Seismic Images of Pressurized Sources and Fluid Migration Driving Uplift at the Campi Flegrei Caldera During 2020–2024

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

See the attached review.

Comments for author File: Comments.pdf

Author Response

Reply to the comments of Reviewer #1

 

The authors thank the reviewer for their comments. The revisions have been made in accordance with the provided suggestions.

However, in response to the requests from the other reviewers, the introduction has been revised to better align with the objectives of Geohazard journal. The volcanological section has been condensed, and information about the urban context of the case study area has been added. This revision aims to highlight the negative impacts of seismic activity beyond those of an eruption, in a densely urbanized environment. Additionally, the section on time-lapse tomography, originally part of the introduction, has been relocated to Paragraph 3.2.

Below is a detailed point-by-point response. All amendments are highlighted in yellow in the manuscript, while those requested by other reviewers are colored in sky blue for Reviewer 2 and intense pink for Reviewer 3. Additionally, added parts are highlighted in gray.

 

line 10: subsidence phase? The opening statement contradicts the fact that strong uplift (≈180 cm) occurred in 1982-1984 event.

The text has been amended.

 

lines 38: please add Orsi et al. (1996).

We have added the reference to Orsi et al. (1996).

 

line 47: I suggest writing "...leading to the formation of Monte Nuovo phonolitic cone (e.g. Peccerillo, 2005)".

We have included the term "phonolitic cone" and referenced Peccerillo (2005).

 

line 51: please write "The first three recent events..."

The text has been amended.

 

line 56: please write "Following this, a 20-years long period of subsidence..."

We have modified the sentence.

 

line 62: please specify which magnitude you consider. For instance, the Time Domain Moment Tensor Catalogue (http://terremoti.ingv.it/tdmt) provides a quite lower moment magnitude (MW = 3.66) for the 20 May, 2024 event.

We have now clarified that the type of magnitude used in our analysis is the "duration magnitude, Md."

 

lines 200-201, 204-205: SimulPS-Vanorio, tomoDDPS-SimulPS-Vanorio, tomoDDPS-Battaglia are exceedingly elaborated terms. I suggest using throughout the text simpler names as Vanorio model, Vanorio scheme and Battaglia scheme respectively.

We have now replaced the suggested simple names.

 

line 421: Please complete the sentence "...progressive and extreme in other geothermal systems such as The Geysers and Larderello-Travale (e.g. [32] [...])." [...] stands for De Matteis et al. (2008), who carried out a complete 3D tomographic caracterization of the Larderello-Travale geothermal field, highlighting the various steam-saturated zones.

We have incorporated your suggestion and included the reference to De Matteis et al. (2008).

 

lines 547-548: Please complete the sentence "...such as The Geysers and Larderello-Travale [32] [...], where...". Again, [...] stands for De Matteis et al. (2008).

We have incorporated your suggestion and included the reference to De Matteis et al. (2008).

 

line 566: Please add Woo & Kilburn (2009), which have modeled the vertical and horizontal ground deformation data, recorded during the 1982-1984 uplift phase.

We have added the reference to Woo and Kilburn (2010).

 

Special issues

 

• Now the tomographic study include a more complete analysis considering the Vp*Vs product, derived from the obtained P-wave and S-wave velocity models. In the results (paragraph 3.1) was introduced an explanation of Vp*Vs product and briefly discussed the results. Following in the discussion (paragraph 4.0). The Vp*Vs product has provided additional constraints for the identification of the causes of some observed seismic velocity variations.

 

• Regarding this aspect, in the paper we reference the works of Petrillo et al. (2013) and Caliro et al. (2025). Specifically, Caliro et al. (2025) present a schematic in its Figure 16 that highlights the role of volcano-tectonic structures (taken from a WSW-ENE section of Orsi et al., 1996) in controlling the migration of fluids (CO₂ gas and aqueous solutions), as inferred from the physical model of the Campi Flegrei hydrothermal system (Petrillo et al., 2013). Petrillo et al. (2013) conducted simulations of a prolonged injection period of a hot H₂O–CO₂ gas mixture at the base of the hydrothermal system, with a composition similar to that used in previous studies, until steady-state conditions were reached.

We agree that a more refined model of a precise flow mechanism could be valuable. Our findings and the new velocity structure beneath Campi Flegrei could contribute to developing such a model. However, this falls beyond the scope of the present manuscript.

