A transition in dry-snow slab avalanche triggering modes

We show that two different types of dry-snow slab avalanches may result from a tensile failure of the slab crown triggered by either a quasi-static or an unstable shear propagation of a basal crack at the slab/substrate interface. The triggering conditions for both modes are derived analytically. The latter only takes place if the depth of the top layer is above a critical value, and in a "window" centred around a critical slope of 35.3 degrees, if residual friction on the basal crack surfaces is neglected. The width of this window increases with snow thickness. The introduction of a residual friction on the basal crack surfaces increases the critical slope.