Snow Slab Fracture Propagation

Snowpack & Slope

Slope angle ψ 36°

Slab thickness D 0.50 m

Weak-layer fracture energy w_f 2.0 J/m²

Trigger flaw size a₀ 1.0 m

Fracture State

Stable — no trigger yet

Driving stress τ_d

— Pa

Critical length a_c

— m

Crack length a

0.0 m

Propagation speed

— m/s

How it works

A slab avalanche releases in two stages: a local weak-layer collapse under load (a skier, cornice fall, or explosive), then either an arrested flaw or a self-propagating fracture that severs the slab from the weak layer across the whole slope. Whether the crack keeps growing follows an anticrack / Griffith energy-balance criterion (Heierli 2008, Gaume et al. 2017 — the physics behind the field Propagation Saw Test):

Driving stress:   τ_d = ρ·g·D·sinψ·cosψ
Critical length:  a_c = sqrt( 2·E'·w_f / (π·τ_d²) )
Propagation speed: v_p ≈ 0.35·sqrt(E'/ρ)

ρ ≈ 250 kg/m³ and E' ≈ 2 MPa are fixed here as typical wind-slab values. τ_d is the shear stress the slab's own weight applies to the buried weak layer; a_c is the crack half-length at which the elastic energy released by further collapse exceeds the weak layer's fracture energy w_f, so the crack becomes self-sustaining instead of arresting.

a₀ < a_c — the collapse stays local (an "whumpf" with no slide): stable.
a₀ ≥ a_c — the crack self-propagates outward at v_p (tens of m/s, comparable to the slab's elastic wave speed) until it reaches the slope's edges, at which point the whole isolated slab loses its bond to the weak layer and accelerates downhill under gravity net of basal friction, a = g(sinψ − μcosψ).
Raising w_f (a better-bonded, more cohesive weak layer) or lowering ψ/D shrinks τ_d and grows a_c, making the same trigger less likely to propagate — this is exactly why forecasters track slope angle and recent snow loading.

Intact slab Fracturing weak layer Released / sliding

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