 

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript number: Geohazards-3511841

 

Title: Seismic Images of Pressurized Sources and Fluid Migration Driving Uplift at Campi Flegrei Caldera during 2020-2024

 After the 1982–1984 subsidence phase, ground uplift at Campi Flegrei Caldera resumed in 2005, while volcanic-tectonic earthquakes have steadily increased in frequency and intensity since 2020, with a significant intensification observed since 2023. This rise in seismic activity enabled the authors to conduct a new tomography using data collected from 2020 to June 2024. In this work, the authors used a large number of local earthquakes processed with the tomoDDPS code, to improve earthquake locations and velocity models. Compared to previous tomographic studies, these 3D velocity models provide higher resolution images of the central caldera's structure down to ~4 km depth. Additionally, the authors separately inverted datasets for 2020–2022 (moderate seismicity) and 2023–2024 (intense seismicity), identifying velocity variations (ranging from 5% to 10%) between these periods. These changes observed in 2023–2024 support the existence of two pressurized sources at different depths. One, located at 3–4 km depth beneath Pozzuoli and offshore, could represent either a magma intrusion enriched in supercritical fluids or an accumulation of pressurized, high-density fluids—a finding that aligns with recent ground deformation studies and modeled source depths. Furthermore, the upward migration of magmatic fluids interacting with the geothermal system generated a secondary, shallower pressurized source at approximately 2 km depth (beneath the 27 Solfatara-Piscirelli area). Overall, these processes are responsible for the recent acceleration in uplift, increased seismicity and gases from the fumarolic field, and changes in crustal elastic properties through stress variations and fluid/gas migration.

The article is written concisely, and clearly, and summarizes the results of a substantial volume of information. The statements are correctly supported with results and explanations.

I recommend the authors to complete the article with information about:

• Please correct the part in which the authors write about their work with „us„ , „our„ etc. and replace with “authors” and „author’s work” (impersonal)
• Please expand the state-of-the-art part
• Please present more urbanistic information regarding the case-study area(s), not only about seismicity (to understand the negative impact of the seismic activity in the area), maybe even some statistic info
• Please provide more details about the depth-related differences (the authors stated that they might reflect the interplay between deep pressurization effects and shallow fluid phase transitions, but further arguments would be good)
• Please clearly state the limitations of the presented work
• Please state clear the opportunity for the research work and your personal contribution. 

I do recommend the paper for publication after the consideration of the minor revisions because it presents important original information.

Author Response

Reply to the comments of Reviewer #2

 

Dear Reviewer,

The authors thank the reviewer for their comments. The revisions have been made in accordance with the provided suggestions.

However, in response to the requests from the other reviewers, the introduction has been revised to better align with the objectives of Geohazard journal. The volcanological section has been condensed, and information about the urban context of the case study area has been added. This revision aims to highlight the negative impacts of seismic activity beyond those of an eruption, in a densely urbanized environment. Additionally, the section on time-lapse tomography, originally part of the introduction, has been relocated to Paragraph 3.2.

Below is a detailed point-by-point response. All amendments are highlighted in sky blue in the manuscript, while those requested by other reviewers are colored in yallow for Reviewer 1 and intense pink for Reviewer 3. Additionally, added parts are highlighted in gray.

 

• Please correct the part in which the authors write about their work with „us„ , „our„ etc. and replace with “authors” and „author’s work” (impersonal)

The requested changes have been incorporated into the manuscript, and an impersonal style has been adopted throughout.

 

• Please expand the state-of-the-art part

The state of the art has now been expanded in several parts of the manuscript.

 

• Please present more urbanistic information regarding the case-study area(s), not only about seismicity (to understand the negative impact of the seismic activity in the area), maybe even some statistic info.

As mentioned above, we have now incorporated additional urbanistic information in the Introduction to better highlight the vulnerability of urban infrastructure to seismic activity.

 

• Please provide more details about the depth-related differences (the authors stated that they might reflect the interplay between deep pressurization effects and shallow fluid phase transitions, but further arguments would be good)

In the Discussion, we have inserted other arguments regarding the interplay between deep pressurization effects and shallow fluid phase transitions.

 

• Please clearly state the limitations of the presented work

We have now better stated the study's limitations of results in the Supplementary Materials together with the synthetic tests. It is well known that tomographic resolution is limited by factors such as the spacing between seismic sources and receivers, the number of earthquakes that determine the adequacy of ray coverage, and the inherent constraints of the ray-tracing based approach. Additionally, the predominance of shallow hypocentral locations further restricts the reliability of our findings, particularly for deeper structures.

 

• Please state clear the opportunity for the research work and your personal contribution.

