On transient thermally induced stability changes in high-mountain permafrost rock walls: A semiquantitative modeling approach applied to recent landslides at Rasac (Cordillera Huayhuash, Peru, 2023) and Blatten (Swiss Alps, 2025)

Abstract. Climate-induced warming affects physical, mechanical and hydraulic properties of permafrost rock walls in cold mountain regions. While detailed understanding of the complex interaction between rock structures, ice and water remains challenging, the overall effect of thermally induced stability reduction seems evident and best explains the recent increase in the number of large rock-ice avalanches. Time-dependent modeling of thermal conditions in the pre-event failure zone of two recent events at Rasac ridge (2023) in the Cordillera Huayhuash, Peru, and at Blatten (2025) in the Swiss Alps documents marked subsurface warming during the past about 150 years down to about 100 meters or more together with a remarkable inertia of the associated temperature change. Both investigated mass movements must have detached from quite cold permafrost with permafrost depths in places exceeding 200 to 300 meters but with pronounced asymmetric thermal conditions as is characteristic for sharp mountain ridges. In the Blatten case, increasing water infiltration from the warmer sunny side may have contributed to the release of an already weak slope which must have developed subcritical rock-mechanical conditions over much longer time.

The large amount of heat already now stored deep below the surface constitutes a strong long- term commitment concerning the future stability of permafrost rock slopes. Ongoing atmospheric and subsurface temperature rise are likely to further enhance related stability reductions. Hazard and risk assessments concerning cold mountains must adequately consider such strongly time-dependent aspects.