Seismic measurement of fracture speed in a weak snowpack layer

Fractures in weak layers of a snowpack can propagate over distances ranging from meters to kilometers. Occasionally, these weak layer fractures are triggered on low-angled terrain and propagate into steep terrain to release an avalanche. Avalanches released in this way are considered remotely triggered avalanches. This paper describes a new field technique that uses geophones (velocity motion sensors) to measure the speed of a fracture propagating in a weak layer through a horizontal snowpack. In February 2000, six geophones were placed approximately 5 m apart along a transect on the surface of the snow. A fracture in a 10-mm-thick weak snowpack layer was artificially triggered and the motion of the snow surface indicates a propagation speed of 20 m s−1. The detected flexural wave in the slab supports a recent theory for fracture propagation due to collapse in a weak snowpack layer across horizontal terrain.