Evaluation of a snow avalanche possibly triggered by a local earthquake at Vallée de la Sionne, Switzerland

Snow avalanches are moving sources of infrasonic and seismic energy. They can be triggered by many different mechanisms that include the shaking produced by earthquakes. The forces induced by an earthquake can cause an increase in the load down the slope and can also decrease the shear strength and both effects can cause the release of an avalanche. This phenomenon represents an important hazard associated with earthquakes in snow-covered mountain areas with high seismicity. ecember 2010 a snow avalanche was released at the experimental site of Vallée de la Sionne (VDLS) in Switzerland seconds after a local earthquake of magnitude ML 3.1 with the hypocenter in France, approximately 43 km from the avalanche starting zone. The seismic and infrasound signals generated by the earthquake and the snow avalanche were recorded by an array of sensors installed at VDLS. This paper analyses these data and shows that the avalanche was possibly triggered by the earthquake. This analysis also allows us to determine the characteristics of the avalanche (type and path). The infrasound data shows that the time of the avalanche release coincided with the arrival of the seismic waves of the earthquake. We calculate the values of the ground vibration parameters (PGD, PGV, PGA, PSA, Ia and TD) measured at the release area of the avalanche and compare them with those of two other earthquakes that did not trigger an avalanche. To evaluate the influence of the snowpack stratigraphy with the effectiveness of the earthquakes to trigger an avalanche, we simulate the snow cover using the one-dimensional snow cover model SNOWPACK. The weather and snow cover conditions of the days on which these events occurred are compared and used to evaluate the snowpack stability and the consequent likelihood of avalanche activity. The snowpack stability is the primary factor that determines whether an avalanche may be triggered by minor earthquakes. We conclude that when the snowpack is only marginally stable then the displacement caused by even a small earthquake could be enough to trigger an avalanche. Furthermore, the analysis of the other two, even stronger, earthquakes shows that in stable conditions no avalanche was triggered.