Rock glacier kinematics and driving processes at depth: seasonal to decadal trends in borehole records from the ice-rich Murtèl rock glacier

Understanding rock glacier creep is essential for assessing mountain permafrost change in a warming climate. Although the monitoring of rock glacier creep began about a century ago, most studies have been limited to annual rock glacier deformation measurements at the surface. There is a lack of high temporal resolution data to observe seasonal changes in rock glacier deformation, and even more so at depth. As rock glacier velocity has been added as a parameter within the mountain permafrost ECV, it is important to understand how these surface velocity measurements represent the entire rock glacier movement at depth. The first objective of this thesis is to use the almost 40-year record of borehole temperatures at Murtèl rock glacier to describe the long-term changes in rock glacier permafrost temperatures in relation to climate. The second is to outline the depths at which seasonal fluctuations of rock glacier creep can be observed in a 8-year borehole deformation record. The third is to explore the potential drivers of seasonal rock glacier deformation by combining the borehole deformation data with meteorological and permafrost temperature data. Murtèl rock glacier is a relatively slow-moving rock glacier in the Engadin region, Switzerland, that has been intensively studied for the past decades. The data used in this analysis comes from the borehole drilled in 1987 focusing on temperatures, the borehole drilled in 2015 focusing on deformation, the surface deformation measurements from the GNSS and geodetic stations, and the in-situ meteorological data. Since 1994 the permafrost temperatures at Murtèl rock glacier have been warming at a rapid rate of 0.3◦C/year and are well correlated with the MAAT. The borehole and GNSS deformation reveal a consistent seasonal deformation peak in September that is limited to the active layer (AL), with lower winter deformation rates driven by the shear zone. On average, the AL, the ice-rich core and the shear zone make up 20%, 24%, and 56% of surface deformation. The total surface deformation from 2016 to 2023 is approximately 90 cm and the measurements by the borehole and the geodetic station differ only by 2 cm. The surface annual velocity is not always representative of the annual velocity at depth. From the 2016-23 period, 2021 had the lowest annual deformation while 2022 the highest. The winter of 2020-21 was especially snow-rich and the summer mild, while the winter of 2021-22 was snow-poor, and the summer of 2022 was the warmest since 2016. The seasonal response time of the AL deforma�tion after the snow-melt is especially slow, up to one month. The combination of these findings suggests that the thermal conditions are dominant over the hydrological to drive the seasonal acceleration of the AL for Murtèl rock glacier. The high ice-content and slow deformation rates of Murtèl rock glacier limit the extrapolation of such interpretation; however, they show how, for some rock glaciers, the RGV as an ECV parameter might be limited to the signal coming from the AL movement. Future borehole deformation monitoring efforts, such as the one installed at Muragl rock glacier in the summer of 2024, should consider the approaches used in this thesis for future analysis.