Volcanic risk ranking and regional mapping of the Central Volcanic Zone of the Andes

Reyes-Hardy, Maria-Paz; Di Maio, Luigia Sara; Dominguez, Lucia; Frischknecht, Corine; Biass, Sébastien; Guimarães, Leticia; Nieto-Torres, Amiel; Elissondo, Manuela; Pedreros, Gabriela; Aguilar, Rigoberto; Amigo, Álvaro; García, Sebastián; Forte, Pablo; Bonadonna, Costanza

doi:https://doi.org/10.5194/nhess-2023-225

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https://doi.org/10.5194/nhess-2023-225

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05 Jan 2024

| 05 Jan 2024

Status: a revised version of this preprint was accepted for the journal NHESS and is expected to appear here in due course.

Volcanic risk ranking and regional mapping of the Central Volcanic Zone of the Andes

Maria-Paz Reyes-Hardy, Luigia Sara Di Maio, Lucia Dominguez, Corine Frischknecht, Sébastien Biass, Leticia Guimarães, Amiel Nieto-Torres, Manuela Elissondo, Gabriela Pedreros, Rigoberto Aguilar, Álvaro Amigo, Sebastián García, Pablo Forte, and Costanza Bonadonna

Abstract. The Central Volcanic Zone of the Andes (CVZA) extends from southern Peru, through the altiplano of Bolivia, to Puna de Atacama of northern Chile and Argentina, between latitudes 14–28° S of the Andean cordillera, with altitudes raising up to more than 4,000 m above sea level. Given the large number of active volcanoes in this area, which are often located close to both urban areas and critical infrastructure, prioritization of volcanic risk reduction strategies is crucial. However, the identification of hazardous active volcanoes is challenging due to the limited accessibility. Here, we identify the riskiest volcanoes based on complementary strategies including i) a regional mapping based on volcanic hazard parameters and surrounding density of elements at risk combined with ii) the application of the recently developed Volcanic Risk Ranking methodology that integrates hazard, exposure and vulnerability as factors that increase risk, and resilience as a factor that reduces risk. The method identifies 59 active and potentially active volcanoes that not only highlights the volcanic centers with the most intense and frequent volcanic eruptions (e.g., El Misti and Ubinas volcanoes (Peru)) and the highest density of exposed elements (e.g., the cities of Arequipa and Mequegua (Peru)), but also those with the highest potential impact requiring risk mitigation actions (i.e., Cerro Blanco (Argentina), Yucamane, Huaynaputina, Tutupaca, and Ticsani (Peru)).

Received: 15 Dec 2023 – Discussion started: 05 Jan 2024

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Status: closed

RC1:
'Comment on nhess-2023-225', Anonymous Referee #1, 25 Mar 2024

This manuscript represents a significant contribution to the current sate-of-the-art of the volcanic risk in the Central Volcanic Zone of the Andes, making a deep revision of the available data in the literature, considering many different parameters of the active and potential active volcanoes of the region, and showing the data in high quality and synthetic maps. It is quite well built, however, some parts of the results, discussions and conclusions could be a little bit improved to transmit in a clearer and more effective way, the most important and remarkable findings. Please find attached a pdf document with my specific comments.
Indeed, the paper address relevant scientific questions to catalogue the many potential active volcanoes of the CVZA, considering exposed population, infrastructures, but also parameters like vulnerability or resilience. It is a useful review to put together a lot of data in well-structured article. But sometimes, there is an overuse of tables that sound repetitive and make slow the smoothy reading flow and make easy to lose the thread of the history.
Despite the conclusion are sufficiently supported by results, I think they can be write in a more direct way to highlight the original outcomes of the research. And above all, some parts of the discussions looks repeating the results, the authors could squeeze much better the data and go further with the interpretations, not only "re-read" what the maps and numbers are saying, but giving more breaking statements.
The title, abstract, methods are clear, concise, accurate and reproducibly. The formulas are well used, but in my opinion, in conclusions they shouldn't be cited and force the reader to go back to the methods chapter to understand it. The conclusions must stand alone, otherwise the thread is cut. For example, if saying which are the riskiest volcanoes depending the 3 and 4-factor VRR, specify that the 3 considers vulnerability and 4 resilience, are only 2 more words and help the readers.
The figures are large enough but the labels and some details are too small (see pdf). The colours of the maps could contrast better. References looks fine. The discussions could be shortened if focusing only in interpretations and avoid repetitions of results text. The technical and english language is good, but can be improved in some parts (see pdf). The supplementary material is appropriate and show a lot of data in which the research is based on.

