Gravitational slope deformation that preceded the Blatten rock slope failure in 2025, Switzerland

The Blatten rock-ice avalanche occurred on 28^th May 2025 and induced a huge disaster downstream. The rock debris from the rock slope failure accumulated on the downslope glacier and destabilized it and finally induced the rock-ice avalanche on 28^th.  We analyzed the topography, geology and the monitoring webcam photographs of the source area of the Blatten rock slope failure that preceded the final rock-ice avalanche and obtained an idea about the gravitational slope deformation process until the rock slope failure event. We used 0.5-m DEMs and aerial photographs of swisstopo and the photographs of monitoring webcam of the canton of Valais (Switzerland).

The topographic features appearing in the aerial photographs before the 2025 event and the monitoring photographs of the source area of the Blatten rock slope failure strongly suggest that the slope was preceded by flexural toppling, which accelerated in May 2025 and finally collapsed to accumulate on the downslope glacier. The flexural toppling had made narrow terraces on the slope probably by preferential shearing along fault crush zones in a long term. There are traces of water gushing out along the terraces at least since September in 2005, which may be due to the damming up of groundwater by the probable fault crush zones and the following spill over. Such a sequence of flexural toppling, water pressure build up by fault crush zones, and landslide generation has been reported by Yokoyama (2020, Geomorphology) and Chigira (2024, Slope Tectonics) in Japan. Flexural toppling is probably continuing in the slopes on the west of the source area, preparing for the next rock slope failure.

The source area of the Blatten rock slope failure is located within the zone of permafrost (Islam et al. 2025) but recent climatic change is supposed to have possibly warmed permafrost and increased the water (Farinotti et al., 2025). This is consistent with the presence of the water gushing traces.

The faults stated above are important but are inferred from the topographic features, so need to be investigated at the site.

Recent climate change has warmed the permafrost in high mountains, potentially increasing rockslope failures like the Bratten incident in many places..