the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Feb 2026
Surface icequakes and basal stick-slip events reveal daily grounding line migration and seawater intrusion at a marine-terminating glacier in East Antarctica
Abstract. As they reach the ocean, Antarctic outlet glaciers transition from grounded to floating at their so-called grounding lines (GL). This transition is known to be mechanically controlled by tides, which induce ice flexure visible at the surface from satellite and ground geodesy and often used as a proxy for grounding line position. Here, we use a dense seismic node array to study the spatial and temporal dynamics of surface, crevasse-induced icequake activity and basal, sliding-induced seismicity at the grounding zone of the Astrolabe Glacier, a fast-moving outlet glacier in East Antarctica. We observe that surface icequakes mimic the expected, tide-induced, ice flexure pattern, as they delineate the grounding line position inferred from previous geodetic studies, and migrate landward as tides rise. We show, however, that the mechanical grounded to floating transition is better evidenced by the spatial distribution of basal sliding-induced stick-slip events, occurring on a limited number of clusters and which depict a grounding line position that is offset inland compared to that identified from the surface. These basal events undergo tidally-driven cycles of activation and de-activation, consistent with sea water intrusion inland over at least 3 kilometers at high tides. Following these results, we propose that the monitoring of stick-slip events could be used as the most accurate means of tracking grounding line retreat over long timescales.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-76', Nathan Stevens, 13 Apr 2026
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AC1: 'Reply on RC1', Tifenn Le Bris, 21 Jun 2026
Dear Nathan Stevens, thank you for reviewing and commenting on this manuscript. We took notice of the general and specific comments and found them pertinent and helpful to improve the scientific quality of the manuscript. We agree that a better delineation between Le Bris et al. (2025) and this paper should be done in order to better focus on new findings, particularly those related to basal stick-slip events. We are also aware that we do not fully dive into the source processes of these basal events. This is mainly because we lack constraints to confidently characterize source mechanisms. However, we integrated more references on this to discuss the physics behind these source processes. Please, find below each of your comments, answers and details on how we are modifying the manuscript accordingly (preceded by '>>' and in bold and italic).
Specific Comments
Greater delineation from Le Bris et al. (2025)
Large portions of this manuscript reiterate content presented in Le Bris et al. (2025) that can be effectively covered by a citation. For example:
- lines 136–140. Are these (shallow) ice-quake detections not the same ones characterized in Le Bris et al. (2025)? As written, it suggests these are previously unpublished results.
- Lines 206–238: How much of this analysis is new relative to results published in Le Bris et al. (2025)? In cross-referencing with your earlier work, it seems like it is only the kriging?
>> We agree with the reviewer that we should make a clearer distinction between results from Le Bris et al., 2025 and the present results. We went through the content and identified which paragraphs required deeper explanation and which can be covered simply by quoting the source. The method section 3.1 has been modified to specify that the detection process and occurrence rate computation have been carried out in Le Bris et al. (2025) and that the new methodologic addition starts with the kriging. We overall tried to find the right balance between referencing and repeating some methodological aspects to make the text easier to understand.
Additional cryoseismology & sliding stability references
The broader contextualization of observations and interpretations in this manuscript are hampered by a lack of a comprehensive grasp of the cryoseismology and basal processes literature of the last decade. I know the assembled author list is very much in-tune with this evolving body of work, and I find it troubling that this is not reflected in the writing and referencing within the submitted manuscript. As such, I have included a reasonably scoped set of references that should be considered foremost for their added context, and subsequently as cited references should their content be illuminating.
Foremost, major cryoseismology reviews are not referenced (Aster & Winberry, 2017; Podolskiy & Walter, 2016). I note relevant segments of these works in my comments below.
There is a growing numerical, experimental, and observational literature constraining oscillatory loading effects on sliding stability for both ice-on-rock (De Diego et al., 2022; Helanow et al., 2021; McCarthy et al., 2022; Skarbek et al., 2022; Stevens et al., 2025; Lucas K. Zoet et al., 2021) and ice-on-till (Hansen et al., 2024; Morgan‐Witts et al., 2025; Warburton et al., 2023; Lucas K. Zoet & Iverson, 2020). This body of work extends into the seismic expression of slip stability, which includes several references cited in this manuscript. The authors might additionally consider findings in Lipovsky & Dunham, (2016), Lipovsky et al. (2019), Zoet et al., 2020), and Stevens et al. (2024).
Disclosure: I authored two suggested references (i.e., Stevens et al., 2024; 2025). For these works, I emphasize that they should only be included as references if the authors decide their content are relevant to this manuscript.
>> Thank you for the suggested references. In the revised version of the manuscript, we addressed this issue. We improved the general introductive view concerning cryoseismic events and the related literature. We provided more references and more context in the discussion, in order to have a more comprehensive description of cryoseismic and basal processes in particular.
Confounding use of the term "icequake"
“Icequake” is a generalized term for wavefield excitation caused by glaciologic process. For the sake of clarity throughout the manuscript, I strongly suggest including modifiers upon “icequake” to specify association to source regions and/or specific processes. E.g., shallow icequakes, basal icequakes, englacial icequakes, crevasse-sourced icequakes, slip-sourced icequakes. Podolskiy and Walter (2016) and Aster and Winberry (2017) provide a reasonable framework for event classification nomenclature.
Clarifying the source regions and underlying processes throughout the manuscript will help to better convey the noteworthy insights on grounding line dynamics from both shallow/ice-fracture-sourced and deep/slip-sourced icequakes. For example, take lines 151–152. It might be clarified as “Basal stick-slip events are pervasively present in the seismic data but have amplitudes [generally] three times smaller than ice-fracture-generated events.”
>> We agree on the importance of being explicit concerning the origin of the events. We modified the manuscript accordingly in order to be more exhaustive in the naming of the cryoseismic signals by including ‘surface’ or ‘crevasse-induced’ or ‘ice-fracture-generated’ upon “icequake” in sentences where it was particularly relevant to specify it (e.g., line 162, 224, 305, 400, 451).
Consideration for till
Much of the interpretation for basal icequakes’ source processes and mechanics in this manuscript are underpinned by the assumption that seismogenic slip is occurring on an ice-bedrock interface (e.g., lines 46–49). There is abundant evidence that tidewater/outlet glaciers and ice-streams are till-bedded, and regularly form grounding zone wedges (e.g., Aitken et al., 2023; Simkins et al., 2017). Lipovsky and Dunham (2016) and Lipovsky et al. (2019) provide a compelling treatment of seismogenic sliding for ice-on-till systems that strikes me as relevant for this setting and observations. Give attention to Figures 4 and 5 in Lipovsky et al. (2019), related text, equations, and citations.
