Monitoring the mechanics of mountain permafrost using ambient noise seismology

Since last decades, coda wave interferometry (CWI) from ambient seismic noise has become an efficient method to probe continuous temporal changes of mechanical properties of the subsurface and crust. This method has successfully been used for environmental seismology issues, in a view of investigating the response of subsurface to environmental changes, in particular hydrological and thermal forcings (2). More, it has contributed to monitoring instabilities such rock slopes or landslides (3). Applying these methods to permafrost is then relevant to assess and monitor its mechanical response to environmental forcings. As lobate or tongue-shaped superficial landforms composed of frozen rock debris, active rock glaciers are widespread features of mountain permafrost (4), potentially causing emerging hazards linked to permafrost thawing and debris flows. Passive seismic instrumentation has been deployed for several years at Gugla, Tsarmine (Valais, Switzerland) and Laurichard (Hautes-Alpes, France) rock glaciers. CWI has been applied to compute daily averaged dV/V (or relative change in velocity of the surface waves). For the three sites studied, seasonal variations of shear stiffness have been measured, associated with freeze-thawing cycles (5) (6). We located these daily fluctuations in depth by using a 1D coda wave inversion scheme. We also tracked water-induced power spectral density (PSD) and we detected microseismic events, highlighting the role of water inputs in changing the mechanical state, thus accelerating the whole rock glacier body. Also, we developed a viscoelastic model to explain the seasonal variability of the kinematics of rock glaciers. Combined with other geophysical methods, environmental seismology paves hence the way to deeply understand the mechanical response of mountain permafrost landforms to thermo-hydrological forcings. References Richter, T., SensSchnfelder, C., Kind, R., Asch, G. (2014). Comprehensive observation and modeling of earthquake and temperaturerelated seismic velocity changes in northern Chile with passive image interferometry.Journal of Geophysical Research: Solid Earth,119(6), 4747-4765 Le Breton, M., Bontemps, N., Guillemot, A., Baillet, L., Larose, . (2021). Landslide monitoring using seismic ambient noise correlation: challenges and applications.Earth-Science Reviews,216, 103518. Haeberli, W., Hallet, B., Arenson, L., Elconin, R., Humlum, O., Kb, A., ... Mhll, D. V. (2006). Permafrost creep and rock glacier dynamics.Permafrost and periglacial processes,17(3), 189-214. Guillemot, A., Helmstetter, A., Larose, ., 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, 501529, https://doi.org/10.5194/tc-15-501-2021, 2021.