Characteristics of human-triggered avalanches

In order to find characteristics of human-triggered dry snow slab avalanches, 10 years of avalanche occurrence data from the Swiss Alps have been analysed. Avalanche release and snowpack patterns were studied. Avalanches triggered by recreationists contribute to about 90% of the avalanche fatalities in Switzerland. Nearly exclusively dry snow slab avalanches were triggered. The slab detached of a human-triggered slab avalanche is (median values given) 50 m wide, and 80 m long; the overall avalanche (path) length is 150 m. The fracture depth is 45 cm, and the inclination in the starting zone is 38°. The slab failure is at the boundary between storm snow and old snow in only 38% of the cases. The other failures are due to weak layers or interfaces within the old snowpack. A weak layer was found in 50% of the cases. In all other cases, the failure was between two adjacent layers, a so-called interface failure. The thin (1 cm) weak layer is usually soft, found between one or two harder layers (above and below) and consists primarily of large crystals (≥2 mm) with plane faces: surface hoar, faceted crystals and depth hoar. The analysis of the 90 profiles available did support most of the mainly unstructured knowledge used in stability evaluation based on snow profiles and supports the simple model of skier triggering.