Dynamic response of the Chamousset rock column (Western Alps, France)

Abstract

This paper investigates the variation of the first resonance frequency of the Chamousset limestone column (21,000 m3, Vercors, French Alps) before its collapse in November 2007. The site was instrumented with seismometers and extensometers during a 4‐month period with some gaps in the measurements. Experimental results and numerical modeling showed that the resonance frequency of a prone‐to‐fall column can be derived from the spectra of continuous seismic noise records. At the Chamousset site, the evolution of the resonance frequency appeared to be strongly controlled by the temperature. When temperatures were positive, slight resonance frequency variations correlated well with thermal fluctuations. Irreversible damage can occur during freeze‐thaw cycles and to a lesser extent during strong wind. It coincided with a significant drop in resonance frequency, which was interpreted as the result of rock bridge breakage. This hypothesis is supported by fresh rupture observations after the collapse, seismic event records, and numerical modeling. This study suggests that seismic noise recording could be used for assessing the potential failure of unstable columns in rigid rocks.