Location of Seismic Signals Associated with Microearthquakes and Rockfalls on the Sechilienne Landslide, French Alps

The Sechilienne rockslide, in the French Alps, has recently been instrumented with three seismic arrays. This network has recorded numerous rockfalls and local microearthquakes. Because the media is highly fractured, it is difficult to identify and pick first arrivals. Beam-forming methods were therefore used to locate these events. The method has been adapted to take into account the heterogeneity of seismic wave velocities. The location accuracy has been estimated to be about 50 m for epicenters by applying the method to calibration shots. Depth is less constrained due to uncertainties on the velocity model and due to the seismic network geometry. This method of location has then been applied to rockfalls and microearthquakes. Most rockfalls initiate in the most active part of the rockslide called Les Ruines; the others are located on a recent eroded area aside from Les Ruines. The network also allows the estimation of the rockfall trajectory and propagation speed. Finally, 55 microearthquakes have been located within two zones. Microearthquakes are located within the first 250 m below the surface. Most microearthquakes are located in Les Ruines, the most active part of the rockslide, where the velocity has increased from 0.5 m/yr in 1996 to 1.4 m/yr in 2008. These events are located close to three faults that delimit a mass of about 3.6 million m3. Other events are located close to the summital scarp, in a zone moving at a few centimeters per year. The western part of the rockslide, which moves more slowly, did not produce any event large enough to be detected simultaneously by all stations. This seismic monitoring suggests that only a very small fraction of the deformation is released through seismic events.