Avalanches occur when a porous, relatively weak layer of snow buried underneath a cohesive snow slab experiences mechanical failure and cracking. If the slab is on a sufficiently steep slope, friction will no longer support the slab’s weight, and shear forces will propel it to slide away. But that mechanism, first published in a 1979 paper, doesn’t explain why avalanches can originate on relatively flat landscapes and why some are triggered by remote disturbances.
To explain those scenarios, in 2008 a second theory was proposed that applied deep-earthquake thinking to avalanches on Earth’s surface. When a porous material is under pressure, like the weak buried layer above a snow slab, its volumetric collapse leads to anticracking—a type of compressive fracture that closes the faces of cracks. The lower layer’s collapse then bends and mobilizes the overlying slab to create an avalanche.
Despite efforts to reconcile the two mechanisms, results from small-scale experiments and large-scale observations have remained stubbornly distinct. That sort of scale dependence suggests a transition from one fracturing regime to another. In seismology research, a similar transition—from rupturing at speeds near that of Rayleigh waves to supershear crack propagation—was first considered by Robert Burridge in 1973 and D. J. Andrews in 1976. In the since-accepted Burridge–Andrews model, supershear crack propagation is characterized by daughter cracks that travel faster than a parent crack and at a velocity higher than the seismic shear-wave velocity.
Now Bertil Trottet (EPFL in Lausanne, Switzerland), Johan Gaume (EPFL and the Swiss Federal Institute for Forest, Snow and Landscape Research in Davos), and their colleagues have found evidence of the Burridge–Andrews mechanism in numerical simulations of snow fracture and large-scale avalanche measurements. The results indicate that avalanches may be more similar to earthquakes than previously thought.
The researchers designed and performed snow-fracture experiments during the winter of 2015–16 in Davos. Data collected from flat terrain and from a 30° slope showed that the weak snow layer had cracks that propagated below the shear-wave velocity, in agreement with their simulations. On that spatial scale and slope, no transition point to a supershear crack propagation regime was observed.
To study larger-scale cracking and search for a regime change in nature, the researchers monitored fracturing of the weak buried snow layer in the Alps after a professional snowboarder performed a jump from a ridge in Wallis, Switzerland. The impact from the jump triggered a dry-snow avalanche, shown in the top photo and captured in the video above, on a 42° slope. The high-speed camera footage confirmed a regime of supershear propagation. That means that the transition between the two types of cracking regimes is expected to occur for slope angles above 30°, the values at which static friction can no longer hold the snow.
The new findings could affect hazard management of mountainous regions prone to avalanches. For steep slopes, models of avalanche prediction may only need to focus on the supershear regime and wouldn’t require explicit modeling of the anticracking regime found in shallower-sloped terrain. (B. Trottet et al., Nat. Phys., 2022, doi:10.1038/s41567-022-01662-4.)
