Analysis of Rock-Fall and Rock-Fall Avalanche Seismograms in the French Alps

This study reviews seismograms from 10 rock-fall events recorded between 1992 and 2001 by the permanent seismological network Sismalp in the French Alps. A new seismic magnitude scale was defined, which allowed us to compare and classify g round-motion vibrations generated by these Alpine rock-falls. Each rock-fall has also been cha racterized by its ground-motion duration t30 at an epicentral distance of 30 km. No relation was found between rock-fall parameters (fall height, runout distance, volume, potential energy) and rock-fall s eismic magnitudes derived from seismogram amplitudes. On the other hand, the signal duration t30 shows a rough correlation with the potential energy and the runout distance, highlighting the co ntrol of the propagation phase on the signal length . The signal analysis suggests the existence of at le ast two distinct seismic sources: one corresponding to the initial rupture associated with an elastic r ebound during the detachment and the other one generated by the rock impact on the slope. Although the fall phenomenon includes other complex processes (fragmentation of the block, interaction with topography, plastic deformation during and after impact) 2D finite-element simulations of thes e two seismic sources are able to retrieve the main seismogram characteristics.

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