| 29 Aug 2025
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.
Thank you for sharing these interesting insights into the kinematics of the Murtèl rock glacier. The variety of processes involved in permafrost creep is impressively demonstrated, and I am sure that this dataset will be a valuable contribution to our understanding of periglacial displacement processes. However, significant improvements are necessary before the work can be published.
You will find many detailed remarks below, many of which can be resolved through more precise and concise writing. However, there are also several issues that require substantial additional work. One of the most important issues is the need for a more careful treatment of the SAA dataset. A general assessment of the challenges and limitations of this measurement system, along with an accuracy analysis, is sorely lacking. Inconsistencies in some of the given values are evident. Several statements in the manuscript appear to be insignificant. Also, the total station data should be reprocessed when analysing the total displacement.
Numerous hypotheses were formulated that could easily be tested, but the study did not do so. This is a pity, as additional work would clearly strengthen your results. (See detailed comments for examples, such as Glens Law, GNSS toppling energy balance...)
Room for improvement is also evident in the discussion of possible processes that cause the observed displacement pattern. As the authors state, many observations are unusual and call for a better physical explanation. Why is there so much deformation in the AL? What is the physical reason? Why is the basal creep component so small? Why does hydrology play a minor role here? You will find some inputs to these questions in the detailed remarks.
On top of it all, this broader discussion of the driving processes could lead to a final, overarching interpretation of your results from the Murtèl site, which I would personally find very interesting. Murtèl is a very famous permafrost study site. How representative is it for rock glaciers in general? Given its massive pure ice core, it is possible that Murtèl is glacier-derived. Perhaps it formed as a result of a single rockfall event onto glacier ice? Does it represent a distinct type of rock glacier? Or is it simply one extreme of a continuum? Should we compare it to other rock glaciers, or is it even justified to compare its dynamics to those of cold glaciers? Have you looked at this?
All the best
Robert Kenner
Detailed comments.
Line 8-13: I see that surface water input does not play a major role for Murtèl, but here in the Abstract I would phrase it differently and would rather refrain from presenting these factors as generally competing. Both are in general highly interdependent. The mentioned studies about water input mainly refer to pore water pressures in the basal shear/creep layer. Here, high pore water pressures are not necessarily related to high surface water input but are also a function of permeability which might be related to temperature in turn. Moreover, high surface water input is often related to a thick winter snow cover and a thick snow cover also reduces winter cooling. It is thus difficult to distinguish these factors on a statistical basis. As an alternative, you would have to talk about the processes, i.e. temperature driven plastic deformation of ice and pore water pressure in a basal creep layer.
Please also clarify what you mean with “temperature” and “warmest summers”. MSAT? MSGT? I guess ground temperature in a depth corresponding to the analyzed deformation process would be appropriate as a reference. If you refer to air temperatures instead, you must explain the physical connection.
Line 63: Temperatures could be important
Line 63: The lack of high temporal resolution.
Line 63: Agreed with reservations. Of course it is important to have more continuous deformation timeseries. However, the existing sporadic measurements already give some indications. They show variability in the basal shear/creep layer in between the measurements and – except the Murtèl case – a constant parabolic deformation above, pointing towards plastic deformation. If you argue with the temperature dependent deformation of ice, the seasonal accelerations at the surface are often much too strong and have the wrong timing to be explained by a power law. See also my comment on line 107.
Line 66/67: “The third aim is to compare the three measurement systems available at Murtèl to monitor surface kinematics: borehole inclinometer, GNSS and geodetic survey using a total station.”
Comparing an inclinometer with a GNSS sensor/Total station sounds a bit odd. Perhaps comparing the surface displacements measured by system xyz.
Line 107: There is also data from Ritigraben, where we did a similar analysis here: doi: 10.1002/ppp.1953
Line 116: ()
Line 118/119: If you have such a good dataset, why don’t you calculate a (winter) surface energy balance instead of working with such statistical approximations?
128/129: Also here exist perhaps better and more reliable methods to calculate precipitation deficits/surplus on different time scales than such a threshold.
135: It is essential to add a paragraph about the accuracy of the Inclinometer somewhere to proof the significance of the results.
144: “This AL-specific deformation increases with time” What does this mean? The displacements increase (what is trivial) or the displacement velocity increases (If yes how strong?)
Line 146/147: That is why it is so important to add a section about the accuracy and diverse challenges and uncertainties when measuring with an inclinometer.
Line 159: “of” instead of “at “
Line 164ff: I daut that this is significant. Do an accuracy analysis.
171: What are daily deformation data? Be precise with “rate”, “deformation”, “velocity”, “strain”… throughout the text. I think you are talking about total deformation per layer here, right? So, it would be best if you define a term such as deformation velocity at the beginning and use it consequently afterwards.
Later you are talking about “daily deformation rate”, what I assume is the deformation velocity of a certain layer on a daily basis? Again, an accuracy analysis is absolutely essential here. If you later calculate the daily deformation velocity changes in the AL, which deforms 26 millimeter per year, this makes on average 70 micrometer displacement per day and of these 70 micrometers per day you calculate seasonal deviations!? I do not believe that your system is that accurate. In contrast, referring to the findings of Buchli et al. we are far of an accuracy that would allow such interpretations: https://doi.org/10.2136/vzj2015.09.0132
Line 175: rewrite to: During the seasonal peaks, the deformation velocity at the surface is about four times higher than in the shear zone.
