Frictional weakening in the highly mobile 2025 Blatten (Switzerland) rock–ice avalanche

Jiahui Kang , Antoine Lucas, Anne Mangeney, Johan Gaume, Kate Allstadt, Clément Hibert, Liam Toney, Hervé Vicari, Michael Dietze, Mylène Jacquemart, Marc Peruzzetto, Lars Blatny, Michael Lukas Kyburz, Joachim Rimpot, Daniel Farinotti, Fabian Walter

Cascading slope failures in alpine environments are intensifying as glaciers retreat and slope stability adjusts to a warming climate. Yet, the mechanisms governing such large, rapidly evolving events remain poorly understood. The 28 May 2025 rock–ice avalanche from Birch Glacier, Switzerland ($\approx9.3\times10^{6}~\mathrm{m^3}$), which devastated part of the village of Blatten, provides a rare, well-documented case to analyze the full sequence of instability and collapse. We combine state-of-the-art seismic, geomorphological, and geotechnical evidence with numerical modeling to reconstruct the evolution of the avalanche. We show that more than two weeks of accelerating rockfall activity and minor glacier collapses preceded the main detachment. The main collapse happened in four kinematic stages, from initial detachment to deposition. Only with drastically reduced friction can both depth-averaged and three-dimensional flow models reproduce the center-of-mass force history and the deposit geometry. Multiple mechanisms could have contributed to this frictional weakening such as ice content and meltwater. In-situ measurements of fine grain size and low permeability indicate possible elevated pore pressures, which would have facilitated the mobility. This work leverages multi-disciplinary datasets to investigate failure precursors and frictional regimes of catastrophic mass movements in changing climatic and permafrost conditions.