Monitoring diffuse volcanic degassing with seismic ambient noise

Seivane, Helena; Schimmel, Martin

doi:10.5194/nhess-26-2249-2026

Articles | Volume 26, issue 5

https://doi.org/10.5194/nhess-26-2249-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/nhess-26-2249-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 26, issue 5

Research article

|

13 May 2026

Research article |

| 13 May 2026

Monitoring diffuse volcanic degassing with seismic ambient noise

Helena Seivane and Martin Schimmel

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Final revised paper (published on 13 May 2026)
Preprint (discussion started on 14 Oct 2025)

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-5031', Anonymous Referee #1, 07 Nov 2025

Major points:
I didn`t find the main measure used in the paper well-explained. It is not difficult though to give a concise, but precise description of what the ellipticity measure is. I went back recursively to several earlier papers, but frankly, didn`t find any simple explanation. Is it the sum of the two larger eigenvalues of the cross-spectral matrix, normalized by the trace? Is it, or is it something else? After looking through three of their papers, I`m still wondering. That shouldn`t be. The sense of motion in a Rayleigh wave is also indicative of subsurface structure. Is that included in the measure or not?

What type are the seismic stations? One of them is an accelerometer, ok, but what about the others? How is the data processing done?

What time windows are used? How is the stacking done?

Something else that is missing from the paper (and perhaps the earlier ones as well) is what the ellipticity says about the subsurface really:

what is the sensitivity to lateral dimensions? Is it sensitive up to a half wavelength away laterally? How does behave for a nearly monochromatic wave? That probably has a very complicated shape. The discussion of ellipticity constraining vp rather than vs changes is interesting though.

As written above, the "DOP-E method" should be explained in this paper (see above). What are min/max values the measure of ellipticity can assume? Are you using a synthetic reference ellipticity calculation here, or not? Why actually should the ellipticity measure be insensitive to variations in noise sources?

Which tidal component is used in Figure 2? Vertical displacement?

In earlier studies (from other places), the barometric signal at 3/day had small amplitude, which did not allow for such studies at 3/day. It is surprising that the 3/day component is so large on the Canary Islands. Was the same time window used for barometric data from ERA5, as for the ellipticity.

What line 217 say in comparison with the 2/day component "..about 5 times smaller" "should say "..about 10 times smaller", right?

It is a pity that only one one among the three seismic stations considered (in the vicinity of the volcano that is erupting) shows a clear pre-eruption signal, and incidentally the one the most distant from the volcano (which should show the smallest effect). The statement that the 3 cpd seismic component "represents a reliable diagnostic of pressure-driven processes" (line 448) seems too bold to me. I don`t think that the paper has demonstrated this yet.

Unfortunately there has been only a short time window of outgassing measurements at one station, and that does not really agree well with the gain measurements. Line 309 states a "moderate agreement" between the lines in Figure 4a and A2. It is obvious what making such a statement would require: averaging the gain data to annual values, and comparing that quantitatively with the CO2 release. What is the statistical significance of the comparison (quantitatively)?

Figure 3 is the first time the seismological data appear, but only in a heavily-processed and very specialized way. Is it really not possible to

simply show the pertaining seismological data themselves. Neither the raw seismological data, nor the ellipticity traces show up in the paper anywhere. Don`t you think that this is missing?

Line 313/314: that is pure speculation.

The paper is extremely dense and not easy to read. Figures are of good quality, but the complexity of the

measures and the complexity of the observed phenomena are complicating reading the paper.

The seasonal variation is real, but it is not clear which process causes that.

The anomaly is intriguing, but the other two stations don`t corroborate it. (Figure 4 contains label "anomalies" but they appear at different times on the different traces (!). You can`t be serious.

The arguments of why the two closer stations don`t show the effects invoke different geology, hydrology etc. Sure, that could perhaps be true,

but the case for convincing that this "represents a reliable diagnostic of pressure-driven processes" is not strong.

Line 460: "a slight increase is observed" seems to optimistic to me: to me that peak look just like noise.

The Appendix shows DSI. That requires a few words saying what it is (one minus the crocor at 0-lag).

Minor points:

Line 57: put a dot at the end.
Line 115: replace "result from" by something like "are affected by". Otherwise the sentence is wrong.
Line 148: define c_m" also.
Figure 2: write "Normalized Spectral Amplitude"
Line 259: since you`re not given a significance level, I would rather use "show" than "prove" (a strong word).
Figures 4 and A2: "annual" instead of "anual".
Figure 5: It would be nicer if the same frequency could be used as for the other two stations.

Line 303: Put a "(MSC)" behind "Magnitude-squared coherence".

Figure 4: label a), b), c) are missing.

Line 367: put a dot at the end.
Line 630, 635 ff: "HVSNR" should be "HVSR".

Citation: https://doi.org/10.5194/egusphere-2025-5031-RC1
- AC1: 'Reply on RC1', Helena Seivane, 04 Jan 2026
  
  Dear Reviewer,
  
  Thank you very much for your thorough and constructive review. Your comments have been helpful in identifying aspects that required clarification. We provide detailed responses to all your remarks in the attached document and indicate the corresponding changes in the revised manuscript.
  