>> We are aware that the nature of the substrate (whether till or hard bedrock) alters the properties of the seismic signal and its propagation signature. However, this potentially goes beyond the scope of the article. Indeed, it does not affect how we examine the basal events (spatial and temporal characterization). This will be an area for improvement in the future, particularly if we wish to discuss and study in greater detail the processes underlying basal events (wave propagation, reflection coefficient, source mechanism…). Ongoing work on noise correlation should provide vertical seismic profiles and therefore constraints on the nature of the subglacial layers, but the results are not yet available. We added the following sentence in the discussion (line 365) to acknowledge this point: “The nature of the substrate, whether deformable till or hard bedrock, influences the seismogenic sliding properties and therefore affects the mechanisms of shear stress accumulation and release (Lipovsky & Dunham, 2016; Lipovsky, 2019). Ongoing work seeks to better constrain the basal interface conditions beneath the glacier, which will be essential for a more detailed interpretation of seismic source processes and for characterizing patterns of stress release.”
Consideration for seismogenic slip mechanics
The ability for an asperity (a zone of ice-bed contact in this case) to accumulate and release strain seismogenically is modulated by the interplay of material properties of the interface, effective contact stresses, and loading rates (slip velocity). Figure 5b-c indicate higher rates of basal icequakes as tides rise, which is consistent with higher localized stresses on basal asperities. This warrants further discussion when presenting these findings (c. lines 300–301), and in their interpretation (lines 350–364). Tidal forcing modulates both slip velocities and subglacial water pressures, and relative timing and influence of each in the context of observed basal seismicity should be discussed.
>> We agree that we didn’t put enough focus on discussing slip mechanisms, stress release, and strain accommodation at the base of the glacier. This is a particularly important point that we intend to highlight further in the results and discussion sections. In the revised version, we added a graph to Figure 5 showing vertical and horizontal surface ice velocities issued from GNSS stations to complete the analysis of seismic activity during tidal cycles. We also developed the discussion regarding variations in stress load and seismic release of deformation above the asperities during tidal cycles, as well as how parameters such as water pressure, slip rates, and asperity properties are interconnected and how this influences the occurrence of basal seismicity.
Advocacy for splitting clusters in analysis
The presented results provide a rich catalog of icequake activity from shallow and basal sources, which are admirably located and characterized given the inescapable limits of characterizing seismicity outside an array’s footprint. I am disappointed that the analysis in the submitted manuscript so abruptly lumps all seismogenic activity spatially given the additional rich context of geophysical constraints on basal topography, ice flow field, and tidal forcing (couched in the “statistical approach” of line 142).
>> We understand the frustration regarding the lack of more precise spatial constraints on seismic activity, as we too would have liked to have access to them. Concerning the surface icequake locations, we have located a number of icequakes (the largest), but due to the very low magnitude of most of them, we lose the consistency of waveforms between seismic stations, which prevents any precise localization. We therefore introduced this “statistical approach” rather than providing locations with significant uncertainties that could potentially mislead interpretations. The ice flow field is well known, and we incorporated it in Figure 5 as mentioned in the previous comment. On the other hand, we are limited by the poor definition of the basal topography, which is highly uncertain. Finally, we believe we have explored tidal forcing and its impact on seismic activity in great detail, which is in fact the core of the study.
Focal mechanisms
Generalization of a single focal mechanism for all basal icequake clusters does a disservice to the quality of observational data presented in this manuscript. The presence of a transverse-to-ice-flow component of slip is likely due to ice convergence across Astrolabe Glacier’s shear margin, which the authors note is the primary source for observed basal seismicity (i.e., remarks on lines 285–286). Figure 4b might be better clarified by separating first-motions observations into three bins: eastern shear-margin, main-trunk, western shear-margin. If the latter two bins do not show significant double-couple patterning, consider adding them to the supplement and clarify in the main text that the first-motions stereonet is sourced from the eastern shear-margin.
I strongly disagree with the assumption stated in lines 322–323. Focal mechanisms should vary across the array as a function of subglacial topography (fault orientation) and basal slip velocity vector field (slip direction/rake on the basal fault). There are sufficient independent constraints on basal topography (e.g., Le Meur et al., 2014) and flow field (Provost et al., 2024) to place constraints on fitting slip- and auxiliary-planes to first motions observations for individual (or at least physiographically lumped) clusters.
>> We acknowledge that assuming a single focal mechanism is a strong generalization. It might indeed be worthwhile to present three of them depending on their locations (eastern margin, main trunk, and eastern margin). We therefore modified Figure 4 to show 3 focal mechanisms, one for each area. We agree on the fact that these focal mechanisms should depend on the fault geometry and orientation. At the end, they exhibit the same pattern: double-couple patterns with a horizontal slip plane. In the revised manuscript, we switched to a region-by-region analysis but we are still limited by the absence of well known topography of the bedrock to discuss in more detail the source processes of these stick-slips. The paper of Le Meur 2014 provides indeed only sparse radar profiles. We are also revising lines 322–323 to remove the stated assumption.
Technical Corrections
>> We have considered all these technical corrections. We added comments where some were necessary. If there is no comment, it means that we took it into consideration and made the modification accordingly.
A (very) recent publication that may be of relevance for manuscript revisions: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2025JF008509
https://apis.ebsco.com/public/linkout/v1/ftf?ref=e7050c8b-7abd-4748-b40a-fe491c87d2ec&id=229668
Line 44: Remove “indeed”
Line 69: “e.g.” is missing a trailing “,” – check throughout manuscript: I.e., “i.e.,” “e.g.,”.
Line 70: Consider citing (Mikesell et al., 2012).
Line 79: Should probably include Hudson et al. (2023) here?
Lines 114–115: use superscript for ordinals, remove leading “0” from “02nd Feb. 2023”.
Line 144: affects -> effects
Line 158: Please be specific here, or in the supplement, regarding the waveform template (pre)processing pipeline. Waveform template-based detection is highly sensitive to preprocessing decisions (Chamberlain et al., 2017).
>> Templates are simply processed and trimmed using Obspy. A frequency band filter is applied to have clear P- and S- waves and we take a 0.5s window around the P wave. We added this information in the text, line 160.
Line 159: template matching detection (not a proper noun).
Line 185: “perform a grid search in six dimensions” – cleaner idiomatic English.
Lines 185–187: (COMMENT) This feels like a lot of bespoke code and methodologic development that could have been accomplished by playing around with NonLinLoc (Lomax et al., 2000), with particular attention to the LOCGAU (generalized observation uncertainties) and LOCGAU2 (generalized velocity model uncertainty) control statements, and some reasonable estimation of arrival time uncertainties from all the hard-won manual picking of events.
>> We are aware of the power of NonLinLoc, but given the simplicity of the system and the poor seismic velocity constraints, we preferred to be able to explore the parameters more broadly. Indeed, since we don’t have many constraints on the seismic velocities in the glacier ice, it is thus easier to use a grid search to explore velocities across a wide range. We agree that there could be more uncertainties, but we ended up with a good location estimate and comprehensive results. In the future, when we should have more information to better constrain the velocity and geometry models, we may for sure return to NonLinLoc technique to provide uncertainty estimates for the velocity models, etc.
Line 246: “seismically active gliding” -> “seismogenic sliding”
Figure 5 – the changing association of event type to colors throughout this figure make it very hard to follow. Please consider using a single color for each of the following throughout all five sub-figures: tide amplitude, basal icequakes, shallow icequakes, surface velocity.
