Seismic analysis of the detachment and impact phases of a rockfall and application for estimating rockfall volume and free‐fall height

International audienceWe analyzed 21 rockfalls that occurred in limestone cliffs of the Chartreuse Massif (French Alps). These rockfalls were detected both by Terrestrial Laser Scanning or photogrammetry and by a local seismological network. The combination of these methods allowed us to study relations between rockfall properties (location of detachment and impacts areas, volume, geometry, propagation) and the induced seismic signal. We observed events with different propagation modes (sliding, mass flow, free-fall) that could be determined from Digital Elevation Models. We focused on events that experienced a free-fall after their detachment. We analyzed the first parts of the seismic signals corresponding to the detachment phase and to the first impact. The detachment phase has a smaller amplitude than the impact phase, and its amplitude and duration increasewith rockfall volume. By measuring the time delay between the detachment phase and the first impact, we can estimate the free-fall height. We found a relation Es=aEpbbetween the potential energy of a rockfall Ep and the seismic energy Esgenerated during the first impact, with parameters a=10-8and b=1.55 and with a correlation coefficient R²=0.98. We can thus estimate both the potential energy of a block and its free-fall height from the seismic signals. By combining these results, we obtain an accurate estimate of the rockfall volume. This relation was then tested on different geological settings and for larger volumes using Yosemite and Mount Granier rockfalls. We also compared our results with a data set of controlled releases of single blocks (Hibert et al., 2017) in order to investigate smaller volumes