Dear Editor,

I have completed a thorough review of the manuscript entitled “Brief Communication – Vent Opening at Campi Flegrei: Clues from Dyke Propagation Patterns” by Selva and Mangone.

This work provides a concise overview of the current understanding of vent-opening maps at Campi Flegrei. It introduces a novel empirical methodology that incorporates the spatial distribution of past vent locations along with the influence of topographic relief on dyke trajectories. The results, presented as vent opening probability maps, suggest two concentric annular maxima at approximately 2 km and 4 km radial distances, with a general peak toward the northeast and additional local maxima corresponding to topographic highs. The authors also note a qualitative correspondence between their results and the epicentral distribution of recent seismicity. While this point is briefly mentioned in both the abstract and the discussion, it is not explored in depth, particularly regarding the significant variation in the depth distribution of seismicity across different sectors of the caldera.

Overall, the manuscript is clearly written and logically structured. Nonetheless, there are several areas in which the presentation could be improved, particularly concerning the manuscript's structure, the accuracy and placement of literature citations, the clarity of the figures, and certain aspects of the wording.

The proposed approach is both interesting and promising; however, the manuscript would benefit from a more explicit articulation of the underlying assumptions. For instance, the authors adopt a similar assumption to that of Rivalta et al. (2019) regarding the dyke source, coinciding with the axis of radial symmetry inferred from deformation data. While this assumption is consistent with certain modeling efforts, it simplifies the broader volcanological literature, which often supports the existence of a sill-like source. Furthermore, the presumption of a fixed source depth of 3 km for all post-NYT eruptions may lack robust petrological justification. The model also assumes predominantly lateral dyke propagation, a simplification that may not hold uniformly throughout the post-NYT eruptive period. It would be beneficial for the authors to state these assumptions more explicitly at the outset.

The referencing of literature requires refinement, as several citations appear either outdated or incorrectly positioned—particularly those pertaining to the structural evolution of the caldera and its eruptive history. More recent studies have revised the temporal framework of Campi Flegrei’s activity, extending it beyond 200 ka, and have updated interpretations of the caldera structure. For instance, the structural rims are now understood to be primarily associated with the CI eruption and subsequently reactivated during the NYT phase (e.g., Natale et al., 2022, Journal of Structural Geology). Additionally, recent investigations into the physical properties of dykes within the Campi Flegrei system should be acknowledged (e.g., Buono et al., 2025, AGU Advances; Natale and Vitale, 2025, Nature Communications).

In summary, I find the manuscript to be of overall high quality and recommend it for publication pending moderate revisions.

Please find below line-by-line minor suggestions and comments.

Best regards

Abstract

Line 9: Consider adding the term “azimuth” to clarify the reference to the orientation of the dykes.

Line 10: The two principal peaks should be more clearly emphasized in Figure 2, which is currently difficult to interpret.

Introduction

Line 14: The earliest volcanic activity observed in outcrops within and beyond the caldera is estimated at approximately 80 kyr, based on Pappalardo et al. (1999) and Scarpati et al. (2013). However, recent studies of widespread tephra layers in Italy and the Mediterranean extend the chronology of CF activity to nearly 200 kyr (e.g., Monaco et al., 2022 - GPC; Fernandez et al., 2024 - QSR). This includes a recently identified large-magnitude eruption at 109 ka (e.g., Fernandez et al., 2025 – Communications Earth and Environment). Tephra deposits of comparable age and composition have also been retrieved from boreholes (e.g., Sparice et al., 2024 – JVGR). These more recent contributions should be acknowledged.

Line 15: Please include a reference to the Campanian Ignimbrite eruption/age.

Line 15: The statement that all eruptions occurred within the CI caldera rim is not accurate. For example, eruptions took place at Procida Island (De Astis et al., 2004) and at CFc, such as the Torregaveta eruption.

Line 17: The citation of Sbrana et al. (2021) is inaccurate, as these authors attribute the caldera structure solely to the CI eruption.

Line 18: The term “inner caldera” has a specific structural meaning, yet many post-NYT vents are located beyond this boundary. Orsi et al. (2004) is incorrectly referenced here, as it does not address volcano-tectonic structures.

Line 19: A citation to Di Vito et al. (1999) would strengthen this statement.

Line 19: Consider modifying the sentence to read “comprises at least 33 eruptions.”

Line 19: Correct “spaning” to “spanning.”

Line 20: Adjust 15,000 to 14,000 for consistency with line 16.

Line 22: Revise 9.200 to 9.100.

Line 23: Revise 28 to 26.

Lines 23–25: Consider dividing the sentence after “3,800 years ago.” A more precise description could be: “Its activity was predominantly concentrated in the northeastern part of the caldera (Agnano area), secondarily in the northwestern sector (Averno area), and concluded with peripheral distal eruptions (Nisida, Capo Miseno, and Fossa Lupara).” Recent works provide additional context (Natale et al., 2025 – GSA Bulletin).

Line 27: Update the reference to Di Vito et al. (1987 – Bulletin of Volcanology).