Line 403 – Remove extraneous parentheses around Pirli et al., 2018.
Line 404–410: Maintaining year-round seismic arrays in polar settings is cost and labor intensive, even for a few seismic sensors close to an established base. As demonstrated here, and in earlier works (Kohler et al., 2019; Mikesell et al., 2012; Vore et al., 2019), the temporal resolution provided by seismic monitoring is a critical contribution to multi-method observation of grounding zones.
>> We modulated the difficulty of maintaining permanent observations on an Antarctic glacier. We are indeed aware of the difficulties of getting continuous time series in such harsh environments but we are currently in discussion with our polar agency to develop such year-long observations on the Astrolabe Glacier. In the revised manuscript, we changed the last sentence to : “However, while it is an interesting prospect, maintaining a permanent seismic array at the grounding zone remains challenging due to logistical constraints, harsh polar conditions, and the need for regular servicing.”
Citation: https://doi.org/10.5194/egusphere-2026-76-AC1
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AC1: 'Reply on RC1', Tifenn Le Bris, 21 Jun 2026
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RC2: 'Comment on egusphere-2026-76', Meghan Sharp & Erin Pettit (co-review team), 15 Apr 2026
Advisor/ ECR co-review: Dr. Erin C Pettit & Meghan A. Sharp, MSc.
General Comments
This paper does a really nice job of presenting a study of grounding zone dynamics from the perspective of a passive seismic network. The authors aim to improve our understanding of grounding zone dynamics by showing that stick slip behavior at the ice-bed interface occurs much farther upstream (as much as 3km in this case), which they argue results from seawater intrusions during rising tides. These observations suggest that the grounding zone behavior initiates farther upstream than the standard flexure-derived grounding zone definition. As grounding zones are notoriously difficult to delineate, track, and model, this paper provides a significant contribution to the field of glaciology within the scope of TC.
This paper is well written and provides a nice analysis of the data set. We have a few specific comments and then some technical suggestions for improving the manuscript.
Specific Comments1. Longitudinal coupling: The abstract and main message in the paper suggest that these events are the "most accurate means" of tracking grounding line retreat. That is a strong statement and while we appreciate that these events likely relate to the most upstream effects of tidally modulated basal slip, we question if this is really the best definition of "grounding zone" for most uses. Studies have shown tidal influence on glaciers quite a ways up, especially for soft bedded glaciers, and the stick slip behavior in many of these cases does not necessarily mean the ice has experienced local intrusion of water or is near enough floatation to experience tidal flexure commonly associated with grounding zone behavior. Importantly, upstream stickslip behaviour could be modulated by downstream intrusions of seawater due to longitudinal coupling within the ice.. Defining a grounding zone based on stick slip behavior may be useful in some cases, but to generalize this, a discussion of longitudinal coupling length and its variations for different glaciers (e.g. colder undamaged ice has longer coupling lengths) should be included. We understand and appreciate the value of detecting migration of the upstream effects of the tides using this technique, but we think the overall value of this paper does not need to hang on such strong universal statements about how we define the grounding zone.
2. Grounding Zone assumptions: When comparing the geodetic flexural GL to the seismic stick-slip mechanical GL, this study implicitly makes the assumption that the GZ remained in a constant location between 2014 and the study period (2023). We’d like to see more of a discussion on how realistic that assumption is based on existing literature on changing ocean conditions in this region (if any), and your result of the icequake flexural GL. We think the icequake patterns back-up this assumption as they appear to match the geodetic flexural GL position, suggesting the offset with the stick-slip mechanical GL must be due to the difference in definition/ method, but it is not stated or discussed explicitly in the manuscript.
3. Other relevant data - We would like to see some of the relevant other data sets integrated more seamlessly. Comparing results to glacier velocity and acceleration, for example, plotted in other papers puts the onus on the reader to retrieve those other papers, when the key data could just be plotted along with the results from the seismic network. Specifically, velocity or acceleration that were discussed in the paper.
4. For an audience of cryosphere scientists, all encompassing, we think the text in the introduction/ methods section would greatly benefit from an additional figure showing the difference between an icequake vs basal stick-slip waveform (e.g. an example waveform from each of your catalogs).
5. Structure & framing of paper: We would argue that maintaining even a small network of seismometers is not a “low cost, low maintenance” tool for most of Antarctica, a low-maintenance system would require a sophisticated system that could telemeter at high data rates even after being buried in the snow or do onboard processing. A low-cost system would likely have to be visited every year or multiple times a year to download data (which means high logistics costs). We don’t mean to dismiss the value of having seismometers monitoring changes in basal conditions, we’d just suggest the authors be careful to overstate the ease of this in the future. We don’t think the value of this paper needs to hang on this being a possible low-cost, low-maintenance method. It is a hugely valuable method for learning about the dynamics of a grounding zone!
Additionally, a suggestion to improve the significance of your work: As this paper is framed as a novel method for delineating GZs, but it seems like the spatial coverage of the seismic network in this study was too small to observe the whole range of seawater intrusions, can you comment in your discussion on some recommendations for how a future seismic network should be designed given observations that can be made remotely, beforehand? We recognize that this specific question is out of the scope of the study, and are not asking for a detailed network design, rather a few sentences on how you would re-design the array to be more likely to capture the entire range of GZ dynamics, if you had the chance to repeat this study?
As per the overall structure of the manuscript itself, there is a significant portion of the methodology described within the results, particularly the stick-slip results sections. For clearer flow of the paper, we recommend separating these and putting them into the methods sectionTechnical Corrections
Title
We would suggest changing “daily grounding line migration” to “tidal grounding line migration”, or include the tidal aspect in some way as it was a significant component of this study.
Abstract
The abstract is clear, concise, well-written, and highlights the key results of this study. However, the strong statement at the end may not be true.
Introduction
The introduction identifies a clear knowledge gap to motivate the study. However, we suggest considering a broader perspective on citations. Nearly all of the citations in the introduction are from the last decade (and even less), which would lead a reader to believe that research into grounding line dynamics and related seismicity is a new area of study. While methods and capabilities have evolved over the years, I’d suggest the authors situate their investigation in the whole history of the field of research.
Specifically, the authors cite Freer et al 2023 and Rignot et al 2024 in a sentence related to the definition of the grounding zone, we do not believe they the first or most important papers to define the grounding zone, which is what this sentence is about. Those paper did offer new insight into grounding zones, and if that is the goal in citing them, then state what new insights they provided, rather than giving them credit for defining the grounding zone. Similarly, Christman et al 2021 and Yan et al 2024 were not the first to show how basal sliding is modulated by coupling at the ice/bed interface; yet that is the sentence in which they are cited. We would suggest being more specific about what those papers contributed and/or include some of the earlier papers to study ice/bed coupling. Please review all citations to ensure that the paper better places the work, methods, and concepts in the existing body of research. .
Line 35 - I’m not sure that the acronyms GL and GZ are really that necessary for such simple phrases. They do not necessarily make the paper easier to read, and we do not believe this paper is subject to such strict word limits that writing out “grounding line” is prohibitive.
