitivity on initial co-seismic softening may be insignificant, while it severely affects the subsequent shear band propagation and runout. The ratio of the affected area approaches six as soil sensitivity is increased from two to five. In the canyon system, several fully softened regions may be caused independently on steep canyon flanks. These regions are then progressively expanded and merged during the earthquake. The sliding masses move along several canyon channels before converging at the gully junctions. Sediment downslope is removed during the initial slope failure, thereby impairing the support provided to the upslope sediment and suggesting more retrogressive failures even after the earthquake. As a result, a peak velocity of 40.8 m/s and an affected area of 23.4 km2 are predicted.