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

Abstract

[1] This paper investigates the variation of the first resonance frequency of the Chamousset limestone column (21,000 m 3 , 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 freezethaw 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.

Seismic monitoring of precursory fracture signals from a destructive rockfall in the Vorarlberg Alps, Austria

“…Levy et al (2011) observed on a rock column in the French Alps that the largest numbers of events fit with minima in the temperature curve, after severe temperature drops. This result suggests a temperature control on rupture events and fracturing, what is supported by resonance frequency analysis (Levy et al, 2010). …”

Towards a standard typology of endogenous landslide seismic sources

“…At the Chamousset rock column, the source mechanism is retrieved by the P -and S-waves amplitude ratio (Lévy et al, 2010). Shear events are found to be located at the bottom of the column while tensile opening is occurring in the upper part (Lévy et al, 2010). To the best of our knowledge, for soft-rock landslides, no source mechanism was modeled.…”

“…Simple slopequakes 5 are currently assumed to be generated by shear movement along a plane or tensile opening of cracks (Spillmann et al, 2007;Helmstetter and Garambois, 2010;Gomberg et al, 2011). At the Chamousset rock column, the source mechanism is retrieved by the P -and S-waves amplitude ratio (Lévy et al, 2010). Shear events are found to be located at the bottom of the column while tensile opening is occurring in the upper part (Lévy et al, 2010).…”

“…In the case of Chamousset (Figure 10.b), the presence of this monochromatic coda is explained by the resonance of the rock column after the occurrence of the rock bridge breakage (Lévy et al, 2010). The resonant coda is rapidly attenuated with the distance and is not recorded by all the sensors (Lévy et al, 2010). Considering the distance between the main scarp and the seismic arrays (> 300 m) at the (2011)).…”

“…Presently, few studies have proposed inversion of the source tensor (Lévy et al, 2010). Therefore, the focal mechanism of the sources remain uncertainly known.…”