Line 45 - the word “indeed” is not necessary and this sentence is long, we suggest breaking it up.
Line 50 - The last sentence in this paragraph needs a citation.
Line 90 - Just a slight confusion on our part: if the fjord walls are only 500-600m deep, how is the floating glacier ice able to be 700m thick?
Methods
Paragraph 135 & 155: Please add a citation for your description of icequake/ stick-slip events (i.e. typical frequency, P&S characterization, etc.)
Line 136-137: Can you summarize the key components of the Le Bris et al (2025) method? I.e. What is special about the Le Bris 2025 method compared to a standard STA/LTA algorithm? If there is some important information from that paper, it would be helpful for the reader to just state what that important information is so the reader doesn’t need to go to LeBris 2025 to understand the method.
How do the thresholds compare to the environmental noise? Does the noise vary in space or time? If these events are near the noise threshold, how do you know you are not biasing the event catalogue because of variable environmental background noise (e.g. Carr et al 2020).
Line 145: It might be helpful when you discuss averaging over 24 hours to specifically state that you center each diurnal tidal signal on the high tide, so it is clear you are averaging 7 days of data, but in hours relative to each high tide time.
Line 170: Similar to the comment above, how does your detection strategy avoid bias with variable environmental background noise?
Results
Throughout the results, we were quite confused as to the geographic references. It looks to me like the ground versus floating are West and East (mostly), respectively and that the shear margins are north and south, but the text is not consistent (e.g. see Line 250 comment below)
Line 212: Be specific on which grounding line (i.e. the geodetic one) these icequake locations are consistent with
Line 245: “The striking feature is that all the detected clusters are located at the periphery or outside of the network with no event detected beneath it”. This is a key result, but we find the wording unclear. Figure 4 shows some events within the network, but we agree that they are on the periphery. Consider rewording for clarity of one of the main messages in this paper: e.g. “directly beneath the instruments, except for in the northern corner”
Line 250: Here is an example of reference to a “east” shear margin, but the map to us looks like shear margins are south and north.
Could you explain more what you mean clusters being “aligned” along or across glacier flow? How do you define alignment and how well do you know what this “alignment” is?
Line 255: Please discuss the depth uncertainty more. What do you expect to be causing seismic velocities outside those of normal ice? Is it taking different pathways?
Line 262: “but has no impact on the following of the discussion” should read “but has no impact on our interpretation”
Line 275: “and then quantify four key parameters variation” should read “and then quantify the variation of four key parameters”?
Line 326: “This coherent pattern confirms our initial hypothesis that all stick-slip clusters share a similar focal mechanism”: this statement requires more explanation on why it confirms your hypothesis
Line 279: If it really is clear, you shouldn’t have to say “clearly.” We suggest just deleting that word.
Line 280-300: We’d suggest breaking this paragraph up, the interevent time on line 289 is a new topic. And the event amplitude on line 294 is a new topic.
Discussion
Line 332: Are you assuming that all of the events that you categorize as icequakes are surface icequakes, or are you actually estimating their depth before categorizing them this way? Do you also see basal crevassing?
Line 348: Line “… surface flexure is a proxy that may sign with some offset with the processes…”: We find the wording “may sign with” unclear. Consider changing to “may depend on”, “likely depends on”, or some re-phrasing of your choice.
Line 353: “true mechanical grounding line positions” what makes it “true”? See the “general comment” above about the role of longitudinal coupling length. If the authors wish to argue that the grounding zone starts at the most upstream effect of the ocean including the ice experiencing indirect effects through longitudinal coupling, then a brief discussion of this, and the controls on longitudinal coupling length for this glacier, is needed.
Line 375: figure reference should be “Figure 6”, not “Figure 6a”
Lines 406 & 407: For sentence clarity/ flow, change “little” to “few”
Line 408: line “The eventual disappearance of stick-slip clusters could sign a grounding line retreat…”: change “sign” to “signal”
Conclusion
Line 424: The word “clear” is not necessary.
Figures
Figure 1:
- The font of DDU and DUMG is difficult to read
- Can you plot the GNSS stations on panel c for context relative to the seismometers?
- Having a coordinate reference frame in this figure would be helpful.
- Labeling the GPS sites would be helpful. We realize they are labeled in figure 3, but since the nodes are labeled in figure 1 it would make sense to have the GPS labeled as well.
Figure 3:- We suggest swapping the location of the colour bars and panel (g) in this figure. It isn’t immediately clear which colour bars correspond to which figures.
- Can you label what you are interpreting as the minimum range of seawater intrusions?
Figure 4: Need a legend item for the black circles
Figure 5:- Panel c: Delete the word “cumulative” as this isn’t cumulative event count but rather, event count in 30min bins
- We would like to see glacier velocity/acceleration shown alongside these data.
Figure 6:- Indicate in the caption that the hypothetical seismic event sources are coloured by their occurrence within the tidal cycle
Concluding Remarks
Overall, we really enjoyed this paper and think it is a nice contribution to The Crysosphere. We recommend it for publication with the minor edits suggested above.
Review References:Carr CG, Carmichael JD, Pettit EC, Truffer M. The influence of environmental microseismicity on detection and interpretation of small-magnitude events in a polar glacier setting. Journal of Glaciology. 2020;66(259):790-806. doi:10.1017/jog.2020.48
Citation: https://doi.org/10.5194/egusphere-2026-76-RC2 -
AC2: 'Reply on RC2', Tifenn Le Bris, 21 Jun 2026
Dear Dr. Erin C Pettit & Meghan A. Sharp, thank you for reviewing and commenting on this manuscript. Please, find below each of your comments, answers and details on how we took your comments into consideration and are modifying the manuscript accordingly (preceded by '>>', written in bold and italic).
Specific Comments
- Longitudinal coupling: The abstract and main message in the paper suggest that these events are the "most accurate means" of tracking grounding line retreat. That is a strong statement and while we appreciate that these events likely relate to the most upstream effects of tidally modulated basal slip, we question if this is really the best definition of "grounding zone" for most uses. Studies have shown tidal influence on glaciers quite a ways up, especially for soft bedded glaciers, and the stick slip behavior in many of these cases does not necessarily mean the ice has experienced local intrusion of water or is near enough floatation to experience tidal flexure commonly associated with grounding zone behavior. Importantly, upstreamstickslip behaviour could be modulated by downstreamintrusions of seawater due to longitudinal coupling within the ice.. Defining a grounding zone based on stick slip behavior may be useful in some cases, but to generalize this, a discussion of longitudinal coupling length and its variations for different glaciers (e.g. colder undamaged ice has longer coupling lengths) should be included. We understand and appreciate the value of detecting migration of the upstream effects of the tides using this technique, but we think the overall value of this paper does not need to hang on such strong universal statements about how we define the grounding zone.
