One of the greatest risks of winter ski tours away from secured pistes is certainly slabs of snow, which can develop into avalanches and sweep people away. According to a computer simulation, these avalanches could occur in a similar way to certain earthquakes. This is what Johan Gaume from the École Polytechnique Fédérale de Lausanne and his working group write in »Nature Physics«.
For their study, the team ran hundreds of simulations over days to model how large masses of snow break loose and then race down the valley. The focus was on slab avalanches, which most often lead to fatal accidents. A layer of snow loosens on the slope and quickly begins to slide downwards, taking other layers of snow with it. The snow boards often break off when a load acts on them from above; the crack along the edge can propagate at up to 300 kilometers per hour.
A weak, poorly consolidated and porous layer of snow usually collapses beneath a harder, heavier layer, which then slides over this subsoil. It starts slowly at first, but after a few meters, gravity creates tension in the compact layer of snow, which eventually causes it to break into smaller pieces: the avalanche picks up speed. Gaume and co compared these simulations to video of an avalanche set off by snowboarder Mat Schaer, a former Gaume student. Analysis of the film then confirmed the simulation.
A similar process also takes place in earthquakes that occur when plates move laterally. Here a daughter crack can form ahead of the main fracture front and propagate at high speed while preparing the way for the large rock fracture. Gaume and his team hope their study will help improve avalanche forecasting. This could be used to identify zones in the Alps, for example, in which these avalanches primarily occur.