2001: Measurement of natural frequencies and damping of speleot�ems.- RIVIERA 2000, Tectonique active et géomorp�ologie, Villefranc�e-sur-Mer, Revue d’Analyse SpatialNo

Broken speleothems (stalagmites, stalactites, soda-straws, etc.) can be interpreted as an indicator of past earthquakes. The dating of such events, up to several thousands of years, could then allow an evaluation of the seismic activity in an area, up to ages much older than that is possible from historical catalogues. This is of particular interest for the determination of long return-period events. One might also consider unbroken speleothems as an indication that no event greater than a certain level has occurred in the region. In order to evaluate the vulnerability of speleothems from earthquakes, it is necessary to know the range of their natural frequencies. However, there had been, up to now, no experimental in-situ measurements made to obtain these frequencies. The work presented here was aimed at the measurement of the fundamental natural frequencies and the damping of a representative population of speleothems. Measurements were made in the caves of Choranche and Antre de Vénus, in the Vercors Mountains (France), using a high-resolution laser interferometer. This study made it possible to show that most of the speleothems do not undergo dynamic amplification phenomena of the seismic motion, since their fundamental natural frequencies are higher than the range of seismic excitation. Only thin and long speleothems (such as long soda-straws) may suffer amplification phenomena. A fundamental frequency higher than the seismic frequencies means that the speleothem moves, with its basement, as a rigid structure. Consequently, most of the broken speleothems are a direct indicator of the peak ground acceleration. In order to use "speleoseismic " traces for seismic hazard assessment in a given area, the magnitude of the corresponding earthquake has to be evaluated. Further studies are proposed with the aim of a quantitative estimate of the confidence level that could be reached in the determination of the magnitude of "speleoseismic " events