  We hope that these revisions adequately address your concerns,
  
  Sincerely,
  
  Helena Seivane and Martin Schimmel
  
  Citation: https://doi.org/10.5194/egusphere-2025-5031-AC1
RC2:
'Comment on egusphere-2025-5031', Anonymous Referee #2, 27 Nov 2025

Dear authors,

I found your approach very interesting, as well as potentially relevant to the community. However, I think that, in its current state, the paper does not convey this message clearly enough. This is most likely due to the way the paper is currently set up. I will outline my main points first and provide more detailed comments later.
Main Points:
I think the current structure of the paper is unbalanced and not ideal. Some sections are extremely long, while other, more important sections are rather short and confusing. Even after reading the paper several times, I found the main message somewhat difficult to follow. I think the authors should consider restructuring the paper and its focus.
The authors use the RWE measure, which seems to be relatively new to the community (there are not many references to its application in monitoring). Since this is the key metric, I think the authors should explain it in more detail. They should explain how the processing is done and how the RWE is determined. They should also consider showing the original RWE series, as well as the processed ones. Once they have done so and established the main features that are known to the community, they can discuss the short-term behaviour.
If I understand correctly, the authors want to measure changes in degassing caused by atmospheric tides using RWE. While I found this idea very interesting, I became confused when the time scales switched back and forth between short-term Fourier analysis and long-term interpretation. I think the idea and the goal should be outlined more clearly.
Where is the high-frequency noise coming from? What are the sources of the noise? These questions should be discussed.
Some aspects mentioned in the paper are not fully relevant to the study and its interpretation. I would suggest focusing more strongly on the relevant aspects (e.g. seismicity is an important factor in volcanic unrest, but perhaps not for this study or the authors mention HVSR, but it is not used in this study). Revising or cutting these sections could help to streamline the manuscript.

Other things, such as a description of atmospheric tides and their origin, are missing. I think the authors should provide a comprehensive yet concise summary of these effects (comparable to Kramer et al., 2025).
For me, it is unclear why the authors present Rayleigh wave ellipticity as a measure when V_p is more affected. For an inexperienced reader, this might sound contradictory. This needs to be discussed more clearly.
Why is the 8-hour tide used? For example, a 4-hour tide has a similar magnitude according to the spectrum. In general, the testing signal is quite weak. I’m wondering why the authors are not using 2cpd instead. Solid Earth tides have a 2cpd component, but I don't expect this to have a significant impact on the near-surface zone. The authors could easily test this and compare the results.
While I am convinced by the author's claim that RWE is affected by the atmospheric tide, I am not so sure about the degassing part. I did not understand whether RWE is small or large, e.g. before an event, or what influence a greater release of gas has on RWE.
The anomalies identified by the authors appear to be of a similar magnitude to noise or other events. How did the authors classify the anomalies?
Minor Points:
The authors claim that RWE is similar to dv/v, yet no comparison is made, for example on line 266. Recent studies (e.g., Oakley 2021 or Kramer et al., 2023, 2025) show a similar correlation, just for dv/v. In general, I think the body of literature could be more comprehensive. Some of the claims should be supported by references, even the commonly used metric dv/v and the proposed RWE are not exactly the same.
I found the use of terms such as 'RWE' and 'RWE gain' confusing. What is the difference between them? Why are they not consistent?
Why is the tidal spectrum suddenly being calculated when it's about pressure changes?
I have decided not to mention minor language inconsistencies in order to focus on the content of the discussion for now. Again, I think it could be a valuable contribution, but it needs to be simplified and clarified. I look forward to hearing your thoughts.

Citation: https://doi.org/10.5194/egusphere-2025-5031-RC2
- AC2: 'Reply on RC2', Helena Seivane, 04 Jan 2026
  
  Dear Reviewer,
  
  We thank you for your insightful review. Your remarks have been very helpful in improving the clarity and structure of the manuscript.
  Several of the points you raise overlap with those of Reviewer 1, and we have updated the manuscript to address these aspects.
  
  We provide detailed responses to all your remarks in the attached document and indicate the corresponding changes in the revised manuscript.
  
  We hope that these revisions satisfactorily address your concerns,
  
  Sincerely,
  
  Helena Seivane and Martin Schimmel
  
  Citation: https://doi.org/10.5194/egusphere-2025-5031-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (24 Jan 2026) by Giovanni Macedonio

AR by Helena Seivane on behalf of the Authors (24 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (16 Feb 2026) by Giovanni Macedonio

RR by Anonymous Referee #2 (04 Mar 2026)

Suggestions for revision or reasons for rejection

Dear authors,

I would like to thank you for your detailed responses to the comments I raised during the first round of revisions. You have done a fantastic job of improving the manuscript, particularly regarding its logical structure. I can now follow the arguments and the proposed technique much more easily. I find the approach you have proposed here very interesting!
However, I still have some minor comments that should be addressed. I will list them below. I hope the authors still find them helpful.

