Thierry Oppikofer, University of Lausanne (Switzerland)
Martina Böhme, Geological Survey of Norway (Norway)
Lars Harald Blikra, Geological Survey of Norway (Norway)
Michel Jaboyedoff, University of Lausanne (Switzerland)
Aline Saintot, Geological Survey of Norway (Norway)
Åknes is a complex rockslide of about 30-40 million m3 located within Proterozoic gneisses of Western Norway. Its failure might cause a catastrophic tsunami in the fjord. Since 2004, this rockslide is part of the Åknes/Tafjord Project (www.aknes-tafjord.no) and one of the most intensively monitored sites in the world. The measured displacements reach about 4 cm/year, but higher movements (7-20 cm/year) are recorded on a fast-moving ridge in the uppermost part of the rockslide.
This study focuses on the conceptual model of the Åknes rockslide based on surface displacements measurements, high-resolution digital elevation model (HR-DEM) analysis, terrestrial laser scanning monitoring, and field investigations. The understanding of the instability mechanism is crucial to implement suitable monitoring and early-warning systems.
The observed displacements permit to divide the rockslide into several parts that move with different velocities and/or directions. Field surveys and HR-DEM analysis show that the geometry and movement of the blocks are controlled by structural features such as main schistosity (S1), folds, joints and faults.
Since most of the sliding surfaces reactivate S1, folds and undulations create changes in the sliding direction between S and ESE. The sliding surface is not continuous, but stepped by sub-vertical fractures perpendicular to the main sliding direction, that have been clearly identified on the topography in the vicinity of Åknes landslide.
The main folds have a gently plunging ESE-trending axis crossing obliquely the landslide body. Vertical S1 near the hinge zone creates weaknesses and favourably orientated planes that lead to the formation of extension failures acting as back-cracks. On the top of the landslide, the opening of such a fracture caused the creation of the fast-moving ridge and an approximately 30 m wide, debris filled trench.
The detailed displacement analysis performed on sequential terrestrial laser scanning point clouds of this ridge reveals translational movements of 6-7 cm/year towards 185° with a plunge of 55°, as well as toppling (towards 327° by 0.03°). A model for this upper-most part of the Åknes rockslide implies a combination of planar sliding along S1, subsidence due to stepped failure surface and toppling towards the opened graben structure.
The rockslide compartments are laterally delimited by sub-vertical NNE-SSW trending faults, acting as transfer surfaces. Towards the SW and the NE, the rockslide body is limited by two kilometric regional NNW-SSE faults.
This interpretation gives a coherent framework of the movements obtained by various methods and leads to a reinterpretation of the morphology, indicating that several rockslides occurred in the past. Rockslide scars on the slope are exactly located at the folds hinges with sub-vertical S1. The future research topics are to establish the failure surface topography and to define the blocks most susceptible to failure.