Line 176: peaks in deformation velocity
Line 177: it will increase the readability and clarity a lot if you stick to a predefined term such as deformation velocity instead of daily deformation rate or other paraphrases
178/179: This is weird. A deformation can’t become negative. A directional displacement can but it won’t happen in this case. What you describe here is a measurement artefact which calls for an accuracy analysis even before presenting any results
Line 180: With 0.25 mm/day deformation velocity you end up with 9.1 cm deformation per year, which deviates strongly from the 5.9 cm given as annual deformation rate. This shows again that an accuracy analysis is very important to distinguish a measurement signal from noise and systematic errors.
Line 180:/181 missplaced and out of context. This is a figure caption.
Line 182: overestimated by which system? What is your reference? And how can you say that one system is more precise than the other?
Line 190 ff: Have you defined warm and cold phases somewhere?
Line 198/199: unclear
Line 206: The discussion about the displacements in the active layer is rather poor. The fact that so much displacement occurs in such flat terrain in such a shallow depth (most likely no high pore pressures) is very remarkable and calls for an explanation. A possible explanation was given in the study of your colleague Marcel Frehner et al., which convincingly explains the formation of ridges and furrows in the active layer on top of the ice core, due to a compressional flow regime. The formation of such a structure is of course a major reason for deformation in the active layer.
Line 214: Vertical strain rate? If I understood right, you measure horizontal strain in a vertical borehole.
Line 233/234: If you bring that into connection here, you should not only cite Glens law but apply it to show, if the implied connection is indeed reasonable. 3.2 mm Y^(-2) is a deformation increase by 10% per year in the ice core. Can this be caused by 0.03°C/y warming?
Table 2: If you like you can also include Ritigraben. See previous comment.
Line 254: Anisotropy is not the right term for what you describe here.
255/266: Unclear, language, reasoning?
258/259: “The shallower shear zones are more likely to be above the depth of zero annual amplitude, and hence experience […] more deformation.” Logic?
271/272: This is more results and misses interpretation. This might be because the ice core is a heat sink and damps the temperature signal in both directions due to its high heat capacity, while the AL is much closer coupled to atmosphere
273ff: As mentioned before, an energy balance would show the effect of snow cover much better than the 70 cm threshold.
277: mild is not a precise adjective here.
293: mild?
322: Melting and not only warming the ice at the bottom of the AL. At one point, the refrozen water (independent if it originates from the snowmelt or from the previous summer) must melt again, if the AL depth stays constant. And as you wrote before, frozen debris with low ice contents deforms slower than ground with high ice contents. Here however, you measure higher strain rates in the ice poor AL than in the ice rich core. The acceleration is thus likely to be triggered by melting of ice in the AL. This melting strongly weakens the shear resistance of the water saturated fine materials at the base of the AL and this is the explanation for the increasing strain rates. The argumentation in this paragraph is very lengthy but does not get to the point. You can shorten this clearly. Be more precise and only show correlations which are relevant.
325: delete these cross-references throughout the paper
336: Another cliff hanger cross-ref. This is obsolete with an accuracy analysis at the beginning.
4.3: It is important that you finally say at least something about challenges of the measurement system. The section is however rather confusing with a lot of speculation. Since this dataset is the core of your paper, this section must be transformed into an extensive analysis of the measurement system, including its accuracy and limitations. Therefore, a literature review is necessary and this section must be placed before the result section and not in the discussion. All results must be reconsidered under the light of this evaluation of the measurement system.
4.4: This is all very descriptive and speculative. Why do you suggest correcting the rotational component in future studies but don’t do it yourself? You could then easily say if tilting is the main reason for the observed differences between GNSS and SAA displacements!
Good that you have summed up the daily GNSS displacements. I have not checked in Cicoira how GNSS was processed but I am sure velocities are somehow calculated over a longer basis than daily. Otherwise the difference between the sum of daily displacements and total displacement would have been more than 11cm. It is impossible to calculate significant daily velocities using GNSS. GNSS can’t measure submillimeters, your device not even subcentimeters. However, the same applies to SAA!
At least we know that GNSS can measure the total displacement over the entire monitoring period accurate to a (few) centimeter. So 101 cm (102 cm in figure 8, what is correct??) total displacement is the benchmark!
89 cm for the total station (~13cm error) reflect the sum of annual errors. You have to recalculate the total displacement from the raw coordinates instead of summing up the annual observations. This gives a horrible error propagation. The fact that GNSS and the surveyed total station prism were mounted on the same boulder, strongly indicates that the 13 cm are indeed a measurement error.
The 87 cm for the SSA (~15cm error) total displacement perhaps reflect the accuracy of the SAA if rotation of the GNSS wasn’t particularly strong and gives an alternative explanation for the difference.
Line 381: Noise filtering should decrease velocity peaks (smoothing) but you are claiming the opposite here!?
Conclusions
Must be rewritten after revision. Total station & SAA match well but are probably similar inaccurate.