2012-2016 Modelling of LANDslides and generated earthQUAKES for detection and understanding of gravitational instabilities
Gravitational instabilities such as debris flows and landslides play a key role in erosion processes on the Earth's surface and represent one of the major natural hazards threatening life and property in mountainous, volcanic, seismic and coastal areas. Despite the large amount of work devoted to this problem, the mechanisms that govern flow dynamics and deposit in a natural environment are still unclear and key questions remain unanswered, such as the origin of the high mobility of some natural flows. Two severe limitations prevent a full understanding of physical processes involved in landslide dynamics. First, numerical models do not take into account the static/flowing transition in granular flows and the co-existence and interaction of fluid (water) and solid phases within the flowing mass, both playing a key role in natural instabilities. Second, field measurements relevant to the landslide dynamics are scarce, making it quite impossible to validate the models. Recent studies have shown that the analysis of seismic signals generated by natural mass flows provides a unique diagnostic of these flows and therefore a way to validate flow models and to discriminate between the physical processes at work. Indeed, the stress applied by the landslide to the topography, which generates seismic waves, is highly sensitive to the flow history and therefore to the rheological and physical properties during mass emplacement. Based on new results obtained by our group, we propose to take a significant step forward in understanding, characterizing and detecting natural gravitational flows by simultaneously developing flow models taking into account more realistic physical processes and simulating, analyzing and inverting the seismic signal generated by these flows. As required by such a project, we will develop an ambitious interdisciplinary approach involving mathematical modelling, numerical methods, laboratory experiments, seismology and geomorphology. To alleviate the high computational costs related to the precise description of the real topography, which plays a key role in landside dynamics, thin layer models will be developed. The static/flowing transition and the solid/fluid mixture will be appropriately described for granular flows over a complex 3D topography, starting from relatively well-established 3D models (partial fluidization, visco-plastic and discrete element models) for which efficient numerical methods will be developed. The main challenge will be to find mathematical formulations and numerical solutions to new models that involve a free interface, derived from non-Newtonian dynamics of visco-plastic granular materials by an asymptotic analysis. The seismic waves generated by the flow on the terrain will be simulated by coupling landslide models to state-of-the-art wave propagation models. An ambitious objective in this proposal is to develop efficient coupling methods, in particular using a consistent fluid-solid variational formulation. Simulated seismic waves will be compared with seismic emissions generated by granular flows in laboratory experiments. Geophysical flows and associated seismic signals will then be simulated, making use of underexploited high-quality seismic and geomorphological data. The sensitivity of the seismic signal to landslide characteristics, topography and the physical processes involved will be investigated and seismic data will be inverted using time reversal techniques. One of the ultimate goals is to develop operational codes for flows over 3D topography, taking into account erosion/deposition processes and the presence of a fluid phase, for distribution to the community. We expect to design and validate new methods for detection of gravitational instabilities, and interpretation of seismic data that could be used both for fundamental research and for monitoring/forecasting of landslide activity at the regional and global scale.