1 Introduction

The valleys and fjords of western Norway are characterized by steep-walled, over-deepened glacial valleys. This Alpine relief, combined with the abundance of brittle and ductile bedrock structures, seasonal heavy precipitation, snow melting, frost cracking periods, and freeze-thaw cycles make many fjords and valleys of western Norway particularly prone to rock slope failures (Blikra et al. 2006). Many rock slope failures occurred shortly (i.e. 1000 to 3000 years) after deglaciation (Blikra et al. 2006; Böhme et al. 2015) and a large number of gravitational deformation structures were formed, possibly caused by the sudden unloading of the steep slopes due to glacial erosion and rapid glacial retreat. These gravitational deformation structures delimit unstable rock slopes, which by continuous deformation might lead to a catastrophic failure and form rock avalanches.

This study presents an inventory of 80 unstable rock slopes located in the Møre & Romsdal County in western Norway (Fig. 1), and investigates their different failure mechanisms and stages of progressive failure evolution. The term " unstable rock slopes" depicts slopes that show signs of significant post-glacial deformation (Hermanns et al. 2012). These investigations are part of the systematic mapping of unstable rock slopes in Norway made by the Geological Survey of Norway and financed by the Norwegian Water Resources and Energy Directorate (Hermanns et al. 2014).

2 Inventory of unstable rock slopes

The national database on unstable rock slopes in Norway (Oppikofer et al. 2015) provides the necessary data for this study. Most field investigations and analyses were made by the Geological Survey of Norway. These comprise geomorphological and engineering geological field mapping (structural measurements), kinematic failure mechanism analysis, periodic displacement measurements (using for example differential GPS, extensometers, but also satellite-based InSAR measurements and terrestrial cosmogenic nuclide (TCN) dating) and estimation of the along-slope elongation (ratio between total displacement and length of slope) (Oppikofer et al. 2017).

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