Transition from sub-Rayleigh anticrack to supershear crack propagation in snow slab avalanches: A numerical and theoretical analysis

Dry-snow slab avalanches are among the most destructive alpine hazards and account for the majority of avalanche-related fatalities worldwide. Their release involves a sequence of mechanical processes: localized failure in a weak, porous snow layer, onset and dynamics of crack propagation, tensile fracture of the slab, and subsequent sliding. Although the initiation of crack propagation is now much better understood, the dynamic fracture processes governing propagation, transition, and arrest at the slope scale remain poorly constrained. This knowledge gap limits our ability to predict avalanche size and dynamics. In particular, the discrepancy between the rather slow crack speeds measured in small-scale experiments and the much faster values inferred from full-scale avalanches suggests the existence of a transition in propagation regimes that has not yet been explored.

The first part of this thesis combines elastoplastic simulations using the Material Point Method (MPM) and Critical State constitutive models, snow fracture experiments, and full-scale avalanche observations to demonstrate the existence of a transition from sub-Rayleigh anticrack to supershear crack propagation within weak snow layers. This transition follows the Burridgeâ Andrews mechanism, whereby a supershear daughter crack nucleates ahead of the main front and propagates faster than the shear wave speed. Crucially, it can occur even when the shear-to-normal stress ratio is below the static friction coefficient, due to frictional weakening induced by weak-layer collapse. This reveals a fundamental analogy between avalanche fracture processes and supershear ruptures in strike-slip earthquakes, providing a new mechanistic basis for understanding avalanche release dynamics. Moreover, we show that slab fractures in soft slabs can suppress the supershear regime on steep slopes, whereas in hard slabs, fracture occurs after the supershear transition, leaving the crack dynamics unaffected. These findings highlight the importance of the coupled mechanical properties of both the weak layer and the slab when evaluating avalanche release sizes. The second part develops a new theoretical framework for the stationary propagation of anticracks that captures both the classical anticrack and the supershear regimes. By modeling the slab and weak layer as deformable Timoshenko beams, the analysis highlights the roles of weak-layer compaction and elasticity in modulating propagation speed. Energy storage and its sudden release at the crack tip can explain the high velocities observed in steep, slope-scale events. This extended framework overcomes key limitations of earlier analytical models by explicitly incorporating weak-layer mechanical properties, thereby providing a more robust physical basis for predicting the characteristics of different crack propagation regimes. Eventually, an unconventional type of avalanche was identified through independent research.