MODELLING TRANSIENT GROUND SURFACE TEMPERATURES OF PAST ROCKFALL EVENTS: TOWARDS A BETTER UNDERSTANDING OF FAILURE MECHANISMS IN CHANGING PERIGLACIAL ENVIRONMENTS

Despite the rising interest in mountain permafrost due to climatic changes and a noticed increase of registered rockfall events in the European Alps and other mountain ranges, little is known about transient thermal conditions in the detachment areas of rockfalls. Temperature conditions prior to the rockfall events of 144 past events in the European Alps were modelled with a physically based ground temperature model. To minimise the impact that uncertainty has on interpretations, only relative values were used, that is, percentiles obtained from cumulative distribution functions of the modelled ground surface temperatures from the beginning of the meteorological measurement series up to the event dates. Our results suggest that small and midsized rockfalls (volumes up to 100 000 m(3)) from high elevation occurred mainly during short-term periods of unusually high temperatures. This was neither found to be a result of the seasonal distribution (most analysed events in higher elevations occurred from July to September) nor of the longer-term temporal distribution (most analysed events occurred after 2000) only. Plausible explanations are either a destabilisation related to advective thaw or failure due to stress redistribution caused by large temperature variations. Large deep-seated rock slope failures (>= 100 000 m(3)) in high elevation occurred all year round.