Changes in Ground Displacement Anticipated the 2021 Cumbre Vieja Eruption (La Palma, Spain)
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors quantify ground deformation before the 2021 eruption of Cumbre Vieja volcano, by analyzing Sentinel-1 data. The results show that ground deformation can be considered a precursor of the eruption. I suggest some minor changes to the paper to improve the presentation of the results and their discussion.
Comments for author File: 
Comments.pdf
Author Response
Comment: This portion of the text is probably referred to the panel c. If, this is the rigth position, please clarify the sentence
Answer: correct, thank you for noticing.
Comment: Can you explain why you selected the 30 December 2020 as cut off date beetween the two maps?
Answer: we have clarified this in the text by adding the following explanation “The cut-off between these two periods has been selected to mark the trend change, as the first period is dominated by subsidence and the second one by uplift”. Now we marked this in the caption of fig 3.
Comment: This caption is different from the figure description in the text. From the text and the figure it seems that both panels show the vertical velocity evaluated at different time spans.
Answer: You are right, we modified the caption.
Comment: I suggest moving this text in the introduction because it does not discuss the findings of your work. Here you can discuss that your work firstly correlated the measured ground deformation with the eruption and it gives a volcanological explanation.
Answer: we have removed this part from the discussions as suggested.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for Authorssee the attached cover letter
Comments for author File: Comments.pdf
Author Response
Comment:
Dear authors,
I have completed the review of your manuscript. The proposed study focuses on ground deformation analysis at the Cumbre Vieja volcano, La Palma (Spain) using InSAR techniques. The SAR data cover both the pre-eruptive and co-eruptive stages and indicate a possible uplift of the ground several months before the eruption. The article is well written and the data are very interesting, highlighting the key role of ground deformation analysis in identifying precursor signals of volcanic eruptions. I have added only minor remarks, mostly related to text style, such as avoiding repetition and improving clarity in some sentences. My main request is to better clarify the subsidence rate you estimated in the overall context of the volcanic island’s evolution.
As a suggestion, it would be valuable to include some information (even just a few sentences in the discussion) about the post-eruption phase. What is the current ground deformation? I believe readers would be interested in understanding how the volcano has evolved (or is still evolving) in response to the eruption in terms of deformation patterns.
The article is suitable to be published after a minor revision.
Answer: thank you for your consideration and suggestion. Unfortunately, we do not have new data besides the ones that we discussed in our paper.
Comment:
Abstract
Please Avoid acronyms and use of brackets if is not extremely needed
Answer: we have modified as suggested.
Comment: Keywords:
Some proposed keywords are repetitive, please think to change some of that with other terms
Answer: thank you, we have changed two keywords.
Comment: Introduction
Rows 67-69: likely associated with a magmatic intrusion at 15-25 km depth [15, 19], at a depth known to storage pre-eruptive magma [20-26]. These swarms originate from a sub-horizontal volume aligned along a EW direction at a depth of 25 km. The sub horizontal volume is the pre eruptive magma chamber? If yes, it is a repetition of the concept explained in the row 67 please rewrite this sentence
Answer: indeed it is a repetition so we merged the sentence into a single one like this: “in October 2017 and February 2018 two seismic swarms were recorded in the Cumbre Vieja area [5, 16-19], concomitant with anomalous gas emission, likely associated with a magmatic intrusion at 15-25 km depth [16, 20], in correspondence of a sub-horizontal volume aligned along a E-W direction that is known to storage pre-eruptive magma [21-27] and from where these swarms are originated”
Comment: Discussion
Rows 191-193. This is already said in the intro. Remove it or reduce it (from 11 to 16 September 2021, with displacement of magma at 15-25 km depth with consequent seismic crisis; [27]) and co-eruptive (from 17 to 22 September 2021, with seismic crisis with opening of the eruptive fracture on the surface; [27]),
Answer: agreed. We have removed this sentence from here.
Comment: Rows 196-197
While the subsidence of that sector of the island of La Palma was a known fact and had been interpreted as the effect of the volcano's weight on lower density volcaniclastic sediments [9-11]. What about the “normal” subsidence rate for the La Palma volcano without considering the last volcanic events. This value is close to your findings (i.e 5 mm in 4 months). Are your data indicative about an increasing of subsidence in concomitance of the seismic swarm of 2019? If yes, what is your interpretation?
Answer: as you noted, our sentence point at the fact that the real signal is not the subsidence but the uplift; we attribute the subsidence to compaction, consistently with literature. Indeed, the subsidence even decreased before the eruption because compensated by the uplift caused by the pressurization.
Comment: Conclusion
In the conclusion is preferable avoid putting references if this is not a clearly needed or the proposed results are compared with other studies.
Answer: done.