Citation: https://doi.org/10.5194/nhess-2023-225-RC1
- AC1: 'Reply on RC1', Maria-Paz Reyes Hardy, 20 May 2024
  
  We appreciate the valuable feedback on our manuscript. We have reorganized the introduction and improved the results, discussion, and conclusions to best present our findings. We have also compiled a rebuttal letter to address each specific comments that we will upload when appropriate.
  It is important to clarify that, as explained in the Methods section, we compared six catalogues that had already classified volcanoes by type, and we used their best selection to compile our list of active and potentially active volcanoes. We then considered four risk factors (i.e., hazard, exposure, vulnerability, and resilience) to find the volcanoes of the CVZA associated with the highest risk. We appreciate that tables 1-3 may seem repetitive. Therefore, we moved Tables 1 and 3 to Supplementary Material 2, as suggested. We have kept Table 2 (now Table 1) in the manuscript, since we considered that this table helps the reader understand the different road classification in different countries.
  We have also restructured both discussion and conclusions to highlight the original outcomes of the research and avoid repeating the results. We have also expanded some parts of the discussion and introduced some specific recommendations on how to reduce vulnerability and increase resilience for target volcanoes. In particular, the discussion about active and potentially active volcanoes has been shortened and some parts have been redistributed between introduction, methods and results. Following your suggestions, we have modified the text when referring to the VRR equations to make the reading smoother.
  Finally, colors of the number of Holocene eruptions on the regional maps, as well as labels of all maps have been modified to improve visibility. All spelling and grammar errors have been corrected.
  
  Citation: https://doi.org/10.5194/nhess-2023-225-AC1
RC2:
'Comment on nhess-2023-225', Pablo Grosse, 27 Mar 2024

The MS is an interesting and important contribution concerning volcanic risk in the Central Volcanic Zone of the Andes. It identifies 59 active / potentially active volcanoes in the region, analyses a wealth of data and presents two main results or products: a set of maps that depict the spatial relations between the volcanoes and elements at risk (population, infrastructure), and volcanic risk rankings that integrate hazard, exposure, vulnerability and resilience. I think that the MS can be considered for publication after a moderate revision. Below I list my main comments and in the annotated PDF I have added several other minor/moderate comments.
-Throughout the MS I find that there is too scarce a consideration with regards to absolute ages. Given that most volcanoes in the CVZ lack historical eruptions, having good radiometric ages is key to evaluate how active or potentially active a volcano might be. However, there are few young ages, in part because of the arid climate that makes it difficult to carry out C14 dating. But there are however available ages, not only Holocene ages (for example Lastarria has some C14 ages, Socompa has a Holocene zircon double-dating age) but also ages < 100 ka which are of interest (i.e., a ‘Pleistocene’ volcano with ages < 100 ka I think should be considered more potentially active than a ‘Pleistocene’ volcano lacking ages or with ages of, say, 500 ka or 1 Ma). I think that some of this should be presented and discussed in the MS. Also, in Sup. Mat. 1 it would be good to add the available radiometric ages for each volcano.
-It is not clear why 1000 years is chosen as the limit to consider volcanoes for the VRR. Also, why 3 signs of unrest and not only one or two? As stated in section 5.2, you are excluding possibly highly impactful volcanoes with these constraints. Maybe it would be worth stating that in the future it is desirable to carry out the VRR on all 59 volcanoes.
-Although I understand that details have been given in Reyes-Hardy et al. (2023), I think that some more information should be provided as to how and from what sources the data given in Sup. Mat. 1 was obtained. Maybe a doc file with this information could be added as a supplement. Some questions concerning Sup. Mat 1: why aren’t radiometric ages given? What are the sources of volumes and areas? A footnote says that the number in parenthesis is number of eruptions confirmed by GVP, then what is the other number, eruptions according to who? What are the sources for the Holocene products lists of ‘yes’ or ‘no’? How was sector collapse potential evaluated? Also, shouldn’t columns of unrest signals be added to this table?
-Tables 1 to 3 are not really necessary in the main text, they could go as supplementary material.
-Table 4 is rather poor, it would be much better to give a more complete table for the main text, with several columns, summarizing the Sup. Mat. 1 table, i.e.: lat-lon, country, type, age, if had eruptions in last 1000 years, signs of unrest, number of Holocene eruptions, maximum VEI.
-Most of Section 5.1 is not really a discussion but rather more for intro, methods or results. It is not clear what is the point or goal of this section, what is the take-home message? I think the issue of how to identify active and potentially active volcanoes in this region is important and warrants a discussion, but it should be re-written, keeping in mind the problems of arid climate (difficulty in determining how young volcanoes are based on morphology, difficulty in C14 dating), un-even intensity in glaciation (more intense towards the north, almost non-existent towards the south of the CVZ), scarce young ages, the fact that many/most volcanoes in the region have not been historically active and in turn volcanoes in this region can have very long repose periods. Something that could be highlighted is that, given the challenges, the list of 59 volcanoes (and also the list of 19 considered as ‘more active’) is not final or definitive, but can change depending on future studies, in particular new ages, that can push volcanoes in or out of the lists.
-Section 5.2 is nice, but in parts a bit repetitive of the results, it could be shortened a bit and in turn some further interpretations/discussions could be added (see comments in PDF).
-Section 5.3 is nice, see a few comments in PDF.
-The first four bullet points of the conclusions should be re-written (see comments in PDF).

