Geological Model of Potential Rockslide Based on Structural Mapping, Surface Geophysical Data and Borehole Logging at Aaknes, Western Norway

Unstable rock slopes possess a threat to the inhabitants along Norwegian fjords, where pre-historic and historic rock avalanches have caused tsunamis, some causing severe casualties (Blikra et al. 2005). The presented site, Aaknes, is a large potential landslide of minimum 30 - 45 million m3 rock mass. Continuous creep of the rock mass and the fact that Aaknes is situated in the vicinity of one of Norway's most visited tourist attractions the Geiranger fjord, listed on the UNESCO`s World heritage list, have triggered a comprehensive mapping program. The overall aim is to assess the likelihood that the landslide accelerates into a rock avalanche. The potential landslide area at Åknes has been mapped by structurally mapped in detailed, whereas subsurface data come from 2D resistivity, Ground Penetrating Radar (GRP), refraction seismics, core drillings and geophysical logging of the boreholes. In symphony, these data give a detailed 3D geological model of the area, in which the depth to and the geometry of the basal slide surface(s) can be identified. A grid of 2D resistivity profiles indicate an undulating slide surfaces that can be followed from the large tension fracture in the back to the foot of the mapped slide area. Geophysical borehole logging including resistivity, water conductivity, gamma ray of bedrock, and sonic log are consistent with the properties of the bedrock found in the 2D resistivity profiles and in the drill cores. When correlated with drill cores, the sliding surface coincide well with intensely fractured bedrock and layers of fault rocks, such as gouge and breccia. Fracturing along the foliation of the host rock in combination with reactivation of fracture sets controls sub- block sizes as well as the pattern of movement, the latter consistent with a wedge failure model. Trends of fractures follow major trends of lineaments in the area, of which the most pronounced lineaments coincide with major fjords. The importance of fracturing along the foliation in the bedrock can be seen by the geometry of the back fracture. This composite structure is steep to sub vertical, but changes along strike as the foliation is folded. Farther down-slope, the foliations dips moderately towards the fjord and has an undulating geometry. There, the developed subsurface sliding plane shows a similar geometry, basically being sub parallel to the topographic slope. In a regional context, it has been documented that rockslides are more common where the foliation is sub parallel to the slope (Henderson et al. 2006). Through the locating of the sliding surface of the rockslide, a precise estimate of the volume is possibly. Further, it opens for a better understanding of the sliding mechanism(s) of the area. New work, such as borehole monitoring, will contribute to locating the sliding surface more precisely and yield additional quantified data regarding spatial and temporal sliding velocities. The aim of the project is to achieve a state of readiness with direct monitoring of movement, so that the local communities are able to evacuate in time.