The contribution of the manuscript aims to understand the subsurface structure of the Campi Flegrei caldera, providing new insights into the seismic properties and processes driving the unrest, while also supporting other disciplines.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

The article titled as Seismic Images of Pressurized Sources and Fluid Migration Driving Uplift at Campi Flegrei Caldera during 2020–2024 was investigated the changes on the seismic activity in this region. They obtained the tomoDDPS and compared with previous studies. This study was written with the help of data obtained over many years. Devastating earthquakes occurring around the world reveal the importance of precautions that can be taken before an earthquake. Monitoring seismic activity and scientific interpretation of the obtained data are important in this context. In this context, I think that this study will make important scientific contributions. Some corrections must be made before the study is published. Suggested corrections.
1. It is suggested that some information be added to the beginning of the study to reveal its importance. For example, the large-scale loss of life and property caused by earthquakes and the importance of precautions that can be taken before an earthquake can be stated. For example, studies on the effects of the 2023 Kahramanmaraş earthquakes on Adıyaman, Gölbaşı and similar places can be used in this regard.
2. In addition, a literature section should be added to the introduction section regarding the monitoring of seismic activity and how and for what purpose the obtained data is used.
3. Taking into account similar and different types of publications made in the mentioned region is important in order to reveal the innovation/difference of the study.
4. It would be more appropriate to transfer a large part of the introduction to the second section.
5. CO2 Please pay attention to superscript and subscript.
6. Vp and Vs Please add a long explanation of the abbreviations when they first appear.
7. Please also add some technical explanation for Vp and Vs and similar ones.
8. It is recommended to add a location map of the study area.
9. Is it possible to measure other parameters than the measured parameters? Please clarify this.
10.Please add detailed information about the validation of the measurements. At the same time, what will affect the measurements is more debatable.

Yours Sincerely

Author Response

Reply to the comments of Reviewer #3

 

Dear Reviewer,

The authors thank the reviewer for their comments. The revisions have been made in accordance with the provided suggestions.

However, in response to the requests from the other reviewers, the introduction has been revised to better align with the objectives of Geohazard journal. The volcanological section has been condensed, and information about the urban context of the case study area has been added. This revision aims to highlight the negative impacts of seismic activity beyond those of an eruption, in a densely urbanized environment. Additionally, the section on time-lapse tomography, originally part of the introduction, has been relocated to Paragraph 3.2.

Below is a detailed point-by-point response. All amendments are highlighted in intense pink in the manuscript, while those requested by other reviewers are colored in yallow for Reviewer 1 and sky blue for Reviewer 2. Additionally, added parts are highlighted in gray.

 

1. It is suggested that some information be added to the beginning of the study to reveal its importance. For example, the large-scale loss of life and property caused by earthquakes and the importance of precautions that can be taken before an earthquake can be stated. For example, studies on the effects of the 2023 Kahramanmaraş earthquakes on Adıyaman, Gölbaşı and similar places can be used in this regard.

As mentioned above, we have now incorporated additional information in the Introduction to better highlight the vulnerability of urban infrastructure to seismic activity.

 

2. 2. In addition, a literature section should be added to the introduction section regarding the monitoring of seismic activity and how and for what purpose the obtained data is used.

The text has been amended.

 

3. Taking into account similar and different types of publications made in the mentioned region is important in order to reveal the innovation/difference of the study.

In the conclusion, we have emphasized the innovations and unique contributions of this study compared to previous ones, particularly in terms of the tomographic code used, the volume of data analyzed, and the level of detail in the resulting tomographic images.

 

4. It would be more appropriate to transfer a large part of the introduction to the second section.

Now, the part of the introduction regarding the time-lapse tomography (4D) has been relocated to Section 3.2.

 

5. CO2 Please pay attention to superscript and subscript.

The correction has been made.

 

6. Vp and Vs Please add a long explanation of the abbreviations when they first appear.

The full explanation of the abbreviations Vp and Vs has been provided.

 

7. Please also add some technical explanation for Vp and Vs and similar ones.

We believe it is sufficient to state that Vp and Vs are the propagation velocities of P-waves and S-waves, respectively.

 

8. 8. It is recommended to add a location map of the study area.

Now, in Figure 1, an inset small map has been added illustrating the regional setting of the Phlegraean Volcanic District, highlighting its main tectonic and volcanic structures.

 

9. Is it possible to measure other parameters than the measured parameters? Please clarify this.

As also requested by Reviewer #1, the tomographic study now includes the Vp*Vs product.

 

10. Please add detailed information about the validation of the measurements. At the same time, what will affect the measurements is more debatable.

We have now better stated limitations of the analysis performed in this study both in the conclusion section and in the Supplementary Materials. In introduction we inserted: “It is well known that tomographic resolution is limited by factors such as the spacing between seismic sources and receivers, the number of earthquakes that determine the adequacy of ray coverage, and the inherent constraints of the ray-tracing based approach. Additionally, the predominance in the Campi Flegrei of shallow hypocentral locations further restricts the reliability of our findings, particularly for deeper structures.”

To validate the results of our study, we performed synthetic tests (Supplementary Materials). These tests were designed to assess the accuracy and reliability of our measurements and inversion models. In the manuscript, we refer to these synthetic tests to provide additional confidence in the results.