Citation: https://doi.org/10.5194/nhess-2023-225-RC2
- AC2: 'Reply on RC2', Maria-Paz Reyes Hardy, 20 May 2024
  
  We are grateful to the reviewer for insightful comments on our paper. We integrated changes to reflect all suggestions provided.
  Regarding absolute ages, we agree it would have been interesting to include this aspect in our database. However, as explained in the Methods section, we compared six catalogues that had already classified volcanoes by type and activity, and we used their best agreement to compile our list of active and potentially active volcanoes. Based on these volcanoes, we applied the existent Volcanic Risk Ranking methodology that considers the recurrence rate as being more informative of the volcanic hazard than absolute ages. For this reason, we have not included absolute ages but recurrence rate and records of the last eruption in our study.
  The criteria of selection of volcanoes to apply the VRR methodology has been expanded, as it was suggested. Nieto-Torres et al., 2021 found that results for Mexican volcanoes were independent on the analyzed time window of eruption occurrence (i.e., <1 and <10 ka). Additionally, Guimaraes et al. (2021), who first applied this methodology on Latin American volcanoes, used the threshold of 1ka as these eruptions are the best constrained and described in the eruptive records. The grouping of volcanoes based on the age, most recent eruptions and eruption periodicities, has also been previously used to rank volcanoes in a general order of “decreasing concern” (e.g., Bailey et al., 1983) and currently the occurrence of eruptions within the last 1,000 years represents one of the controlling factors in developing strategies to increase resilience (Nieto-Torres et al., 2021). In this work, we applied the same methodology to the CVZA. However, we appreciate that focusing on the VRR analysis of the last 1,000 years of volcanic activity might exclude potentially impactful volcanoes. Therefore, we constrained this aspect by integrating into the VRR analysis all the volcanoes presenting records of 3 signs of unrest (i.e., Uturuncu, Lastarria, and Cerro Blanco). Unrest signs include seismic, ground deformation, and degassing (details are available in the Supplementary Material 1). Due to lack of information for many of these volcanoes, the analysis of all 59 volcanoes remains a task to be addressed in the future as our knowledge of the CVZA volcanoes and its surroundings improves (we state this limitation in the discussion).
  Following your suggestion, the sources for the number of eruptions are specified in Table 4 (now Table 2). All other sources considered in Supplementary material 1 are specified in the open access report of Reyes-Hardy et al. (2023) (“Report – Version 2” in https://archive-ouverte.unige.ch/unige:172413). This report includes a brief one-sheet description of each of the 59 identified volcanoes, describing physical characteristics, eruption frequency and hazard types (with associated references).
  Tables 1 and 3 were moved to Supplementary Material 2. Table 2 (now Table 1) was kept in the main text in order to homogenize the different datasets. Table 4 (now Table 2) has been modified to show all suggested characteristics.
  We have also restructured some sections as suggested. In particular, most of Section 5.1 was moved to the introduction, methods and results. We modified parts of Section 5.2 to avoid repetition and expanded on the explanations/interpretations of our main results, as well as on the criteria of volcano selection for the VRR method.
  Finally, we modified the conclusions and better discussed the criteria behind volcanic risk rankings.
  
