The dynamic response of four prone-to-fall compartments exhibiting diverse morphology, rupture mechanism, geological context and volumes has been studied using the resonance frequency technique. All sites exhibit well-defined spectral energy peaks, both in specific directions and at given frequencies. A predominant peak has been systematically measured at the lowest frequency, which has been interpreted as the first resonance frequency (f1) of the unstable compartment. The observation that the vibration direction at f1 is perpendicular to the rear main fracture at the four sites supports this interpretation and suggests that the first vibration mode is probably bending. These results show that the spectral analysis of the seismic noise might provide valuable information on unstable compartments, in various geological contexts. The first resonant frequency has also been monitored over a few months and did not show irreversible variations linked to damaging. By contrast, all sites exhibited reversible changes in fundamental frequency, clearly related to temperature variations but showing different patterns. These results suggest that the origin and control of the resonance depends on the site characteristics (volume, geology, morphology and rupture mechnism).


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