As transport routes and population centres in mountainous areas expand, risks associated with rockfalls grow at an alarming rate. As a consequence, there is an urgent need to delineate mountain slopes susceptible to catastrophic collapse in a safe and non-invasive manner. For this purpose, we have developed a 3-D tomographic seismic refraction technique and applied it to an unstable alpine mountain slope, a significant segment of which is moving at 0.01-0.02 m/year. First-arrivals recorded across the exposed gneissic rock mass have extraordinarily low apparent velocities. Inversion of their traveltimes produces a 3 D tomogram that reveals the presence of a huge volume of very low quality rock with ultra-low to very low P-wave velocities of 500-2700 m/s. Such low values likely result from the ubiquitous presence of dry cracks, fracture zones and faults at a wide variety of scales. They extend to more than 35 m depth over a 200 x 150 m area that encompasses the mobile segment of the slope and a large part of the adjacent stationary rock mass. Although hazards related to the mobile segment have been recognized since the last major rockslides in 1991, those related to the adjacent low quality stationary rock mass have not.


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