  Citation: https://doi.org/10.5194/nhess-2023-225-AC2
RC3:
'Comment on nhess-2023-225', Francisca Vergara-Pinto, 03 Apr 2024

This manuscript is well documented and provides an important contribution to the Central Volcanic Zone of the Andes, which is approached from a disaster risk perspective that is relevant to the aims and scope of the Journal. The regional mapping of active (and potentially active) volcanoes and applying a ranking that considers the interaction of hazard, exposure, vulnerability and resilience to assess volcanic risk contribute to broadening the understanding of disaster risk reduction in volcanic areas. In this sense, this work offers an opportunity to rethink the ways of assessing risk regarding the broad environmental, economic and social contexts with which a volcanic eruption interacts. This understanding deserves the general attention of the volcanology audience.
Therefore, I recommend its publication with moderate revision. To further enhance this work, I suggest some moderate changes that could be considered. The suggestions and comments (see PDF) are organised according to the structure of the paper to make the audience more familiar with the topic:
ABSTRACT: The summary concludes with a result. I recommend adding a sentence suggesting, as volcanologists, some ways to mitigate the risk in the identified areas, bearing in mind that such mitigation actions will typically fall on decision-makers and development processes. Therefore, the novelty of this ranking and methodology could also be translated into implemented recommendations. The same applies to the last sentence of the conclusion.
INTRODUCTION: In addition to the transboundary nature of this volcanic area, the socio-cultural and geographical composition that existed (and exists) prior to the states' formation and the area's transboundary nature is also important. The volcanic areas of this zone are characterised by being ancestrally and historically inhabited (but not only) by indigenous populations (Aymara, Quechua, Licanantay, etc.), and some authors have already studied this aspect in relation to volcanic risk (Melcher 2004; Romero & Albornoz, 2013; Petit-Breuilh, 2016; Ramos & Tironi, 2022; among others). Therefore, I suggest starting the introduction by mentioning that the central volcanic zone has been inhabited for centuries, even before the division of borders. Both qualities today involve "significant socio-cultural and cross-border challenges". Including this kind of background will bring this work on volcanic risk to the Earth and social science audience as well.
It is perhaps inappropriate to begin the paper by stating that it is "one of the most active areas in the Andes" compared to other volcanic zones, as its main characteristic is instead that it is an area that encompasses "a number of previously unrecognized, geologically young, potentially active giant ignimbrite centers and caldera systems, both in the Central Andes" (Stern, 2004), along the border of these countries. Moreover, the importance of the CVZA can be underlined by the fact that it has the "long record of voluminous silicic pyroclastic activity' associated with these giant systems 'has important implications for the safety of several major cities near the mouths of Andean canyons" (Stern, 2004, p.164).
The objective is missing in the introduction. I suggest stating the main objective of this work, what VRR's most relevant findings were, and why it is considered necessary to use it as an approach or method for the CVZA. This will give the reader an idea of how this new ranking differs from others.
METHODOLOGY: While the reader is asked to refer to the supplementary material for information on these concepts, the definition of Hazard and Resilience must be mentioned at least once in this part of the Methodology. They are the central elements of this new approach to risk ranking that the authors are trying to promote and should be present.
RESULT: Regarding using the 4-Factor VRR formula, could the authors explain the relevance of assessing each factor for each volcano? For example, it is not very clear that Misti tops the ranking of Normalized Exposure and Resilience simultaneously. On the other hand, it also needs to be clarified to determine a value (of H, V, E, R) for a volcano as a unit because depending on the distribution of the population in different flanks or sectors, it will vary. Finally, the authors need to mention how resilience is associated with a specific volcano, as resilience is the human or social characteristic within the formula, so there would be more or less resilient groups or villages around volcanoes. Moreover, resilience alone does not reduce risk; it must be associated with vulnerability reduction.
DISCUSSION: I suggest moving part of the discussion to the introduction for presenting conceptual definitions of active and potentially active volcanoes.
Here, it is mentioned that the regional mapping carried out can contribute to further risk analysis. Do the authors consider that they are already providing a risk analysis with these results and the method applied? If yes, it should be mentioned. On the other hand, "the VRR method provides an in-depth risk analysis", but some interpretations by the authors on the combination of the 4 factors are missing, apart from those done individually. I recommend then to improve this part of the discussion to mention "What is the risk for the different segments of the central volcanic zone?" or "Why does the identification of active or potentially active volcanoes not considered in other classifications contribute to a better understanding of CVZA risks?".
It is also mentioned that each country evaluates specific volcanoes. This idea is central to the paper. The authors should emphasise that they are interested in demonstrating the value of assessing risk on a regional scale and that, in the event of an eruption of CVZA volcanoes in a border zone, this type of "regional" risk analysis can significantly aid collaboration between countries in managing a volcanic emergency that may affect their populations (in different ways probably, e.g., tephra fall or PDCs).
CONCLUSION: What are the riskiest volcanoes in the CVZA? The authors could mention the 5 or 10 most risky volcanoes (or a number they consider appropriate) in the conclusion without breaking them down again by factor or element.
Finally, it would be ideal to conclude why it is important to rethink volcanic risk classifications (in this case, at the regional level), and how this could reflect DRR and mitigation measures tailored to the regional reality of the area: for instance, it is an area with very explosive volcanoes, with volcanoes whose history is partially unknown and may be potentially active, and with diverse populations around them that indeed share some socio-cultural and economic characteristics because they have been there since before the division of borders. Finally, because some of these volcanoes are located on the border, it is necessary to rethink the volcanic risk of the Central Andes at a regional level.

