Advanced seismic methods applied to the study of snow avalanche dynamics and avalanche formation

[eng] Snow avalanches are extended moving sources of infrasonic and seismic energy. The acoustic and seismic wave fields generated by an avalanche is a complex natural phenomenon produced by the interaction of the flow with its environment. Seismic waves are mainly generated by friction and the impacts of the flow on the snow cover and terrain features; whereas the infrasound waves are generated by the interaction of the powder cloud with the air. Nowadays, avalanches are recorded using seismic and infrasound sensors that provide data that can be used to obtain information about the characteristics of the source and the basis processes that govern avalanche dynamics such as erosion and deposition. So far, the knowledge of how all these dynamical processes and the inherent characteristics of avalanches affect the signatures of the signals is limited and thus more quantitative data are require. Additionally, seismic and infrasonic monitoring systems can provide information on the avalanche triggering mechanisms such as, for example, the shaking produced by an earthquake. Few earthquake-generated avalanches have been documented to date, all of them visually or statistically identified. Hence, a complete dataset of such events does not exist. This study aims to enlarge the current application of seismic methods, mainly for snow avalanche research, which in turn, is relevant to improve monitoring systems. A catalogue of thirty-three snow avalanches of different natures is analysed using the seismic signals recorded with a set of seismic sensors at the Vallée de la Sionne test site in Switzerland. A comparative analysis of the seismic measurements and data acquired with other instrumentation such as infrasound sensor, several frequency-modulated continuous wave radars, and weather stations, is presented in each case to complement and validate the results. This thesis presents novel contributions using seismic methods in the research field of avalanche dynamics and in the field of avalanche formation, specifically in the field of avalanche induced by earthquakes. As a first step, the joint analysis of seismic and infrasound data, correlated with radar measurements, provides data of the onset of an avalanche with the arrival of an earthquake. Seismic data were used to quantify energy parameters and changes in the elastic stress field within the snowpack due to the earthquake. This event was compared with two stronger earthquakes that did not trigger any avalanche. The study was complemented by nivo-meteorological data and snow cover simulations. I 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. In addition, the analysis of the other two, even stronger, earthquake shows that in stable conditions no avalanche was triggered. Furthermore, in order to better understand the connection between seismic signals and avalanche dynamics, I quantify the seismic signals in a set of seismic indices. For each seismic signal, the duration, peak ground velocity of the envelope, and both the intensity and the frequency content were compared with the avalanche flow regimes and the thicknesses of the snow cover measured using radar measurements, as well as with the avalanche size. The frequency content of the seismic signal can be used to infer the avalanche flow regime and to characterize its internal parts. Furthermore, the seismic signal duration can be correlated to the avalanche size. If the snow cover absorption does not significantly weaken the intensity of the signal, the avalanche size can also be deduced from the peak ground velocity and its intensity. These results show that avalanche characteristics can be feasible inferred using only seismic data. Such analysis can be usefully employed in avalanche monitoring and management.[spa] Las avalanchas de nieve son extensas fuentes móviles de energía sísmica e infrasónica. El campo de ondas acústicas y sísmicas generado por ellas es un fenómeno natural complejo producido por la interacción del flujo con su medio ambiente. Actualmente, las avalanchas son monitorizadas mediante sensores sísmicos y de infrasonido proporcionando datos para caracterizar las avalanchas y los procesos dinámicos que las gobiernan. Además, estos sistemas de monitoreo también proporcionan información sobre los mecanismos de desencadenamiento de avalanchas tales como, por ejemplo, las vibraciones producidas por terremotos. El objetivo de este tesis es ampliar las aplicaciones actuales de los métodos sísmicos principalmente para investigar avalanchas de nieve, y que a su vez, los resultados sean relevantes para mejorar los sistemas de monitoreo actuales. Se analiza un catálogo de treinta y tres avalanchas de nieve registradas mediante sensores sísmicos en el sitio de test de Vallée de la Sionne (Suiza). Un análisis comparativo de los datos sísmicos y datos adquiridos con otra instrumentación (datos de infrasonido, radares de frecuencia modulada y estaciones meteorológicas) se presenta en cada caso para complementar y validar los resultados de la tesis. Los resultados obtenidos presentan aportaciones novedosas dentro del campo de investigación de la dinámica de avalanchas de nieve y de su formación, específicamente en el campo de avalanchas inducidas por terremotos. En este campo, se analiza un conjunto de datos únicos de una avalancha que coincide temporalmente con un terremoto, evaluando la posibilidad de ser o no desencadenado por las vibraciones generadas por el terremoto. En el campo de investigación de la dinámica de avalanchas, se ha realizado una parametrización de las señales sísmicas de un catálogo de avalanchas de diferente naturaleza. La cuantificación de las señales sísmicas en el conjunto de índices definidos permite inferir directamente características dinámicas de las avalanchas de nieve