>> Thank you for this comment. We agree that the statement has been worded too strongly considering what we are observing in terms of stick slip distribution and activation patterns. They reflect changes in the contact state between the ice and the bedrock which could result from stress load variations due to downstream seawater intrusion modifying the equilibrium of the system further upstream. Following your comment, we mitigated our statement and better emphasized the longitudinal coupling state and stress load evolution during tidal cycles rather than proposing a straightforward grounding zone outline and seawater intrusion. We changed the wording of these statements in the abstract (e.g., “These basal events undergo tidally-driven cycles of activation and de-activation, consistent with stress load variations at the base of the glacier over at least 3 kilometers at high tides”). We also removed in the title the notion of “seawater intrusion”. Finally, we emphasized the aspect of stress variations at the ice-bedrock interface during tidal cycles in the discussion (section constraining grounding zone dynamics).
- Grounding Zone assumptions: When comparing the geodetic flexural GL to the seismic stick-slip mechanical GL, this study implicitly makes the assumption that the GZ remained in a constant location between 2014 and the study period (2023). We’d like to see more of a discussion on how realistic that assumption is based on existing literature on changing ocean conditions in this region (if any), and your result of the icequake flexural GL. We think the icequake patterns back-up this assumption as they appear to match the geodetic flexural GL position, suggesting the offset with the stick-slip mechanical GL must be due to the difference in definition/ method, but it is not stated or discussed explicitly in the manuscript.
>> This is a very interesting point which we were aware of, but indeed did not discuss enough in the paper. We are aware that the geodetic positioning of the grounding line is based on a study from 2014 and therefore one could expect changes in its position during the past decade. This is a limitation that we need to acknowledge more clearly in the discussion and we have taken that into account. We also updated our results with complementary observations. From the time of the submission of this paper, we have now been progressing on using InSAR to access estimated positions of the grounding line to compare with GNSS and seismic data. It first gives us a more up to date position of the grounding line compared to 2014. We further show that InSAR remote observation provides complementary and very consistent constraints compared to our seismic observations. We observe a good match between position of the stick slip clusters and the grounding line based on InSAR data. The added value of looking at basal seismicity is that we also investigate the ice-bedrock interface state and the variation in stress loading across tidal cycles and the migration of the grounding zone during tidal cycles due to changes of the stress state at the base. All in all, we wanted to better emphasize these (new) points in the discussion and have modified this section accordingly by comparing the GNSS, seismic and InSAR data (in the section named “Delineating the grounding zone based on stick-slip clusters”).
- Other relevant data - We would like to see some of the relevant other data sets integrated more seamlessly. Comparing results to glacier velocity and acceleration, for example, plotted in other papers puts the onus on the reader to retrieve those other papers, when the key data could just be plotted along with the results from the seismic network. Specifically, velocity or acceleration that were discussed in the paper.
>> We understand the need of incorporating other datasets into our results without referencing them to external sources. We have directly taken your comment into account and decided to include information on velocity (vertical and horizontal surface velocities, relative to tidal cycles) by adding a panel with these timeseries in Figure 5. In the ‘Discussion’ section, we discussed the interconnection between stress loading, glacier velocity and basal stick slip occurrence.
- For an audience of cryosphere scientists, all encompassing, we think the text in the introduction/ methods section would greatly benefit from an additional figure showing the difference between an icequake vs basal stick-slip waveform (e.g. an example waveform from each of your catalogs).
>> We agree with your comment to show the seismic waveforms of icequakes and stick slip events to better understand and visualize their characteristics and differences. In the revised manuscript, we added two sub figures in Fig1. to display their respective waveforms.
- Structure & framing of paper: We would argue that maintaining even a small network of seismometers is not a “low cost, low maintenance” tool for most of Antarctica, a low-maintenance system would require a sophisticated system that could telemeter at high data rates even after being buried in the snow or do onboard processing. A low-cost system would likely have to be visited every year or multiple times a year to download data (which means high logistics costs). We don’t mean to dismiss the value of having seismometers monitoring changes in basal conditions, we’d just suggest the authors be careful to overstate the ease of this in the future. We don’t think the value of this paper needs to hang on this being a possible low-cost, low-maintenance method. It is a hugely valuable method for learning about the dynamics of a grounding zone!
>> Having read these sentences again, we agree that we may have overstated how easy it would be to set up a small network of seismometers and need to be more careful with our statement and the wording. It would be relatively easy in the sense that, on a glacier such as the Astrolabe Glacier, close to a permanent scientific base, an annual visit could be feasible since it is part of an already existing GNSS and glaciology observatory and is therefore visited every year. But that is very specific to the Astrolabe Glacier. There are indeed usually many challenges regarding servicing, powering and maintenance due to snow accumulation and so on. We toned down this statement in our revised version: Line 410; “However, while it is an interesting prospect, maintaining a permanent seismic array at the grounding zone remains challenging due to logistical constraints, harsh polar conditions, and the need for regular servicing.”
Additionally, a suggestion to improve the significance of your work: As this paper is framed as a novel method for delineating GZs, but it seems like the spatial coverage of the seismic network in this study was too small to observe the whole range of seawater intrusions, can you comment in your discussion on some recommendations for how a future seismic network should be designed given observations that can be made remotely, beforehand? We recognize that this specific question is out of the scope of the study, and are not asking for a detailed network design, rather a few sentences on how you would re-design the array to be more likely to capture the entire range of GZ dynamics, if you had the chance to repeat this study?>> Yes, we agree that it is important to acknowledge the limitations of this study in terms of deployment and network extent, and we discussed further how the design of the seismic network could be better planned using remote data (which we will actually show since we will now include InSAR data in the paper), whilst taking into account constraints regarding the number of stations, spacing, extent of the area of interest etc. We added a few sentences on this point: “[...] The limited spatial coverage of our seismic array prevents us from assessing how far upstream stick-slip activity extends and whether it remains influenced by tidal forcing away from the grounding zone. Resolving these questions would improve constraints on the spatial extent of tidally induced stress variations at the ice-bedrock interface and contribute to better capture the entire range of grounding zone dynamics. Future field campaigns could benefit from deploying several small-aperture arrays distributed across the glacier. Such a network design would optimize the trade-off between precise event localization, through short inter-station spacing within each array, and a broader spatial coverage, without requiring additional stations.”.
As per the overall structure of the manuscript itself, there is a significant portion of the methodology described within the results, particularly the stick-slip results sections. For clearer flow of the paper, we recommend separating these and putting them into the methods section
>> In the revised version, we moved sentences from the stick-slip occurrence and the focal mechanism paragraphs to the Methods section.
Technical Corrections
>> We answered the comments/ corrections where an explanation was useful and/ or necessary. If there is no comment, it means we agree with the correction and make the modification in the revised manuscript.
Title
We would suggest changing “daily grounding line migration” to “tidal grounding line migration”, or include the tidal aspect in some way as it was a significant component of this study.
>> We modified the title to make sure we emphasize the tidal aspect that is indeed central to the paper: “[...]n Tidally driven grounding line migration”
Abstract
The abstract is clear, concise, well-written, and highlights the key results of this study. However, the strong statement at the end may not be true.
>> Thank you, as mentioned previously, we modified the latest sentence, to mitigate the statement that is too strong: “Following these results, we propose that the monitoring of stick-slip events could be used as an accurate means of tracking grounding line retreat over long timescales.”
