Changes in resonance frequency of rock columns due to thermoelastic effects on a daily scale: observations, modelling and insights to improve monitoring systems

Abstract

Summary Slope instabilities, including prone-to-fall rock columns, are known to exhibit clear vibrational modes. The resonance frequencies of these modes can be tracked by seismic instrumentation, allowing the rock column's mechanical and structural properties to be monitored, as well as providing precursors of imminent irreversible failures. In previous studies, superficial thermoelastic effects were assumed to drive daily fluctuations in resonance frequencies, but no qualitative or quantitative evidence was provided. The results presented here corroborate this hypothesis and quantify the physical processes involved. We interpreted daily variations of resonance frequencies in the Les Arches study site (Vercors, French Prealps) using a thermo-mechanical finite-element model. Modelled fluctuations of the resonance frequencies over a day closely matched experimental observations, reproducing the daytime frequency increase of around 2 per cent. In addition, our model provides explanation of the various behaviours observed across study sites: the frequency response strongly depends on solar exposure, as well as the timing and intensity of both radiative and convective heat fluxes. We highlight acousto-elastic constants as key parameters of our semi-quantitative model, although they remain poorly constrained here. For future instrumentation, we therefore recommend the deployment of pyranometers on rocky sites to accurately invert these parameters over time, thus allowing rock fracturing to be quantitatively tracked by acousto-elastic monitoring.