ISTerre seminar

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Supershear snow slab avalanche

Tuesday 27 May 2025 - 11h00

Johan Gaume - ETH Zürich, SLF Davos
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Snow slab avalanches, characterized by a distinct, broad fracture line, are released following anticrack propagation in highly porous weak snow layers buried beneath cohesive slabs. The anticrack mechanism is driven by the volumetric collapse of the weak layer, which leads to the closure of crack faces and the onset of frictional contact. Traditionally thought to propagate at sub-Rayleigh speeds, recent findings demonstrate that avalanche release can also involve a spontaneous transition to supershear crack propagation, where the rupture front travels faster than the shear wave speed. Here, we present a comprehensive study combining snow fracture experiments, artificially triggered full-scale avalanches captured with high-speed imaging, and numerical simulations to investigate this transition. The deformation fields obtained from DIC analysis reveal that sub-Rayleigh propagation is characterized by slope-normal collapse and slab flexure, whereas supershear fracture is associated with slope-parallel deformation and slab tension. Numerical simulations reproduce these observations and confirm that the transition follows the Burridge–Andrews mechanism, in which a supershear daughter crack nucleates ahead of the main anticrack front. Interestingly, our results indicate that supershear propagation can occur even when the shear-to-normal stress ratio is below the static friction coefficient, due to the transient loss of frictional resistance during weak layer collapse. These findings highlight some striking analogies between snow slab avalanches and strike-slip earthquakes, particularly supershear ruptures known for their high energy release and extensive rupture areas. The observation of supershear avalanches provides important new insights into avalanche formation, with significant implications for hazard assessment.

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Organizing team : Mécanique des failles

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères

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