Thierry Oppikofer, University of Lausanne (Switzerland)
Lars Harald Blikra, Geological Survey of Norway (Norway)
Marc-Henri Derron, Geological Survey of Norway (Norway)
Michel Jaboyedoff, University of Lausanne (Switzerland)
Andrea Pedrazzini, University of Lausanne (Switzerland)
In the past centuries, several large rockslides occurred in the Norwegian fjords and caused catastrophic tsunamis. In the Storfjord, historical reports and rock avalanche deposits in the fjord confirm this high activity of approximately one catastrophic rockslide event every 100 years. The last major event was the 1934 Tafjord rockslide that caused an over 60 m high tsunami wave and killed 40 people. In the vicinity of the 1934 Tafjord rockslide scar, the potential rockslides in the Hegguraksla area are now studied within the Åknes/Tafjord Project (www.aknes-tafjord.no).
The 10 km2 Hegguraksla study area lies on the northern flank of the narrow Tafjord, a branch of the Storfjord. Geomorphic, geological and structural maps of the region were carried out by extensive field surveys in combination with the analysis of high-resolution digital elevation models and orthorectified aerial photographs. The terrain is mainly constituted of fine-grained, mica-rich gneisses, with well-developed foliation, overthrusted during the Caledonian orogeny by augengneisses of the Risberg nappe.
Besides the 1934 Tafjord rockslide scar, the terrain morphology reveals at least nine other scars. Deposits in the fjord indicate that some of these past events occurred in the last ∼10'000 years after the deglaciation. The basal failure surfaces generally use the main schistosity S1 dipping south towards the fjord with a dip angle of ∼45°. More mica-rich bands in the gneisses and gouge-filled faults parallel to S1 favoured these rockslides. The basal sliding surface is often stepped by sub-vertical fractures. But the instability mechanism was not planar sliding, since S1 is constrained by sub-vertical ENE-WSW fractures. The resulting wedge intersection trends towards WSW with an axial plunge of 35°.
16 potential instabilities were inventoried on the basis of morphologic, structural and field evidences. Reactivated S1 might again act as basal sliding surface for most of these instabilities. The localization of these instabilities is controlled by the orientation of S1, which varies due to large-scale folds and impedes sliding if it does not daylight. The estimated volumes range from several thousands to millions of m3. Four of these instabilities are in the extension of ancient rockslides scars. This indicates a progressive evolution of the slope using similar instability mechanisms as for past rockslide events.
In addition to these instabilities, two larger unstable areas with several possible instabilities were detected. One of these is the Hegguraksla plateau with a morphology that is characteristic for ancient slope movements. At the front of this potentially unstable area, two instabilities are currently monitored within the Åknes/Tafjord Project, but no displacement is detected. The detailed analysis of these instabilities forms the first step in the rockslide hazard analysis in the Tafjord area.