Modeling submarine slope failure in a gas hydrate stability zone: An example from offshore Fiordland, southwest New Zealand.

Gas hydrates are ice-like species consisting of natural gas (usually methane) enclosed in a regular, stabilizing framework of water molecules. They have been found to be a significant constituent of seafloor sediment on many continental shelf-slope environments around the world. Gas hydrate is stable only within a limited temperature and pressure regime, and dissociation in response to a change in the physical environment can liberate excess gas and elevate the local pore fluid pressure in the sediment. This effect of sediment weakening is interpreted to be a significant contributing factor to a submarine landslide that has been seismically imaged off the southwest coast of New Zealand. Data show a distinct and continuous bottom-simulating reflection (BSR) below the continental shelf from water depths of ∼1650 m to ∼700 m where it intersects the seafloor. Additionally, the outcrop of the BSR on the seafloor corresponds with an apparent landslide scarp.

The geometry of the submarine landslide is well controlled in two dimensions, but the geotechnical characteristics of the material are not constrained, except by interpolation from other work. Representative soil strength parameters have been applied to both limit-equilibrium and finite-element methods of slope stability analysis with respect to the Mohr-Coulomb failure criterion to develop an understanding of the relative sensitivity of the feature to model parameters. Excess pore fluid pressure (suprahydrostatic) has been modeled with realistic material properties of: internal angle of friction, bulk soil unit weight, and cohesion, to show the considerable effect it has on stability. Permeability and cohesion in the overlying sediment have also been modeled so that their relative significance with regards to stability can be gauged.