Multi-annual and seasonal patterns of Murtèl rock glacier borehole deformation, environmental controls and implications for kinematic monitoring

Abstract. Information about rock glacier deformation with depth is crucial for understanding the kinematic processes responsible for variations in rock glacier velocity. The majority of studies on rock glacier kinematics have been limited to surface measurements. Here we present the unique, almost eight-year long record from Murtèl rock glacier of borehole deformation at high temporal resolution. The extracted velocity time series with depth shows that seasonal variations are only observed in the active layer (AL), while in the main ice-rich core and the shear zone deformation rates remain relatively stable. At interannual timescales the variability in movement reaches beyond the AL and into the ice-rich core. The AL, ice-rich core and shear zone make up 20 %, 24 % and 56 % of surface displacement respectively. Compared to previous borehole inclinometer data, we find an unusually high fraction of deformation in the AL at Murtèl for the observation period. There are multiple rock glacier studies which report that water input dominates over temperature as a control for the seasonal variations in velocity. In contrast, at Murtèl we find that the years with the highest seasonal peaks in velocity are the years with the warmest summers; while the years with the highest meltwater input have a lower seasonal acceleration in deformation. The borehole deformation and temperature data suggest that the seasonal cycle in AL deformation is strongly related to thermal processes, rather than controlled by water input. Beyond this, three independent approaches for measuring surface displacement were applied and show that the borehole inclinometer and geodetic measurements agree well over a period of almost eight years. The continuous GNSS surface observations slightly overestimate the seasonal acceleration, but match the general background displacement well. Rock glacier velocity has recently been included in the essential climate variable (ECV) of "permafrost". Our borehole deformation data provide novel insight on how representative surface velocities are for rock glacier deformation at depth and on various timescales.