Dynamics and precursors of the 2025 Blatten rock–ice avalanche: Integrating seismic analysis, granular flow simulations, and field observations

Cascading slope failures in high mountain environments are observed with increasing frequency as glaciers retreat and slope stability is impacted by warmer conditions. On 28 May 2025, a large rock-ice avalanche (~9.3x106 m3) originating from Birch Glacier, Switzerland, destroyed parts of the village of Blatten, and provided a rare, well-documented case of a rapid, highly mobile mass movement.We combine seismic observations, geomorphological mapping, grain size and permeability measurements, and granular flow modelling to reconstruct the evolution of this event, from precursory instabilities to the main collapse. Seismic data scanned with machine learning algorithms reveal a two-week period of increasing rockfall and small glacier failures preceding the main collapse. The main collapse was reconstructed using force history inversion of low-frequency seismic signals from Switzerland’s national seismic network. Numerical simulations constrained by both seismic data and observed deposit extents indicate that an exceptionally low effective basal friction was required to reproduce the observed deposit extent and force history. This and the field observations of low-permeability deposit materials indicate that frictional weakening contributed to the unexpectedly high mobility of the main event.Our results highlight the value of integrating seismic monitoring with field and modelling approaches to constrain the dynamics of complex rock-ice avalanches. The Blatten event illustrates how large alpine slope failures can transition into highly mobile flows. Our study provides one of the first detailed reconstructions of this hazard cascade, including precursory failure activity, and the dynamics and frictional characteristics of the main event. The frictional weakening inferred here provides a much-needed mechanistic basis for predicting runout and deposit geometry in large debris avalanches.