Creep instability of the weak layer and natural slab avalanche triggerings

The weak layer connecting the snow slab with the older snow substrate is modelled as an open cell ice foam made of an array of ice bonds. The bond rupture and rewelding rates under load are derived using simple analytical equations. We show that the weak layer experiences stable flow for a wide range of loads, bond brittleness and bond rewelding rates. However, a critical point may be reached, corresponding to a finite shear rate of the weak layer, beyond which the shear rate increases catastrophically, leading to a natural avalanche release. This critical situation is equivalent to a ductile to britte transition, whose activation energy is one-half of that of diffusion in ice. A measurement of the exponent in the shear strain vs. stress relation may give a hint on the imminence of the avalanche release. A similar calculation performed at the tip of a pre-existing basal crack gives a simple criterion for crack stability and an analytical expression for snow fracture toughness. (C) 2001 Elsevier Science B.V. All rights reserved.