Quantitative probabilistic Hazard analysis of earthquake-induced rockfalls

On May 6, 1976, in the Friuli Venezia Giulia Region (Eastern Italian Alps) a 6.5 Mw earthquake triggered a high number of rockfalls, causing casualties and relevant damages. For ensuring safety to people and infrastructure, as well as for territorial and land use planning, hazard analysis is fundamental. This contribution illustrates a methodology for quantitative probabilistic hazard analysis related to earthquake-induced rockfall events. This methodology is based on Rockfall Hazard Vector (RHV, Crosta and Agliardi, Nat Hazards Earth Syst Sci 3:407–422, 2003), which has been modified to incorporate temporal probability. RHV is defined as the magnitude of a three dimensional vector in the Frequency- Energy-Height space. The parameters involved are calculated for each grid cell by using Hy-STONE, a software developed to perform 3D numerical modeling of rockfall propagation over complex terrain. In particular, rockfall annual frequency for each grid cell is assessed by combining the annual onset frequency with the transit frequency simulated by Hy-Stone. For earthquakes-induced rockfalls, the annual onset frequency is defined based on a magnitude-frequency function, which has been spatially downscaled according to a rockfall susceptibility map. This map was developed by using multivariate statistical techniques (e.g. Discriminant Analysis) with morphological and seismic variables (e.g. slope, curvature, PGA, distance from the epicentre ecc.).