Citation: https://doi.org/10.5194/nhess-2023-225-RC3
- AC3: 'Reply on RC3', Maria-Paz Reyes Hardy, 20 May 2024
  
  Thanks for the insightful comments. Following your suggestions, we have modified the last sentence of the abstract and added some recommendations to mitigate the risk in the discussion (e.g., volcanic hazard and risk/impact assessments, monitoring systems, educational activities, and implementation of early warning systems).
  We modified the introduction to clarify the objectives and underline the importance of our study in the context of CVZA volcanoes.
  In the Results, we better explain the importance of assessing each risk factor separately for each volcano. In fact, each risk factor has different implications, and, therefore, it is important to assess their specific contribution to the final risk ranking. Unlike previous rankings that only assess hazard and exposure parameters, this VRR strategy integrates hazard, exposure, vulnerability, and resilience for each volcano. Whilst the reviewer is correct in stating that the distribution of elements in the flanks of the volcano may not be homogeneous and that resilience can strongly change from one community to another, this VRR strategy accounts for the first appearance within a certain radius around the volcano. It is therefore independent on the distribution.
  Parts of section 5.1 of the Discussion were moved to the introduction to provide conceptual definitions of active and potentially active volcanoes as suggested, and some parts were moved to methods and results.
  We clarified that our regional map of the CVZA provides a first step to quickly identify target areas that require a more detailed risk analysis and could be a helpful approach for stakeholders. We also better explained some key assumptions of the VRR strategy. For example, we explained that the analyzed elements at risk consider population, residential buildings and critical infrastructures potentially exposed within four distance thresholds (i.e., 5, 10, 30 and 100 km). These radii have been considered as they cover the most susceptible area to the impact of the different types of hazards (e.g. tephra, PDCs). In addition, the vulnerability factor help to differentiate volcanic systems with equal or similar threat, while the resilience factor allows to highlight volcanoes that require the implementation of risk reduction strategies. Finally, we better explain that active and potentially active volcanoes had already been identified in the CVZA in previous catalogues.
  Conclusions were expanded with a final paragraph summarizing why it is important to rethink volcanic risk classifications at the regional level. Indeed, we encourage the use of volcanic risk ranking at a regional level to highlight critical aspects. As an example, most CVZA volcanoes are located within less than 25 km from an international border and at least 20 of them are sharing borders. Consequently, a wide diversity of elements at risk might be affected in case of an eruption, which need to be treated at multiple scale. Finally, regional volcanic risk assessments are needed to analyze the level of preparedness especially in the case of cross-border volcanoes. We, therefore, hope that our work motivates further studies at different scales and promotes cooperation amongst CVZA countries.
  