Introduction
The introduction identifies a clear knowledge gap to motivate the study. However, we suggest considering a broader perspective on citations. Nearly all of the citations in the introduction are from the last decade (and even less), which would lead a reader to believe that research into grounding line dynamics and related seismicity is a new area of study. While methods and capabilities have evolved over the years, I’d suggest the authors situate their investigation in the whole history of the field of research.
Specifically, the authors cite Freer et al 2023 and Rignot et al 2024 in a sentence related to the definition of the grounding zone, we do not believe they the first or most important papers to define the grounding zone, which is what this sentence is about. Those papers did offer new insight into grounding zones, and if that is the goal in citing them, then state what new insights they provided, rather than giving them credit for defining the grounding zone. Similarly, Christman et al 2021 and Yan et al 2024 were not the first to show how basal sliding is modulated by coupling at the ice/bed interface; yet that is the sentence in which they are cited. We would suggest being more specific about what those papers contributed and/or include some of the earlier papers to study ice/bed coupling. Please review all citations to ensure that the paper better places the work, methods, and concepts in the existing body of research. .
>> We acknowledge that some citations could be misleading in believing these papers are defining concepts instead of bringing new insights to concepts previously presented in earlier papers. For instance, for Freer et al. it is not placed at the right place in the sentence. Since this comment is also pointed out by the other reviewer, we carefully reviewed the dedicated literature and our citations to be more specific about their original contributions and ensure we give credit for definitions correctly.
Line 35 - I’m not sure that the acronyms GL and GZ are really that necessary for such simple phrases. They do not necessarily make the paper easier to read, and we do not believe this paper is subject to such strict word limits that writing out “grounding line” is prohibitive.
Line 45 - the word “indeed” is not necessary and this sentence is long, we suggest breaking it up.
Line 50 - The last sentence in this paragraph needs a citation.
>> We added Dutrieux et al., 2014 and Rintoul et al., 2016 as citations for this last sentence.
Line 90 - Just a slight confusion on our part: if the fjord walls are only 500-600m deep, how is the floating glacier ice able to be 700m thick?
>> We believe the confusion comes from our original description of the glacier geometry, which was not sufficiently clear. Ice thickness varies from approximately 700 m at the grounding zone to about 300 m at the glacier terminus. At the grounding zone, the glacier is laterally confined within a glacial valley whose rock walls are roughly 500-600 m and are locally overlain by an additional 100-200 m of stable ice. The total valley relief therefore reaches approximately 700–800 m, consistent with the ice thickness at the grounding zone. Beneath the glacier, however, there is a subglacial cavity and the valley walls extend deeper (possibly to 1000m). To avoid ambiguity, we have simplified and clarified the description in the manuscript as follows: “The floating part of the Astrolabe Glacier is laterally constrained within a glacial valley. At the grounding zone, the thickness of the floating tongue has been estimated at approximately 700 m, gradually thinning to around 300 m at its terminus (Le Meur et al., 2014).”
Methods
Paragraph 135 & 155: Please add a citation for your description of icequake/ stick-slip events (i.e. typical frequency, P&S characterization, etc.)
Line 136-137: Can you summarize the key components of the Le Bris et al (2025) method? I.e. What is special about the Le Bris 2025 method compared to a standard STA/LTA algorithm? If there is some important information from that paper, it would be helpful for the reader to just state what that important information is so the reader doesn’t need to go to LeBris 2025 to understand the method.
>> We agree with the point of being clearer in terms of which part of our methodology comes from Le Bris et al (2025) and what are the key aspects of the methods that have to be better delineated. We modified the method section to state the key components of the method from Le Bris et al (2025). There is nothing special about our approach though compared to standard sta/lta, we just want to tell readers what set of parameters we used which are optimized to work efficiently with our seismic dataset.
How do the thresholds compare to the environmental noise? Does the noise vary in space or time? If these events are near the noise threshold, how do you know you are not biasing the event catalogue because of variable environmental background noise (e.g. Carr et al 2020).
>> This point is discussed in Le Bris et al., 2025. In that study, we adjusted the parameters of the STA/LTA to ensure that missed events and false detections were minimised as much as possible. This was achieved through manual verification of detections across several hours of data from different days, in order to account for environmental noise.
Line 145: It might be helpful when you discuss averaging over 24 hours to specifically state that you center each diurnal tidal signal on the high tide, so it is clear you are averaging 7 days of data, but in hours relative to each high tide time.
Line 170: Similar to the comment above, how does your detection strategy avoid bias with variable environmental background noise?
>> We first manually check our template quality (how high is the similarity coefficient given a template for other events of the same cluster). As mentioned from Line 161 to Line 164, we then ensure that we are indeed selecting the events correctly, by also setting a “low” similarity coefficient threshold which enables us to find events even if the background noise is higher. Afterwards, we verify the quality of our detections and filter out the false detections. We also look at all the detections in the timeseries, to make sure we do not miss any periods of time where events from the same family would occur across multiple days, but under different background noise.
Results
Throughout the results, we were quite confused as to the geographic references. It looks to me like the ground versus floating are West and East (mostly), respectively and that the shear margins are north and south, but the text is not consistent (e.g. see Line 250 comment below)
>> The Astrolabe Glacier flows along a direction trending NE making the geographical referencing not easy. The positioning taking into account the direction is rather north- north west and south-south east for the shear margins. And north east for the floating area. Depends from where we look. In the revised version, we will make sure to find the best terms to describe the system and help the reader to be well located.
Line 212: Be specific on which grounding line (i.e. the geodetic one) these icequake locations are consistent with
Line 245: “The striking feature is that all the detected clusters are located at the periphery or outside of the network with no event detected beneath it”. This is a key result, but we find the wording unclear. Figure 4 shows some events within the network, but we agree that they are on the periphery. Consider rewording for clarity of one of the main messages in this paper: e.g. “directly beneath the instruments, except for in the northern corner”
Line 250: Here is an example of reference to a “east” shear margin, but the map to us looks like shear margins are south and north.
Could you explain more what you mean clusters being “aligned” along or across glacier flow? How do you define alignment and how well do you know what this “alignment” is?
>> We define alignment as it is following the direction of the glacier flow (toward the NE), from the continent to the ocean, roughly in the South West -North East direction. We made sure to be more specific in the text.
Line 255: Please discuss the depth uncertainty more. What do you expect to be causing seismic velocities outside those of normal ice? Is it taking different pathways?
>> We hypothesize that ice properties specific to the Astrolabe Glacier could be causing these slower than expected seismic velocities. It could be related to ice damage, for instance very crevassed or cracked areas which impact the pathways and slow down the propagation from source to receivers. Overall, we are missing constraints on the velocity model which would help us understand the low seismic velocity we would use to reach expected ice thickness in the area of study. We added these details in section 4.2.
Line 262: “but has no impact on the following of the discussion” should read “but has no impact on our interpretation”
Line 275: “and then quantify four key parameters variation” should read “and then quantify the variation of four key parameters”?
Line 326: “This coherent pattern confirms our initial hypothesis that all stick-slip clusters share a similar focal mechanism”: this statement requires more explanation on why it confirms your hypothesis
>> We decided to not make this assumption anymore, after comments from Reviewer 1. Indeed, we acknowledge that assuming a single focal mechanism is a strong generalization. Instead we decided to present three focal mechanisms from different areas of the glacier (eastern margin, main trunk, and eastern margin). They all exhibit the same pattern: double-couple patterns. We are also revising lines 322–323 to remove the stated assumption and instead discuss the results more locally.
Line 279: If it really is clear, you shouldn’t have to say “clearly.” We suggest just deleting that word.
Line 280-300: We’d suggest breaking this paragraph up, the interevent time on line 289 is a new topic. And the event amplitude on line 294 is a new topic.
Discussion
Line 332: Are you assuming that all of the events that you categorize as icequakes are surface icequakes, or are you actually estimating their depth before categorizing them this way? Do you also see basal crevassing?
>> Surface icequakes have a short propagation distance and most of them are not detectable at many stations. Which tends to indicate that they are close to the stations recording them and rather shallow. If they were deeper, they should be detected on more stations, which is not the case. We have identified one deep cluster within the ice column but it accounts for a small proportion of all events. Further details can be found in Le Bris et al (2025).
Line 348: Line “… surface flexure is a proxy that may sign with some offset with the processes…”: We find the wording “may sign with” unclear. Consider changing to “may depend on”, “likely depends on”, or some re-phrasing of your choice.
Line 353: “true mechanical grounding line positions” what makes it “true”? See the “general comment” above about the role of longitudinal coupling length. If the authors wish to argue that the grounding zone starts at the most upstream effect of the ocean including the ice experiencing indirect effects through longitudinal coupling, then a brief discussion of this, and the controls on longitudinal coupling length for this glacier, is needed.
>> See the answer to the general comment above.
Line 375: figure reference should be “Figure 6”, not “Figure 6a”
Lines 406 & 407: For sentence clarity/ flow, change “little” to “few”
Line 408: line “The eventual disappearance of stick-slip clusters could sign a grounding line retreat…”: change “sign” to “signal”
Conclusion
Line 424: The word “clear” is not necessary.
Figures
>> We acknowledge and agree with all the technical corrections for the figures and take them into account for the revised manuscript.
Figure 1:
- The font of DDU and DUMG is difficult to read
- Can you plot the GNSS stations on panel c for context relative to the seismometers?
- Having a coordinate reference frame in this figure would be helpful.
- Labeling the GPS sites would be helpful. We realize they are labeled in figure 3, but since the nodes are labeled in figure 1 it would make sense to have the GPS labeled as well.
Figure 3:
- We suggest swapping the location of the colour bars and panel (g) in this figure. It isn’t immediately clear which colour bars correspond to which figures.
- Can you label what you are interpreting as the minimum range of seawater intrusions?
Figure 4: Need a legend item for the black circles
Figure 5:- Panel c: Delete the word “cumulative” as this isn’t cumulative event count but rather, event count in 30min bins
- We would like to see glacier velocity/acceleration shown alongside these data.
We will add a panel to Figure 5 to show the glacier velocity alongside the other data.
Figure 6:- Indicate in the caption that the hypothetical seismic event sources are coloured by their occurrence within the tidal cycle
Citation: https://doi.org/10.5194/egusphere-2026-76-AC2
-
AC2: 'Reply on RC2', Tifenn Le Bris, 21 Jun 2026
Data sets
SEIS‐ADELICE temporary experiment measuring the cryoseismicity of the Astrolabe glacier in Terre Adelie, Antarctica (RESIF‐SISMOB) G. Barruol et al. https://doi.org/10.15778/RESIF.ZR2020
DATA of 'Surface Icequakes and Basal Stick-Slip Events Reveal Daily Grounding Line Migration and Seawater Intrusion at a Marine-Terminating Glacier in East Antarctica' T. Le Bris https://doi.org/10.5281/zenodo.17977426
GNSS data at the Astrolabe Glacier A. Togaibekov et al. https://doi.org/10.5281/zenodo.14003385
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- 1
Designated Reviewer Discussion of:
Manuscript egusphere-2026-76
https://doi.org/10.5194/egusphere-2026-76
“Surface icequakes and basal stick-slip events reveal daily grounding line migration and seawater intrusion at a marine terminating glacier in East Antarctica”
Tifenn Le Bris, Guilhem Barroul, Florent Gimbert, Emmanuel Le Meur, Dimitri Zigone, Maxime Bés de Berc, Armelle Bernard
Reviewer: Nathan T. Stevens, PhD (University of Washington)
General Comments:
The manuscript presented by T. Le Bris and co-authors present novel seismic and geodetic observations from Astrolabe Glacier, East Antarcitca, that builds upon earlier works by the co-authors and others. They make an appreciable extension to published results in Le Bris et al. (2025) by incorporating analysis of basal icequakes to further refine migration of mechanically coupled areas of the grounding zone on diurnal timescales. These findings complement crevasse-sourced seismicity patterns and geodetic observations in earlier works.
Notable portions of the submitted manuscript reiterate findings in Le Bris et al. (2025), which tends to overshadow new findings arising from basal seismicity observations. Improved delineation of icequake source-types and their spatio-temporal patterns throughout the manuscript can help to better communicate these new findings. Interpretation of basal icequake source processes are limited in the current form of the manuscript, despite a growing body of literature relevant to glacier sliding and slip-sourced seismicity in response to diurnal forcing. I believe that reasonable refinement of select elements of basal icequake source characterization, and discussion of these events, can improve the scientific rigour of the manuscript and highlight the importance of these new observations.
Specific Comments
Greater delineation from Le Bris et al. (2025)
Large portions of this manuscript reiterate content presented in Le Bris et al. (2025) that can be effectively covered by a citation. For example:
Additional Cryoseismology & Sliding Stability References
The broader contextualization of observations and interpretations in this manuscript are hampered by a lack of a comprehensive grasp of the cryoseismology and basal processes literature of the last decade. I know the assembled author list is very much in-tune with this evolving body of work, and I find it troubling that this is not reflected in the writing and referencing within the submitted manuscript. As such, I have included a reasonably scoped set of references that should be considered foremost for their added context, and subsequently as cited references should their content be illuminating.
Foremost, major cryoseismology reviews are not referenced (Aster & Winberry, 2017; Podolskiy & Walter, 2016). I note relevant segments of these works in my comments below.
There is a growing numerical, experimental, and observational literature constraining oscillatory loading effects on sliding stability for both ice-on-rock (De Diego et al., 2022; Helanow et al., 2021; McCarthy et al., 2022; Skarbek et al., 2022; Stevens et al., 2025; Lucas K. Zoet et al., 2021) and ice-on-till (Hansen et al., 2024; Morgan‐Witts et al., 2025; Warburton et al., 2023; Lucas K. Zoet & Iverson, 2020). This body of work extends into the seismic expression of slip stability, which includes several references cited in this manuscript. The authors might additionally consider findings in Lipovsky & Dunham, (2016), Lipovsky et al. (2019), Zoet et al., 2020), and Stevens et al. (2024).
Disclosure: I authored two suggested references (i.e., Stevens et al., 2024; 2025). For these works, I emphasize that they should only be included as references if the authors decide their content are relevant to this manuscript.
Confounding Use of the Term “Icequake”
“Icequake” is a generalized term for wavefield excitation caused by glaciologic process. For the sake of clarity throughout the manuscript, I strongly suggest including modifiers upon “icequake” to specify association to source regions and/or specific processes. E.g., shallow icequakes, basal icequakes, englacial icequakes, crevasse-sourced icequakes, slip-sourced icequakes. Podolskiy and Walter (2016) and Aster and Winberry (2017) provide a reasonable framework for event classification nomenclature.
Clarifying the source regions and underlying processes throughout the manuscript will help to better convey the noteworthy insights on grounding line dynamics from both shallow/ice-fracture-sourced and deep/slip-sourced icequakes. For example, take lines 151–152. It might be clarified as “Basal stick-slip events are pervasively present in the seismic data but have amplitudes [generally] three times smaller than ice-fracture-generated events.”
Consideration for Till
Much of the interpretation for basal icequakes’ source processes and mechanics in this manuscript are underpinned by the assumption that seismogenic slip is occurring on an ice-bedrock interface (e.g., lines 46–49). There is abundant evidence that tidewater/outlet glaciers and ice-streams are till-bedded, and regularly form grounding zone wedges (e.g., Aitken et al., 2023; Simkins et al., 2017). Lipovsky and Dunham (2016) and Lipovsky et al. (2019) provide a compelling treatment of seismogenic sliding for ice-on-till systems that strikes me as relevant for this setting and observations. Give attention to Figures 4 and 5 in Lipovsky et al. (2019), related text, equations, and citations.
Consideration for Seismogenic Slip Mechanics
The ability for an asperity (a zone of ice-bed contact in this case) to accumulate and release strain seismogenically is modulated by the interplay of material properties of the interface, effective contact stresses, and loading rates (slip velocity). Figure 5b-c indicate higher rates of basal icequakes as tides rise, which is consistent with higher localized stresses on basal asperities. This warrants further discussion when presenting these findings (c. lines 300–301), and in their interpretation (lines 350–364). Tidal forcing modulates both slip velocities and subglacial water pressures, and relative timing and influence of each in the context of observed basal seismicity should be discussed.
Advocacy for Splitting Clusters in Analysis
The presented results provide a rich catalog of icequake activity from shallow and basal sources, which are admirably located and characterized given the inescapable limits of characterizing seismicity outside an array’s footprint. I am disappointed that the analysis in the submitted manuscript so abruptly lumps all seismogenic activity spatially given the additional rich context of geophysical constraints on basal topography, ice flow field, and tidal forcing (couched in the “statistical approach” of line 142).
Focal Mechanisms
Generalization of a single focal mechanism for all basal icequake clusters does a disservice to the quality of observational data presented in this manuscript. The presence of a transverse-to-ice-flow component of slip is likely due to ice convergence across Astrolabe Glacier’s shear margin, which the authors note is the primary source for observed basal seismicity (i.e., remarks on lines 285–286). Figure 4b might be better clarified by separating first-motions observations into three bins: eastern shear-margin, main-trunk, western shear-margin. If the latter two bins do not show significant double-couple patterning, consider adding them to the supplement and clarify in the main text that the first-motions stereonet is sourced from the eastern shear-margin.
I strongly disagree with the assumption stated in lines 322–323. Focal mechanisms should vary across the array as a function of subglacial topography (fault orientation) and basal slip velocity vector field (slip direction/rake on the basal fault). There are sufficient independent constraints on basal topography (e.g., Le Meur et al., 2014) and flow field (Provost et al., 2024) to place constraints on fitting slip- and auxiliary-planes to first motions observations for individual (or at least physiographically lumped) clusters.
Technical Corrections
A (very) recent publication that may be of relevance for manuscript revisions: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2025JF008509
Line 44: Remove “indeed”
Line 69: “e.g.” is missing a trailing “,” – check throughout manuscript: I.e., “i.e.,” “e.g.,”.
Line 70: Consider citing (Mikesell et al., 2012).
Line 79: Should probably include Hudson et al. (2023) here?
Lines 114–115: use superscript for ordinals, remove leading “0” from “02nd Feb. 2023”.
Line 144: affects -> effects
Line 158: Please be specific here, or in the supplement, regarding the waveform template (pre)processing pipeline. Waveform template-based detection is highly sensitive to preprocessing decisions (Chamberlain et al., 2017).
Line 159: template matching detection (not a proper noun).
Line 185: “perform a grid search in six dimensions” – cleaner idiomatic English.
Lines 185–187: (COMMENT) This feels like a lot of bespoke code and methodologic development that could have been accomplished by playing around with NonLinLoc (Lomax et al., 2000), with particular attention to the LOCGAU (generalized observation uncertainties) and LOCGAU2 (generalized velocity model uncertainty) control statements, and some reasonable estimation of arrival time uncertainties from all the hard-won manual picking of events.
Line 246: “seismically active gliding” -> “seismogenic sliding”
Figure 5 – the changing association of event type to colors throughout this figure make it very hard to follow. Please consider using a single color for each of the following throughout all five sub-figures: tide amplitude, basal icequakes, shallow icequakes, surface velocity.
Line 403 – Remove extraneous parentheses around Pirli et al., 2018.
Line 404–410: Maintaining year-round seismic arrays in polar settings is cost and labor intensive, even for a few seismic sensors close to an established base. As demonstrated here, and in earlier works (Kohler et al., 2019; Mikesell et al., 2012; Vore et al., 2019), the temporal resolution provided by seismic monitoring is a critical contribution to multi-method observation of grounding zones.
Rating Rubric for Primary Criteria of The Cryosphere Peer Review
Criteria
Rating
Originality
Good
Scientific Rigour
Fair
Significance
Good
Presentation Quality
Excellent
Review References
Aitken, A. R. A., Li, L., Kulessa, B., Schroeder, D., Jordan, T. A., Whittaker, J. M., et al. (2023). Antarctic Sedimentary Basins and Their Influence on Ice‐Sheet Dynamics. Reviews of Geophysics, 61(3), e2021RG000767. https://doi.org/10.1029/2021RG000767
Aster, R. C., & Winberry, J. P. (2017). Glacial seismology. Reports on Progress in Physics, 80(12). https://doi.org/10.1088/1361-6633/aa8473
Chamberlain, C. J., Hopp, C. J., Boese, C. M., Warren-Smith, E., Chambers, D., Chu, S. X., et al. (2017). EQcorrscan: Repeating and near-repeating earthquake detection and analysis in python. Seismological Research Letters, 89(1), 173–181. https://doi.org/10.1785/0220170151
De Diego, G. G., Farrell, P. E., & Hewitt, I. J. (2022). Numerical approximation of viscous contact problems applied to glacial sliding. Journal of Fluid Mechanics, 938, A21. https://doi.org/10.1017/jfm.2022.178
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