Abstract
The North Nagano prefecture Earthquake（ Mg 6.7 ） on 12 March 2011 induced surface-snow avalanches, full-depth avalanches, collapse of the block-like snowpack and cracks on the surface of the snowpack over wide areas, where the seismic intensity was 6 or more. Especially, within the area, failure surfaces of the snow avalanche were stepped and irregular. The avalanches filled roads at many locations, some of which occurred at slopes even though avalanche supporting structures had been deployed. Earthquake-induced avalanches of snow mixed with rock and sand flew long distances due to high fluidity. According to the snowpack and weather conditions, the likelihood of avalanche at the time of the earthquake was estimated to be low. We calculated the stability index of the snowpack by using an equation that incorporates the earthquake acceleration. The result shows that the index of the sliding surface for a surface-snow avalanche, consisting of wet melt forms snow, was estimated to be 1.5 or less which supports the possibility of earthquake-induced avalanche. Further research is needed to understand the snowpack collapse mechanism caused by an earthquake.