General Comments:

I think the authors present a natural pump-probe approach here. This should be stated somewhere, along with how it can be used to improve our understanding of the subsurface. I believe this would add significance and help people know what to expect. After a quick search, I can see two natural pumps in seismological research: Solid Earth tides (e.g. Mao et al., 2019; Delorey et al., 2021; Aiman et al., 2023) and atmospheric tides (Kramer et al., 2023; 2025).

When describing the effect of atmospheric tides on the subsurface, the focus is solely on the gas/air phase in the vadose zone. However, according to other publications, water is also involved, e.g. in the form of soil moisture, and, more significantly, in the form of water held above the groundwater by capillary forces. It is reported that this is also affected by atmospheric tides. I think it is important to mention this to provide a more complete description of processes in the vadose zone. Later, you mention hydrological changes in your area of investigation. Therefore, it would make sense to incorporate this.

I now better understand why you chose the 3cpd component. I think the readers deserve better contextualization and information on other observations. For example, I saw that Kramer et al. (2023) observed this 3 CPD, whereas Oakley et al. (2021) did not. It might be worth discussing whether this feature depends on location. Is there any mention of this in the literature? Depending on the answer, the proposed workflow would either be applicable everywhere or only in certain areas. I think this would be important for the discussion.

It would also be interesting to know how large the variation in 3 cpd is in terms of actual pressure change: is it a few Pa or kPa? It is also important to mention this somewhere in order to assess the feasibility of potential other locations.

Minor comments:

The manuscript contains many hyphens that are not always used correctly. I would suggest checking this carefully and only using them if they are correct. I am aware that the authors use LLM to improve the language, and I fully understand this, but the so-called LLM hyphen is an unwanted artefact. Please check this.

L. 155: Comma missing in the reference (Zimmermann, 2018)
L. 165: Comma missing in the reference (Zimmermann, 2017)
L. 178: I believe Kramer et al. 2025 is the better reference here, as it describes the underlying process in more detail.
Fig. 3c: Maybe I missed it, but what is the reason for the sudden increase of the ellipticity amplitude in early November 2010?
L. 332: I think the word “synchronized” is not ideal here.
L. 339: Same as before.
L. 342: I know that your measure and dv/v are not the same, but the same conclusion was reached by Kramer et al. 2023, so it makes sense to refer to that.
Fig. 8: I am wondering, why the authors present different cpd. In my opinion it weakens the arguments made by the authors. It seems there is no need for a specific pump to observe the features, because it is very similar across all selected bands.

Hide

RR by Manuela Köpfli (12 Mar 2026)

ED: Publish subject to minor revisions (review by editor) (26 Mar 2026) by Giovanni Macedonio

Dear Authors,

We have now received the reports from the second round of review. Reviewer #1 is highly satisfied with your previous revisions and commends the improved logical structure. Reviewer #2, who is new to this round, also finds your approach highly interesting and relevant. Both reviewers recommend a "Minor Revision".
To prepare your final manuscript, please address the following key areas highlighted by the reviewers.
Concerning the manuscript structure, the manuscript could be improved by moving methodological and study area descriptions (e.g., sensor types, rock permeability) out of the "Results and Discussion" section and into their respective dedicated sections to avoid interrupting the narrative.
Reviewer #2 strongly suggests adding clear subsections within "Results and Discussion" (e.g., *Precipitation*, *Tidal forcing*, *Degassing*) to guide the reader. Ensure a sharper boundary between the "Introduction" and "Study Area".
About the scientific claims, I invite you to slightly moderate your tone: Since the causal link between RWE variations and CO2 emissions remains largely qualitative and complicated by differences in spatial and temporal resolution, please temper strong assertions like "excellent agreement" or "clear interplay." Be highly explicit about exactly what variables and stations are being compared across paragraphs.
In addition, please, discuss whether the 3 cpd feature is location-dependent based on existing literature. Furthermore, explicitly state the magnitude of the actual pressure change (e.g., in Pa or kPa) to help readers assess the method's feasibility in other environments.
Concerning the barometric pumping, described in Material and Methods, probably, an explicit declaration that your method is based on this approach would be useful (see General Comments of reviewer #1).
When detailing the effect of atmospheric tides on the vadose zone, broaden the scope to include the role of water (soil moisture and capillary water), rather than just the gas/air phase. This directly supports your later discussion of hydrological changes.
Please, ensure the code availability link is explicitly provided within the manuscript text, and carefully proofread the manuscript to remove unwarranted hyphens and ensure correct punctuation.

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AR by Helena Seivane on behalf of the Authors (14 Apr 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (23 Apr 2026) by Giovanni Macedonio

AR by Helena Seivane on behalf of the Authors (23 Apr 2026) Manuscript

Short summary

Mantle-derived gases can seep to the surface before a volcanic eruption, but detecting them is difficult when emissions are weak. We used seismic noise from individual stations at Cumbre Vieja Volcano, La Palma, to monitor shallow gas accumulation being modulated by atmospheric cycles. We find clear links between pressure variations and gas release, with a marked increase months before the 2021 eruption, highlighting potential for early warning and hazard assessment.