Continental slope sediment failures around the epicentre of the 1929`Grand Banks' earthquake have been imaged with the SAR (Syste Áme Acoustique Remorque Â) high-resolution, deep-towed sidescan sonar and sub-bottom pro®ler. The data are augmented by seismic re¯ection pro®les, cores and observations from submersibles. Failure occurs only in water depths greater than about 650 m. Rotational, retrogressive slumps, on a variety of scales, appear to have been initiated on local steep areas of seabed above shallow (5±25 m) regional shear planes covering a large area of the failure zone. The slumps pass downslope into debris¯ows, which include blocky lemniscate bodies and intervening channels. Clear evidence of current erosion is found only in steep-sided valleys: we infer that debris¯ows passed through hydraulic jumps on these steep slopes and were transformed into turbidity currents which then evolved ignitively. Delayed retrogressive failure and transformation of debris¯ows into turbidity currents through hydraulic jumps provide a mechanism to produce a turbidity current with sustained¯ow over many hours.

We present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature, pressure, pore water chemistry, and the mean pore size distribution. The model uses a new formulation based on the enthalpy form of the law of conservation of energy. The developed model shows that due to a temperature and pressure increase, hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water. This original result confirms what has been already shown through experiments. The second part of the paper presents an application of the model through a back-analysis of the giant Storegga Slide on the Norwegian margin. Two of the most important changes during and since the last deglaciation (hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature) were considered in the calculation. Simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the Storegga slope. Once again, the developed model leads to predictions, which are supported by laboratory experiment results, but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone.