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. Understanding which processes lead to the observed displacement signal on the rock glacier surface is important because it is a parameter of the essential climate variable (ECV) “permafrost”. Here, we present a unique record of the nearly eight-year-long borehole deformation data at high temporal resolution from the Murtèl rock glacier in the Swiss Alps. The extracted velocity time series with depth shows that seasonal variations are only observed in the active layer (AL), while in the ice-rich core and the shear zone the velocity remains relatively stable. Interannual variability in deformation is driven by both the AL and the ice-rich core components, while the shear zone shows minimal change over the years. The AL, ice-rich core and shear zone components of deformation make up 20 %, 24 % and 56 % of surface displacement respectively. Compared to previous borehole inclinometer data, we find an unusually high fraction of surface displacement happening in the AL at Murtèl rock glacier. There are multiple rock glacier studies that report a fast response in surface velocity to water input from snowmelt or rainfall. In contrast, at Murtèl rock glacier we find that surface acceleration begins multiple weeks after the snowmelt period ends. The largest peaks in surface velocity coincide with the years with the highest ground surface temperature rather than the years with the most snowmelt. We postulate that the deep ice-rich and cold permafrost core of Murtèl rock glacier prevents the pore water pressure in the shear zone to change at a seasonal timescale. The borehole deformation and temperature data suggest that the seasonal cycle in AL deformation is strongly related to thermal processes and the melting of refrozen seasonal ice. The comparison of three independent approaches for measuring surface displacement shows that the borehole inclinometer and geodetic survey measurements agree well over multi-annual timescales. The continuous GNSS surface observations slightly overestimate the seasonal acceleration, but match the long-term background displacement well. Our borehole deformation data provide novel information on how representative surface velocities are for rock glacier deformation at depth and on various timescales.