Avalanche cracks may appear globally supersonic while remaining locally subsonic

A newly proposed theoretical framework suggests that cracks in snowpacks during avalanches may propagate at supersonic speeds, even though they are locally subsonic in certain conditions. This finding, presented by Nicola Pugno, a professor at the University of Trento, helps reconcile conflicting interpretations in the scientific literature regarding avalanche crack speeds.

Traditionally, snowpacks have been considered homogeneous materials, but in reality, they consist of multiple layers with varying mechanical properties. The new approach reveals a clear transition between a regime governed by the snow slab and another governed by the weak layer or its interface, where the crack may exhibit local stiffness greater than the global one.

This phenomenon occurs due to the presence of a stiffer weak layer or interface, which causes the fracture to propagate at a globally supersonic speed while remaining locally subsonic. The transition between these regimes is explained by a characteristic energy length previously introduced to understand the two propagation regimes observed in numerical studies.

This new theory could aid in better understanding whether avalanches can sustain supersonic crack propagation under specific conditions, such as the snow layering and mass. Furthermore, it could improve predictions of avalanche behavior, including the propagation front and other key characteristics, and potentially contribute to designing structures that may be impacted by the snow mass, like avalanche barriers and exposed buildings.