Abstract
On January 25, 2009, a snowboarder was caught and injured seriously by an avalanche that occurred at Mumeizawa, Japan. The formation process and spatial distribution of the snow instability that caused this avalanche were examined based on the avalanche fracture line profile, snow and avalanche condition records from around the accident site, and snow profile data collected over a wide area. The instability was caused by the snowpack structure, which included a 40-cm-thick slab consisting of new snow and rounded grains, and a 2-cm-thick weak layer consisting of large rime-less plate-like new snow crystals. The weak layer was, formed over a large area in association with the passing of low pressure during January 22-23；however, this weak layer was stabilized immediately by solar radiation at the sunny side slopes. Conversely, the slab was formed within a limited area of the Japan Sea side mountain range in association with the Japan Sea effects snowfall. Therefore, it is presumed that the snow instability that caused the accident existed only over a limited high-elevation area in the shady side slopes. Furthermore, based on the analysis of observations along the Mumeizawa avalanche fracture line profile combined with extra observations and observations which collected from previous researches, it is suggested that weak layers composed of large rime-less plate-like crystals persisted longer than the layers composed of other precipitation particles.
