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Analyse des processus de glissements gravitaires sous-marins par une approche géophysique, géotechnique et expérimentale : cas de la pente continentale de Nice

Abstract : Small submarine landslides, when triggered near the coast represent a major coastal hazard due to erosion of the coastline and marine submersion. In October 1979, a submarine landslide generated a part of the airport platform of Nice (France) to collapse at sea and provoked a tsunami. Because the continental slope off this region is abrupt, close to the coast and subject to moderate seismic activity it is a natural laboratory to study small-scale submarine mass movements processes. This work is based on a multidisciplinary approach allowing a global study of landslide processes in the source area. It integrates data from marine geophysics, sedimentology, geotechnics and numerical modelling. For the first time morphology and architecture of the Var delta deposits are investigated using very high resolution data. It allows identification of numerous small-size gravitational processes signatures as well as their embedding at depth. In the case of the 1979 landslide previously unknown features are identified: 1) eastern and western scars, 2) in-situ blocks and lateral spreading’s traces, 3) in-depth sliding surface, 4) estimation of a total displaced volume, which is different from the evacuated sediment volume. The sedimentary cores are then used to discuss the proximal distribution of deposits, erosion and paleo-landslides records from the deposits overconsolidation. The landslide activity has been estimated over time in terms of return frequencies. The largest landslides (>106 m3) have return frequencies nearing 50 years; the medium-size landslides (105 > V > 106 m3) between 3 and 25 years; and the numerous small landslides (<105 m3) every 1-2 months to 5 years during the most active periods in the last 50 years. Landslides deposits recorded in the source area show return frequencies of 3-7 years during periods of greater activity over the last 400 years. The morphology’s evolution follows successive cycles of sliding/quiescence/reloading. In recent times, two main cycles can be observed (from 1967 to 1999 and from 1999 to 2011) during which triggering are clustered in 5-10 years. On a longer time scale, the turbidites clusters span 20-40 years and quiescence periods ~100 years. Finally, this study brings new constraints on preconditioning and triggering factors acting on the Var delta. The stability of the area seems to be strongly conditioned by the complexity of the topography, the sediments consolidation and the quantity of sediments brought by the Var river. The deposits architecture mainly constrains the depth of the instabilities. Among the external drivers known in the area the extension of the gas-rich zone as well as fluids plumes in the water column have been considered and mapped. Looking at other external drivers, the analysis shows that the magnitude of rainfall and floods, and the alluvial water level would be too low over the past 50 years to act as an isolated triggering factor. In order to destabilize slopes, these external drivers need to be tackled together or associated to earthquakes. Relationships between databases analyses, landslides activity and numerical tests suggest that regional seismicity and historical earthquakes are either too small or too distant from the source areas to generate sufficient peak ground acceleration (0.2 g) and to have an individual impact on the delta slopes.
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Maëlle Kelner. Analyse des processus de glissements gravitaires sous-marins par une approche géophysique, géotechnique et expérimentale : cas de la pente continentale de Nice. Sciences de la Terre. Université Côte d'Azur, 2018. Français. ⟨NNT : 2018AZUR4113⟩. ⟨tel-02102191v2⟩

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