Line 28: It would be useful to cite recent studies on caldera resurgence at La Starza cliff (Isaia et al., 2019 – JVGR; Natale et al., 2022 – Basin Research).

Line 31: The history of ground deformation since 35 BCE has been reconstructed by Di Vito et al. (2016 – Scientific Reports). Vitale and Natale (2023 – Earth, Planets, and Space) describe the long-term deformation pattern.

Line 33: Correct the date to 1982–84.

Lines 34–35: Suggested rephrasing: “which fully recovered the subsidence in 2021, and now exceeds the uplift peaks observed in the last century.” If this statement refers exclusively to deformation, cite Bevilacqua et al. (2024). If it encompasses seismicity and degassing, please include additional references accordingly.

Line 40: Suggested revision: “[…] parameters, focusing on the tectonic structures recognized at that time, to track […].”

Line 45: Update to Charlton et al. (2020), not 2018.

Line 46: Replace with “corresponding to the caldera rim.”

Line 52: Suggested rephrasing: “Rivalta et al. (2019) analyzed the effects of caldera unloading, as well as those of topographic peaks, […].”

Line 55: Insert “may”: “[…] creates a stress field that may favor magma trajectories […].”

Line 56: Add “(unloading effect)”: “[…] of the caldera (unloading effect) significantly […].”

Line 57: Clarify the intended wording: “Geometric centre at a given distance”?

Line 60: Replace with “left a trace.”

Line 66: Correct citation to Amoruso et al. (2014).

Line 67: Refer also to the general comment above regarding assumptions on the dyke source.

Line 68: Verify coordinates, as longitude (Easting) appears to contain one digit too many, while latitude (Northing) appears to be missing one.

Line 76: Please ensure consistent use of terminology (“dyke” vs. “dike”) throughout the manuscript.

Line 85: Among the cited works, only Rivalta et al. (2019) appear to state this explicitly.

Line 88: The assertion regarding topographic control seems to be treated as a fact, whereas it should be presented as a working hypothesis.

Line 101: Consider expanding the statement: “[…] the stress field is mainly controlled by unloading.”

Line 102: Please clarify whether Figure 1c represents simple topographic profiles or averaged swath profiles. The latter would provide a more representative average topography.

Line 107: Use “La Starza marine terrace.”

Line 107: Clarify orientation: If La Starza terrace is located NW of the center, it should correspond to ~340° azimuth, not 50°.

Line 108: Correct to “Camaldoli Hill.”

General: These locations should be shown on a map, as readers unfamiliar with Campi Flegrei may otherwise be confused.

Line 112: This observation indicates an anticorrelation, or at least underscores the need to state at the outset that the spatial distribution of vents is hypothesized to be controlled by topography.

Line 120: Invert the order of this sentence and the next, so that it follows the order presented in the formula.

General comment on Figure 2: Figures could be made more legible by using a lighter, more uniform background (e.g., shaded relief). The seismic hypocentre dots currently obscure the NE peak within the inner circle.

Line 142: Consider renaming this section “Discussion and Conclusions,” as the final sentences are conclusive in nature and a separate Conclusions section is absent.

Line 149: The reference to “E and SE” is unclear. Does it pertain to Baia and Bacoli, or to Bagnoli and Nisida? Please clarify and specify locations.

Line 151: It would be appropriate to include the map from Alberico et al. (2002) to illustrate similarities and differences, rather than requiring the reader to consult that paper independently.

Line 170: The difference in the Solfatara peak is not particularly pronounced; it is already visible in Selva et al. (2012) and Bevilacqua et al. (2016).

Comment on Figure 3F: According to the caption in Tizzani et al. (2024) and cited references, the rims shown in Tizzani et al. (2024) correspond to the CFc caldera rims, not the CI and NYT rims. Revising accordingly would improve consistency throughout the text.

Line 172: The directions are unclear. By “E and SE,” do you, in fact, mean “W and SW,” referring to the topographic highs at Baia and Capo Miseno?

Line 178: Natale et al. (2025) is not included in the reference list.

Final comment: A dedicated “Concluding Remarks” section is missing and should be provided if the name of the section is not updated as suggested above.

I trust that these comments will assist the authors in enhancing the clarity, accuracy, and overall quality of their manuscript.

The manuscript describes an interesting approach to define a vent opening map at Campi Flegrei caldera by splitting the density function in polar coordinates as the product of radial and azimuthal components. While this formulation is promising in studying the vent opening patterns, the manuscript does not provide any convincing argument for assuming independence between the radial and the azimuthal patterns, nor an uncertainty quantification of the output. In fact, splitting the density function in two factors is equivalent to assume they are independent, and that seems a pretty strong assumption, to me, although it may provide an interesting point of view on the problem.

The manuscript, after a general introduction, follows with a description of the empirical distributions of direction and radial distances of past vents from the caldera center. Then, it describes the split formulation of vent opening probability, before a Discussion section in which the new vent opening maps are briefly described and compared to some pre-existing vent opening maps and to the current geophysical unrest pattern.