Comment: Figure 1 The labels of coordinates are too small, please enlarge them.
Answer: you are absolutely right. Indeed we realized that almost all our figures had small writings so, following your suggestion, we have enlarged most of the figures and font sizes.
Comment: Figure3
The caption seems wrong: …Displacement maps showing the vertical (a) and the eastward horizontal (b) components. Instead, fig3 a shows vertical displacement related to the September 2019 to December 2 2020 period , while the fig3 b the vertical displacement related to the 2021 period. Right? Please correct it
Answer: correct, thanks for noticing.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThis paper analyzes ground deformation before the La Palma volcanic eruption using InSAR data. The methodology is clear, and it is a solid piece of research.
I noticed that several key result figures in the paper (e.g., Figures 3, 4, and 5) clearly contain the "TRE Altamira" watermark or label. It needs to be clarified whether the authors obtained authorization from TRE Altamira to use these results.
The authors state that nine months before the eruption, the ground deformation in the area changed from subsidence to uplift, which they interpret as magma migrating from deep (15–25 km) to shallow (approximately 5 km) depths. This explanation sounds reasonable, but could it be further substantiated? I would like to know if this phenomenon has been observed before (in other volcanic eruption cases)?
I suggest that the authors revise the manuscript according to the style guidelines of the Remote Sensing journal. For example, the abstract of papers published in Remote Sensing is not divided into paragraphs.
Author Response
Comment: This paper analyzes ground deformation before the La Palma volcanic eruption using InSAR data. The methodology is clear, and it is a solid piece of research.
I noticed that several key result figures in the paper (e.g., Figures 3, 4, and 5) clearly contain the "TRE Altamira" watermark or label. It needs to be clarified whether the authors obtained authorization from TRE Altamira to use these results.
Answer: one of the authors is from TRE Altamira, as shown in the affiliation.
Comment: The authors state that nine months before the eruption, the ground deformation in the area changed from subsidence to uplift, which they interpret as magma migrating from deep (15–25 km) to shallow (approximately 5 km) depths. This explanation sounds reasonable, but could it be further substantiated? I would like to know if this phenomenon has been observed before (in other volcanic eruption cases)?
Answer: we have added a thorough discussion in the Discussion section. To substantiate our explanation as requested we enhanced the references, both in the discussion and in the methods sections. This is the new addition in the discussion section:
“This switch from subsidence to uplift caused by magma rising from depth and injecting in a shallower reservoir with subsequent pressurization of the chamber and upward deformation of the ground above it is a well-recognized phenomenon that has been observed at several volcanic systems worldwide and is sometimes called resurgence [72] or reinflation [73].
Probably, the most famous case is Campi Flegrei (Italy) where this phenomenon is called bradyseism. Campi Flegrei caldera is a volcanic field hosting a nested caldera complex that historically experiences periods of subsidence (around 1-2 cm per year) occasionally interrupted by rapid uplifts [74], as it occurred in 1950-1952 (74 cm uplift), 1970-1972 (159 cm uplift), and 1982-1984 (178 cm uplift) [75-76]. The latest event, in particular, was associated with 16.000 earthquakes up to 4.0 magnitude [78]. The subsidence resumed following this event and was again interrupted in 2005 with a new uplift phase that was again associated with seismic activity that intensified since 2018 [77].
Among basaltic calderas that displayed uplift and seismicity prior to eruptions (such as Kilauea in 2018 and Bárðarbunga in 2014) the Sierra Negra volcano (Galápagos Islands) has shown one of the highest vertical deformations. Sierra Negra presents a flat-topped sill-like magma reservoir at around 2 km depth below the caldera [79-81]; before the 2018 eruption, magma supply and accumulation to this shallow reservoir drove 6.5 m of pre-eruptive uplift and seismicity over thirteen years [82].
Another example is Piton de La Fournaise, a shield hot-spot volcano in Réunion is-land. The pre-eruptive inflations preceding two eruptions (one in November and another in December 2005) have been linked to the pressure caused by a dyke and/or a shallow source located at a depth around 500-2.300 m [83].
Within the same Canary Archipelago as La Palma, also El Hierro volcano experienced a similar phenomenon. Before the 2011 submarine eruption, around 10.000 earthquakes. The strongest seismic event before the eruption reached a magnitude of ML = 4.3 and was recorded at 12 km depth [84], corresponding to the locations of magma pockets below El Hierro [85]. The pre-eruptive phase was also associated with an uplift of more than 5 cm [84].”
Comment: I suggest that the authors revise the manuscript according to the style guidelines of the Remote Sensing journal. For example, the abstract of papers published in Remote Sensing is not divided into paragraphs.
Answer: We have removed the paragraph subdivision from the abstract and we have used the template provided by the journal.
Author Response File: Author Response.pdf
