What triggers large submarine landslides on <2° slopes? A study on overpressure generation in slow deposition environments

Large submarine landslides that exceed in size landslides on land by up to two orders of magnitude occur on slope angles of less than 2°. Mechanically, failure of soil on nearly flat slopes is only possible when high ratios of excess pore pressure to effective stress prevail which significantly reduce the shearing resistance of the sediment. It has been suggested that the combinations of rapid deposition of low permeability sediment with lateral fluid flow precondition the slope to a tipping point, and that earthquakes act as final trigger to cause failure of low gradient slopes. However, slope failure is also observed at continental margins that experience comparatively low rates of sediment deposition. We develop a fully coupled 2D stress-fluid flow finite element model of an entire continental slope in order to investigate excess pore pressure generation due to consolidation under slow sediment deposition. Different permeability-porosity relationships, permeability anisotropies, compressibility, sediment strengths or inclusion of aquifers are modelled, but resulting excess pore pressures are too low to significantly decrease effective stress anywhere along the slope. This result indicates that we may miss out a dominant pore pressure generating process in areas of slow deposition, or that factors other than overpressure drive slope failure.