Marine gas hydrate distribution and submarine slope failure : an example from offshore Fiordland, southwest New Zealand

Crutchley, Gareth James.

Gas hydrates are ice-like species consisting of natural gas (usually methane) enclosed in a regular, stabilising 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 offthe southwest coast ofNew Zealand (Geodynz line 61c). Data show a distinct and continuous bottom-simulating reflection (BSR) below the continental shelf from water depths of ~2400 m to ~750 m where it intersects the seafloor. The landslide is evidenced by disrupted bedding and anomalously hummocky bathymetry in the area where the slope flattens out to sub-horizontal. 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 limitequilibrium and finite-element methods of slope stability analysis with respect to the MohrCoulomb failure criterion to develop an understanding of the relative sensitivity of the feature to model parameters. Excess pore fluid pressure (suprahydrostatic) has been modelled with realistic material properties of; internal angle of friction, bulk soil unit weight, and cohesion to show the considerable effect it has on stability. The best estimate of excess fluid pressure responsible for failure of a perfectly elastic body above a noncircular failure surface in a static environment is ~ 1700 kPa. Finite element modelling provided an opportunity to model internal deformation and displayed the positive correlation between excess fluid pressure and distribution of shear failure.

105 leaves : ill., maps (some col.) ; 30 cm.

2004Crutchley