Predisposition and triggering conditions at a permafrost-affected rock avalanche site in the French Alps (Étache, June 2020)

Permafrost-affected rockwalls are highly sensitive to rapid climate change, sometimes leading to rock slope failures threatening human lives and activities. Many studies have demonstrated a link between permafrost degradation and rockwall instability, but there is still a need to document destabilization events to improve the understanding of triggering mechanisms to ultimately develop relevant approaches for hazard assessment.   Our study investigates the little rock-avalanche (c. 229,000 m3) that occurred in the Vallon d'Étache (Savoy, France) on June 18, 2020, after several days of heavy precipitation. We try to decipher the preconditioning and triggering factors of the rock avalanche by combining ground surface temperature monitoring, numerical modelling of permafrost evolution, energy balance modelling and geoelectrical survey interpreted with a petrophysical model to bring a detailed description of the hydrological and thermal mechanisms. The results show an intense permafrost warming especially since 2012 (annual trend: +0.06 °C a-1 at 30 m depth), with permafrost transitioning from cold to warm permafrost along the scar plan at a depth of c. 45 m when the event occurred. This warming may have preconditioned the rock avalanche. The geoelectrical soundings (240 to 640m long profiles) confirm that the crest around the scarp is still largely frozen with possible ice-rich layers (high resistivity values; 360 kΩ m). Furthermore, the energy balance model shows that the event occurred during the highest water input from rain and snowmelt, since at least 1985 which may have played as a triggering factor.  It also shows that the ground surface temperature experienced its highest winter and spring values before the event.   This multi-method approach shows that this rock avalanche occurred in still largely frozen bedrock but subject to recent and very intense warming, and that water infiltration may have played a key-role in its triggering, either due to the development of high hydrostatic pressure or to accelerated permafrost thawing along fractures.