The Critical Crack: How Long a Fracture in a Weak Snow Layer Must Grow Before a Slab Avalanche Runs by Itself

Critical crack length for slab avalanche release from the anticrack energy model, computed across slope angles 25-50 degrees, slab thicknesses 0.2-1.0 m, densities 150-350 kg/m3 and weak-layer fracture energies 0.05-2 J/m2. The bare model gives initiation lengths of 0.48-11.08 m (median 2.21 m), overestimating propagation saw test values of 0.1-1 m by about a factor of two; the residual-friction term sets a hard arrest threshold at the friction angle (30 degrees baseline, 25-38 degrees across the plausible range), below which no crack runs away in shear. Skier loading, flat-terrain collapse driving (remote triggering), and a slab wave-speed bound on crack propagation (shear 54-117 m/s, plate 92-197 m/s) are also computed. Computed model, not field-measured.