Submarine spreading
(Source: Micallef et al., 2007; Norsk Hydro AS)
Spreading is a common type of ground failure in subaerial environments. However, this type of mass movement has hardly been documented in submarine settings. In this study we show that spreading covers at least 25% of the Storegga Slide scar area, a giant submarine slide located offshore mid-Norway. The morphological signature of spreading is a repetitive pattern of ridges and troughs oriented perpendicular to the direction of movement. Two modes of failure can be identified: retrogressive failure of the headwall, and slab failure and extension, both involving the break up of a sediment unit into coherent blocks. These blocks are displaced downslope along planar slip surfaces. Limit-equilibrium modeling indicates that loss of support and seismic loading are the main potential triggering mechanisms. The extent of displacement of the spreading sediment is controlled by gravitationally-induced stress, angle of internal friction of the sediment, pore pressure escape and friction. The resulting block movement pattern entails an exponential increase of displacement and thinning of the failing sediment with distance downslope. Sediment properties explain the remaining spatial variation of ridge and trough morphologies associated with spreading.
Micallef, A., Masson, D.G., Berndt, C. and Stow, D.A.V. (2007) Morphology and mechanics of submarine spreading: A case-study from the Storegga Slide, Journal of Geophysical Research – Earth Surface, 112, F03023.
Micallef, A., Masson, D.G., Berndt, C. and Stow, D.A.V. (2007) Submarine spreading: Dynamics and development, in Lykousis, V., Sakellariou, D. and Locat, J. (eds.) Advances in Natural and Technological Hazards Research, 119-128.
Micallef, A., Masson, D.G., Berndt, C. and Stow, D.A.V. (2006) Lateral spreading within the Storegga Slide: Form and process, British Sedimentological Research Group Annual Meeting 2006, Aberdeen, Scotland.
Micallef, A., Masson, D.G., Berndt, C., Blondel, P. and Stow, D.A.V. (2006) A geomorphological investigation of lateral spreading and translational sliding within the Storegga Slide, Geophysical Research Abstracts, 8.
Fractal statistics and morphology of mass movements
This study documents the fractal characteristics of submarine mass movement statistics and morphology within the Storegga Slide. Geomorphometric mapping is used to identify one hundred and fifteen mass movements from within the Storegga Slide scar and to extract morphological information about their headwalls. Analyses of this morphological information reveal the occurrence of spatial scale invariance within the Storegga Slide. Non-cumulative frequency-area distribution of mass movements within the Storegga Slide satisfies an inverse power law with an exponent of 1.52. The headwalls exhibit geometric similarity at a wide range of scales and the lengths of headwalls scale with mass movement areas. Composite headwalls are self-similar.
One of the explanations of the observed spatial scale invariance is that the Storegga Slide is a geomorphological system that may exhibit self-organised criticality. In such a system, the input of sediment is in the form of hemipelagic sedimentation and glacial sediment deposition, and the output is represented by mass movements that are spatially scale invariant. In comparison to subaerial mass movements, the aggregate behavior of the Storegga Slide mass movements is more comparable to that of the theoretical ‘sandpile’ model. The origin of spatial scale invariance may also be linked to the retrogressive nature of the Storegga Slide. The geometric similarity in headwall morphology implies that the slope failure processes are active on a range of scales, and that modeling of slope failures and geohazard assessment can extrapolate the properties of small landslides to those of larger landslides, within the limits of power law behavior. The results also have implications for the morphological classification of submarine mass movements, because headwall shape can be used as a proxy for the type of mass movement, which can otherwise only be detected with very high resolution acoustic data that are not commonly available.
Micallef, A., Berndt, C., Masson, D.G. & Stow, D.A.V. (2008) Scale invariant behavior of the Storegga Slide and implications for large-scale submarine mass movements, Marine Geology, 247, 46-60.
Micallef, A., Berndt, C., Masson, D.G. and Stow, D.A.V. (2007) Fractal statistics of the Storegga Slide, in Lykousis, V., Sakellariou, D. and Locat, J. (eds.) Advances in Natural and Technological Hazards Research, 3-10.
Evolution of the Storegga Slide
(Source: Micallef et al., 2009)
The Storegga Slide, which occurred ~8100 years ago, is one of the world’s largest and best studied exposed submarine landslides. In this study we used novel geomorphometric techniques to constrain the submarine mass movements that have shaped the north-eastern Storegga Slide, understand the link between different forms of failure and propose a revised development model for this region. According to this model, the north-eastern part of the Storegga Slide has developed in four major events. The first event (event 1) was triggered in water depths of 1500 – 2000 m. In this event, the surface sediments were removed by debris flows and turbidity currents, and deposited in the Norwegian Sea Basin. Loading of the seabed by sediments mobilised by the debris flows and turbidity currents resulted in the development of an evacuation structure. Loss of support associated with this evacuation structure, reactivation of old headwalls and seismic loading activated spreading in the failure surface of event 1 up to the main headwall (event 2). In some areas, spreading blocks have undergone high displacement and remoulding. Parts of the spreading morphology and the underlying sediment have been deformed or removed by numerous debris flows and turbidity currents (event 3). We suggest that the higher displacement and remoulding of the spreading blocks, and their removal by debris flows and turbidity currents, was influenced by increased pore pressures, possibly due to gas hydrate dissolution/dissociation or by lateral variability in the deposition of contourite drifts in palaoeslide scars. The fourth event entailed a large, blocky debris flow that caused localised compression and transpressive shearing in the southern part of the spreading area.
Micallef, A., Masson, D.G., Berndt, C. and Stow, D.A.V., (2009) Development and mass movement processes of the north-eastern Storegga Slide, Quaternary Science Reviews, 28(5-6), 433-448.
Submarine landslides - miscellaneous
Mountjoy, J.J. and Micallef, A. (2011) Polyphase emplacement of a 30 km3 blocky debris avalanche and its role in upper continental-slope development, Advances in Natural and Technological Hazards Research.
Foglini, F., Dalla Valle, G., Micallef, A., Berndt, C., Campiani, E. and Trincardi, F. (2011) Seafloor instability and mass wasting processes along the eastern Gela Slope, 5th International Conference on Submarine Mass Movements and Their Consequences, Kyoto.
Micallef, A. (2011) Marine geomorphology: Geomorphological mapping and the study of submarine landslides, in Smith, M.J., Paron, P. and Griffiths, J. S. (eds.) Geomorphological Mapping: Methods and Applications, Elsevier Science Ltd.
