Measurement of the deformation field associated with fracture propagation in weak snowpack layers

Dry snow slab avalanches release as a result of the failure of a weakly bonded layer located below a slab‐like layer of cohesive snow. Traditionally the failure of the weak layer was attributed to the formation and propagation of volume‐conserving simple shear cracks. Over the past decade, however, evidence for slope‐normal subsidence associated with fracture propagation in snow has accumulated, pointing toward a new understanding of fracture propagation in snow. The typically very high porosity of weak layers implies a loss of volume during fracture, as the aggregate of ice grains composing the weak layer collapses and rearranges in a tighter packing order. Therefore, in the new models failure is attributed to the formation and propagation of mixed‐mode anticracks. In order to experimentally investigate the nature of the deformation field associated with fracture propagation through weak snowpack layers we monitored propagating fractures in field tests on natural snowpacks with a high level of detail using high‐speed photography and analyzed the data with particle tracking velocimetry. The collapse of the weak layer was always observed and ranged between 0.3 cm and 4 cm. In all experiments the collapse was found to coincide with the fracture front. A shear crack preceding the collapse front was not detected at any stage of the process. It is concluded that the volumetric collapse of the weak layer, which is characteristic of anticracking, is the rule rather than the exception for the fracture process in stratified snow and therefore for slab avalanche release.