On May 28, 2025, at approximately 15:30 CEST, a 9-million-cubic-meter rock–ice avalanche detached from the Birch Glacier in the Lötschental valley, Switzerland. This event was preceded ten days earlier by a major landslide from a source area higher up in the valley, which impacted and covered a part of the Birch glacier. Concerns about the resulting instability of the glacier prompted a precautionary evacuation of the village of Blatten below, which was indeed partially destroyed by the May 28^th ice-avalanche and caused one fatality. The avalanche’s runout exceeded predictive model forecasts. The event underscores the need for accurate runout prediction for complex mass movements.

To better understand the rock-ice avalanche dynamics, we use granular flow numerical modelling constrained by force histories inverted from seismic broadband data and the deposit extent and thickness. The simulations of landslide runout demonstrate that the force history can be reproduced with both a pure Coulomb and a μ(I) rheology. Notably, the successful Coulomb models require a low friction angle of 7^o, lower than predicted by empirical volume-mobility relationships for a landslide of this size. The presence of substantial ice in the avalanche material may have contributed to this higher mobility. Furthermore, fully explaining the final deposit distribution may require additional late-stage processes that reduce internal friction, such as partial liquefaction. This slow-moving phase of motion would generate forces of lower magnitude and longer period than could be detectable in a seismic force history but is supported by the final deposit’s thickness distribution.