The manuscript showcases interesting ideas, but it is incomplete and unconvincing in providing a new vent opening map. Also, the methodology section is missing and the methods are mixed with the results section, and insufficiently detailed. Another problem is the unclear comparison between past vent opening positions and geophysical unrest patterns, in the discussion.

The text is generally well written, but I am not a native speaker so I may have overseen minor language issues. The key novelty of this research stays in the split formulation, but that would deserve a more extensive analysis and discussion.

In summary, the results are incomplete and the discussion section is disappointing. I strongly suggest re-shaping the manuscript by adding new analyses, separate the methods from results, and rewrite the Discussion. Probably, a plot of the differences between the pre-existing vent opening maps and the results of the manuscript would help to highlight how the previously published maps differ from the manuscript's results. Very importantly, if the authors aim at introducing a new vent opening model, they should provide an uncertainty quantification of the probability density values. Also, having a clear and concise Conclusion section would be really appreciated. Finally, please consider if a “plain” research article would not be a better format for this manuscript.

DETAILED COMMENTS   

L21 – please add a reference for the 12300 BP age, in fact the 2022 book by Orsi et al. reports 11900 to 12200 BP.

L24 – when you say that the third epoch is predominantly focused in the NE part of the caldera and secondarily in the NW sectors, you should also mention the Nisida and Capo Miseno exceptions.

L25 – is the 4550 BP age an average of 4500 to 4600 BP? Add reference please.

L29 – “geometrical center” is unclear.

L33 – why 1984-1986? Should not be 1983-1985?

L40-44 – please mention that both Selva et al. 2012 and Bevilacqua et al. 2015 are vent opening maps with uncertainty quantification, i.e., the probability density values have an uncertainty distribution. This is a key point.

L62 – Why you speak of past dykes? I would find much clearer if you spoke of past vents distances. In fact, we don’t really know if all (or any) dykes actually started from the caldera center.

L74-75 – Is 1000 samples the most appropriate sample size? Did you try with more, or less, samples? What changes? Also, by looking at the Supporting File I see that you probably used a uniform distribution inside the elliptical shapes of Bevilacqua et al. 2015; I can’t find the uniform distribution specified anywhere, tough.

L76, L87 – again, please do not speak about dykes. It is unrequired to assume that all dykes propagated from the caldera center if you just speak about vent opening patterns. It is ok to speak about dykes in the discussion, but it is not necessary here.

L83 – you used the Kolmogorov-Smirnov test many times. For a statistician it is a classical test, but I would strongly suggest adding an explanation of how it works. This would fit very well in the new methods section.

L119 – This formulation is not valid for every 2D density function. You can do that if (and only if) the two components (radial and azimuthal) are independent. This is really a key point that you missed to discuss.

L140 – Please delete the earthquakes from Figure 2. They do not belong to the vent opening pattern analysis that you are describing here and they hide the probability density function values. Please add contour lines. The comparison of vent opening map and seismic patterns could be done in the discussion section, once it is clarified its meaning.

L142 – The result section is unfortunately incomplete. Some examples of required additions are listed below.

1. You assumed that azimuthal and radial distributions are independent. But how the bivariate plots of distance and direction of past vents looked like? If they looked much correlated, it should be a concern for the validity of this formulation in producing a new vent opening map. However, this analysis could be useful anyway to understand that correlations in polar coordinates may play a role in shaping the vent opening patterns. This should be discussed.
2. Why you did not try to produce maps based on the three epochs? I can’t see why you just tested the third epoch after having analyzed the marginal distributions of the three epochs – by doing that, you discarded half of the vent opening dataset.
3. How the differences between E and W sectors could find integration in this analysis? I believe this distinction was important in shaping some of the pre-existing vent opening maps. Please give a look over 10.3389/feart.2017.00072.
4. Why you used empirical distributions and not kernel functions? Is this changing much if you assume a simple Gaussian kernel with an appropriate bandwidth?
5. How the new formulation deals with a uniformly distributed layer that both Selva et al. 2012 and Bevilacqua et al. 2015 assumed inside the caldera?
6. Could you try quantifying the uncertainty affecting all this? You may do that as you wish, but you should not totally oversee that step.

L150-L155 – I would have liked to see a plot of the differences between the new map(s) and the pre-existing maps (e.g., by considering their mean values).

L158 – I can’t see the rings very well in Figure 2, because of the earthquakes plotted on top of it. Please delete them.

L164 – Figure 3: I find this Figure a bit confusing. These are all Figures published in other papers, and I can’t do a quantitative comparison of these heterogeneous data to the new maps (which are also plotted elsewhere, in Figure 2). Also, the data in plots d-e-f are not about vent opening information. Why mix apples with pears?

L174-175 – “striking correspondence” is a qualitative claim. Please make it more quantitative. In particular, please explain how a comparison between your vent opening map and seismic pattern should be read. In general, main active faults are certainly correlated to seismicity, and main active faults are also hypothesized to be correlated to vent locations in several of the pre-existing vent opening maps. What is really the point of this sentence? Is it pointing at the possibility of new vent opening models also integrating seismicity? Or is it somewhat trying validate the current vent opening maps by using geophysical data?