This paper presents a marine model for evaluating and investigating the geological risk induced by massive dissociation of gas hydrates. The model follows two independent paths: 1) analysis of the hydrate stability field on the seafloor, and 2) analysis of geological processes active in the study area. Finally, the paths converge in the analysis and evaluation of susceptibility of geological risk. The model is based on the 3D geographical distribution of the seafloor hydrate stability zone (HSZ) and the analysis of the geological processes (slumps and pockmarks) associated to hydrate dissociation.The model has been erected and applied in the Gulf of Cadiz, a region west of the Gibraltar Strait where (a) Atlantic (cold) and Mediterranean (warm) water masses meet each other; (b) tectonics induces a wide range of geothermal gradients, and (c) there are gas hydrates and a variety of geological structures linked to hydrocarbon seepages.
The thickness of the regional HSZ decreases abruptly in the area where the Mediterranean Outflow Water (MOW) warms the sea floor. The position and thickness of the HSZ is in a delicate equilibrium, very sensitive to changes of sea-level and the warm current, as revealed by the abundance of pockmarks, mud volcanoes and slumps in the continental margin. Warming of the seafloor by the MOW that causes thinning of the HSZ is a critical factor in the analysis of the geological processes derived from hydrate dissociation, as pockmarks and slumps are highly susceptible to a reduction of HSZ thickness.
The warm MOW that sweeps the sea floor of the Gulf of Cadiz introduces a critical factor in the equilibrium of the hydrate stability field, as the increase in temperature causes a sharp decrease in the calculated thickness of the HSZ, and enhances the development of pockmarks and slumps.
Pockmark susceptibility in the Gulf of Cadiz is highest where the factors "gas source and store" and "pathways of fluids escape" converge. This means that they are particularly abundant over diapirs and in the vicinity of faults or lineaments respectively. Slump susceptibility in the Gulf of Cadiz in the limit of the seafloor HSZ is relatively low owing to the gentle slope of the continental margin. Susceptibility is high in diapirs where the steep slopes of pockmarks trigger the slumps.
The marine model presented is valid and of universal application because it works with standard oceanographic data (depth, temperature, geothermal gradient) and geological analysis of the seafloor, where the accuracy of calculus depends on the precision of the acquired data. An additional advantage of the model is that it is dynamic, because the maps of the variables considered (i.e., the standard oceanographic data cited before) come from independent work flows and will easily updated using new data sets gathered in future research cruises. This enables new real time calculations of the model and ever-increasing precission.
