Mapping rockfall hazard and detecting precursory damage in rock slopes with passive seismic: Lessons from the La Praz case-study

We conducted repeated successive passive seismic surveys on the La Praz unstable rock slope (similar to 13,000-15,000 m(3), Savoie, France), spanning over more than ten years. The last survey was conducted two months before its complete collapse in August 2023. We used the amplitude of Horizontal to Vertical Spectral Ratio at peak (a(HVSR(fp))) to discriminate stable from unstable areas and delineate the fractures with major mechanical decoupling effect. We also classified the compartments in terms of rockfall hazard: the most prone-to-fall areas showed the highest a(HVSR(fp)) (in the range [26-157]), compared to [7.5-19] on intermediate areas and 2.7 on adjacent stable rock mass. The frequency of HVSR main peak (f(p)) on three unstable rock compartments showed unambiguous -15%, -14% and -11% drops prior to the collapse. These drops significantly exceed the range of environmentally driven resonance frequency wandering observed on site and generally reported as a limitation in the literature. These drops were interpreted as progressive in-depth slope damage. Interestingly, f(p) losses were detected solely on the three most prone-to-fall rock compartments showing the highest a(HVSR(fp)). The unveiling of in-depth damage processes on such a complex, heterogenous and highly fractured rock slope using passive seismic is a first.