Geological model of the Åknes rockslide, western Norway

Abstract

Åknes is known as the most hazardous rockslide area in Norway at present, and is among the most investigated rockslides in the world, representing an exceptional natural laboratory. This study focuses on structural geology and the usage of geophysical methods to interpret and understand the structural geometry of the rockslide area. The interpretations are further used to build a geological model of the site. This is a large rockslide with an estimated volume of 35–40 million m 3 (Derron et al., 2005) defined by a back scarp, a basal shear zone at about 50 m depth and an interpreted toe zone where the sliding surface daylights the surface. The rockslide is divided into four sub-domains, experiencing extension in the upper part and compression in the lower part. Structural mapping of the area indicates that the foliation of the gneiss plays an important role in the development of this rockslide. The upper boundary zone of the rockslide is seen as a back scarp that is controlled by, and parallel to, the pre-existing, steep foliation planes. Where the foliation is not favourably orientated in regard to the extensional trend, the back scarp follows a pre-existing fracture set or forms a relay structure. The foliation in the lower part, dipping 30° to 35° to S–SSE, seems to control the development of the basal sliding surface with its subordinate low angle trust surfaces, which daylights at different levels. The sliding surfaces are sub-parallel to the topographic slope and are located along mica-rich layers in the foliation. Geophysical surveys using Ground Penetrating Radar (GPR), refraction seismic and 2D resistivity profiling, give a coherent understanding of undulating basal sliding surface in the subsurface. The geophysical surveys map the subsurface in great detail to a depth ranging from 30–40 m for GPR to approximately 125 m for refraction seismic and 2D resistivity profiling. This gives a good control on the depth and lateral extent of the basal sliding surface, and its subordinate low angle thrusts. Drill cores and borehole logging add important information with regard to geological understanding of the subsurface. Fracture frequency, fault rock occurrences, geophysical properties and groundwater conditions both in outcrops and/or drill cores constrain the geometrical and kinematic model of Åknes rockslide.