Failure processes in submarine landslides : a geomorphological approach

Abstract

This thesis presents a novel technique for the quantitative characterisation of bathymetric data sets. The technique integrates three main geomorphometric methods: morphometric attributes and their statistical analyses, feature-based quantitative representation, and automated topographic classification. These methods allow useful morphological information to be extracted from bathymetric data and can significantly enhance submarine geomorphological investigations. The methods are applied to bathymetric data from the Storegga Slide, one of the largest known submarine landslides, to investigate three aspects of submarine mass movements: spreading, fractal statistics and morphology and slide development. The morphological signature of spreading, in the form of a repetitive pattern of ridges and troughs, covers at least 25% of the Storegga Slide scar. Two modes of failure can be identified for submarine spreading. The first involves retrogressive slide development via the unloading of the headwall. The second entails the extension of a thin coherent slab of semi-consolidated material downslope by gravity. Both modes of failure involve the breakup of surface sediment units into coherent blocks and their displacement along planar slip surfaces. The block movement pattern entails an exponential increase of displacement, and thinning of the failing sediment, with distance downslope. Loss of support and seismic loading are the main potential triggering mechanisms of submarine spreading. Analysis of headwall morphologies within the Storegga Slide reveals the occurrence of spatial scale invariance. One explanation for this scale invariance is that the Storegga Slide is a geomorphological system that may exhibit self-organised criticality. Spatial scale invariance may also be linked to the retrogressive nature of the Storegga Slide. The shape and fractal dimension of headwalls, on the other hand, can be used as a proxy for the type and number of the formative mass movements. A detailed reconstruction of the development of the north-eastern Storegga Slide shows that after the initial evacuation of the surface sediment as turbidity currents, the area failed as an extensive spread. The spreading blocks subsequently underwent higher displacement and remoulding, and were partly removed by debris flows and turbidity currents. The renewed instability within the spreading areas may have been related to gas hydrate dissociation and pore pressure increases due in response to the changing overburden, and the distribution of contourite drift deposits within underlying palaeoslide scars.