Stability, mobility and failure mechanism for landslides at the upper continental slope off Vesterålen, Norway

Abstract

Abstract Several relatively small and spatially-isolated landslides with low mobility characterize the geomorphology of the upper continental slope off the Vesteralen islands. Here, we present results from a multidisciplinary study that integrates swath bathymetry data, high-resolution seismic reflection profiles and a multitude of geological and geotechnical laboratory tests from a 12 m long piston core in order to investigate the origin and hazard potential of these shallow landslides. Four of the landslides have their upper headwall around the 500 m isobath, whereas the main escarpments of another four landslides lie around 700 to 800 m. The slip planes of the translational landslides lie within laminated glacial marine clays, overlying a well-defined seismic horizon. These clays have a higher plasticity and water content compared to the surrounding soils (sandy clays), and they exhibit a modest strain-softening behaviour in triaxial tests. The interdisciplinary data set is used as input to various numerical analyses in order to assess the failure and triggering mechanisms for these landslides, as well as their hazard potential. Stability analyses, dynamic analyses and post-earthquake pore pressure dissipation modelling suggest that the margin is essentially stable and that it would require a large magnitude earthquake to trigger landslides. The resulting deformation and excess pore pressure generation occur primarily within the top 10 m of the soil, and they become more pronounced towards the surface. Finally, the run-out distance of these landslides is limited and strongly depends on the volume of displaced material and the slope angle. Mobility analyses reveal that the acceleration phase lasts about 1 min and that peak velocities may have reached up to 17 m/s. Hence, the consequences of such flows during a time of active seabed exploitation or the impact with seabed infrastructure could be devastating.