Pluriannual seismic monitoring of rock glaciers: new insights on their dynamics

Abstract

Among mountain permafrost landforms, rock glaciers are composed of a heterogeneous mixture of rock debris, ice and liquid water. They can reach surface velocities of several m/yr for the most active ones, potentially causing emerging hazards linked to permafrost thawing and debris flows. As a complement to geophysical methods (georadar, active seismics, geoelectrics) providing interesting tools for investigating the subsurface, and to in-situ and remote sensing methods that track kinematics of these instabilities (1), passive seismic instrumentation offers a continuous monitoring at depth.Such instrumentation has been deployed for several years at Gugla, Tsarmine (Valais, Switzerland) and Laurichard (Hautes-Alpes, France) rock glaciers.From seismic ambient noise, Coda Wave Interferometry has been applied to compute daily dV/V (or relative change velocity of the surface waves) (2)(3) which are directly linked to the elastic properties of the medium at depth, and therefore its rigidity and density (4)(5). For the three sites studied, seasonal variations of shear stiffness have been measured, and located by using a 1D coda wave inversion. These changes in mechanical properties of the medium are related to seasonal hydro-thermal forcing.   We developed a simple viscoelastic model to explain the seasonal variability of the deformation rate of rock glaciers. By using observed shear stiffness as a parameter varying over time, we reconstructed well the creep rates observed, strengthening the key role of meltwater and rainfall on rock glacier dynamics at a seasonal scale. In the long term, a pluriannual seismic monitoring allows to detect changes in ice content, by tracking long-term changes in rigidity within the rock glacier body. Such permanent instrumentation paves thus the way to quantify the permafrost degradation.   ReferencesKneisel, C., Hauck, C., Fortier, R., Moorman, B., (2008). Advances in geophysical methods for permafrost investigations. Permafrost and Periglacial Processes 19, 157–178. https://doi.org/10.1002/ppp.616 Guillemot, A., Helmstetter, A., Larose, É., Baillet, L., Garambois, S., Mayoraz, R., & Delaloye, R. (2020). Seismic monitoring in the Gugla rock glacier (Switzerland): ambient noise correlation, microseismicity and modelling.Geophysical Journal International, 221(3), 1719-1735. https://doi.org/10.1093/gji/ggaa097 Guillemot, A., Baillet, L., Garambois, S., Bodin, X., Helmstetter, A., Mayoraz, R., and Larose, E.: Modal sensitivity of rock glaciers to elastic changes from spectral seismic noise monitoring and modeling, The Cryosphere, 15, 501–529, https://doi.org/10.5194/tc-15-501-2021, 2021. Larose E., C. S. (2015). Environmental seismology: What ca we learn on earth surface processes with ambient noise. Journal of Applied Geophysics, 116, 62-74. https://doi.org/10.1016/j.jappgeo.2015.02.001 Roux Ph., Guéguen Ph., Baillet L., Hamze A. (2014). Structural-change localization and monitoring through a perturbation-based inverse problem, The Journal of the Acoustical Society of America 136, 2586; https://doi.org/10.1121/1.4897403