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

Laboratoire de Géophysique Interne et Tectonophysique, CNRS, Observatoire de Grenoble, Université J. Fourier, LGIT, Maison des Géosciences, BP 53, 38041 Grenoble Cedex 9, France denis.jongmans{at}ujf-grenoble.fr

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 that allowed us to compare and classify ground-motion vibrations generated by these Alpine rock falls. Each rock fall has also been characterized 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 seismic 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 control of the propagation phase on the signal length. The signal analysis suggests the existence of at least two distinct seismic sources: one corresponding to the initial rupture associated with an elastic rebound 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 these two seismic sources are able to retrieve the main seismogram characteristics.