Thermoelastic effects on resonance frequency of rock columns at daily scale, and modeling by acousto-elasticity

Among slope instabilities, prone-to-fall rock columns are known to exhibit features of strong vibration modes. Corresponding resonance frequencies can be tracked by seismic instrumentation for monitoring column’s mechanical and structural properties, as well as preventing any irreversible failures. In previous studies, superficial thermoelastic effects were supposedly driving resonance frequencies daily fluctuations, but not qualitatively neither quantitively evidenced[1]. Our work corroborates this hypothesis and quantifies the physical processes involved. We interpret daily variations of resonance frequencies in Les Arches study site (Vercors, French Prealps) [2] through a thermo-mechanical model based on finite-element method. The fluctuations of modelled resonance frequencies along day match closely the observed ones, reproducing the frequency increase at daytime of around 2%. In addition, our model allows explaining the various behaviors observed across study sites: the frequency response strongly depends on solar exposition, as well as timing and intensity of both radiative and convective heat fluxes. For future instrumentation, we hence recommend the deployment of pyranometers on rocky sites in order to accurately invert acousto-elastic parameters along time, thus tracking rock fracturing through acousto-elasticity monitoring.

[1] Colombero, C., Jongmans, D., Fiolleau, S., Valentin, J., Baillet, L. & Bièvre, G. (2021) Seismic Noise Parameters as Indicators of Reversible Modifications in Slope Stability: A Review. Surv Geophys. doi:10.1007/s10712-021-09632-w

[2] Bottelin, P., Lévy, C., Baillet, L., Jongmans, D. & Guéguen, P. (2013) Modal and thermal analysis of Les Arches unstable rock column (Vercors massif, French Alps). Geophys J Int, 194, 849–858. doi:10.1093/gji/ggt046