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

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.

 

 

 

References

• Kneisel, 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