  Citation: https://doi.org/10.5194/nhess-2023-225-AC3

Status: closed

RC1:
'Comment on nhess-2023-225', Anonymous Referee #1, 25 Mar 2024

This manuscript represents a significant contribution to the current sate-of-the-art of the volcanic risk in the Central Volcanic Zone of the Andes, making a deep revision of the available data in the literature, considering many different parameters of the active and potential active volcanoes of the region, and showing the data in high quality and synthetic maps. It is quite well built, however, some parts of the results, discussions and conclusions could be a little bit improved to transmit in a clearer and more effective way, the most important and remarkable findings. Please find attached a pdf document with my specific comments.
Indeed, the paper address relevant scientific questions to catalogue the many potential active volcanoes of the CVZA, considering exposed population, infrastructures, but also parameters like vulnerability or resilience. It is a useful review to put together a lot of data in well-structured article. But sometimes, there is an overuse of tables that sound repetitive and make slow the smoothy reading flow and make easy to lose the thread of the history.
Despite the conclusion are sufficiently supported by results, I think they can be write in a more direct way to highlight the original outcomes of the research. And above all, some parts of the discussions looks repeating the results, the authors could squeeze much better the data and go further with the interpretations, not only "re-read" what the maps and numbers are saying, but giving more breaking statements.
The title, abstract, methods are clear, concise, accurate and reproducibly. The formulas are well used, but in my opinion, in conclusions they shouldn't be cited and force the reader to go back to the methods chapter to understand it. The conclusions must stand alone, otherwise the thread is cut. For example, if saying which are the riskiest volcanoes depending the 3 and 4-factor VRR, specify that the 3 considers vulnerability and 4 resilience, are only 2 more words and help the readers.
The figures are large enough but the labels and some details are too small (see pdf). The colours of the maps could contrast better. References looks fine. The discussions could be shortened if focusing only in interpretations and avoid repetitions of results text. The technical and english language is good, but can be improved in some parts (see pdf). The supplementary material is appropriate and show a lot of data in which the research is based on.

Citation: https://doi.org/10.5194/nhess-2023-225-RC1
- AC1: 'Reply on RC1', Maria-Paz Reyes Hardy, 20 May 2024
  
  We appreciate the valuable feedback on our manuscript. We have reorganized the introduction and improved the results, discussion, and conclusions to best present our findings. We have also compiled a rebuttal letter to address each specific comments that we will upload when appropriate.
  It is important to clarify that, as explained in the Methods section, we compared six catalogues that had already classified volcanoes by type, and we used their best selection to compile our list of active and potentially active volcanoes. We then considered four risk factors (i.e., hazard, exposure, vulnerability, and resilience) to find the volcanoes of the CVZA associated with the highest risk. We appreciate that tables 1-3 may seem repetitive. Therefore, we moved Tables 1 and 3 to Supplementary Material 2, as suggested. We have kept Table 2 (now Table 1) in the manuscript, since we considered that this table helps the reader understand the different road classification in different countries.
  We have also restructured both discussion and conclusions to highlight the original outcomes of the research and avoid repeating the results. We have also expanded some parts of the discussion and introduced some specific recommendations on how to reduce vulnerability and increase resilience for target volcanoes. In particular, the discussion about active and potentially active volcanoes has been shortened and some parts have been redistributed between introduction, methods and results. Following your suggestions, we have modified the text when referring to the VRR equations to make the reading smoother.
  Finally, colors of the number of Holocene eruptions on the regional maps, as well as labels of all maps have been modified to improve visibility. All spelling and grammar errors have been corrected.
  
  Citation: https://doi.org/10.5194/nhess-2023-225-AC1
RC2:
'Comment on nhess-2023-225', Pablo Grosse, 27 Mar 2024

The MS is an interesting and important contribution concerning volcanic risk in the Central Volcanic Zone of the Andes. It identifies 59 active / potentially active volcanoes in the region, analyses a wealth of data and presents two main results or products: a set of maps that depict the spatial relations between the volcanoes and elements at risk (population, infrastructure), and volcanic risk rankings that integrate hazard, exposure, vulnerability and resilience. I think that the MS can be considered for publication after a moderate revision. Below I list my main comments and in the annotated PDF I have added several other minor/moderate comments.
-Throughout the MS I find that there is too scarce a consideration with regards to absolute ages. Given that most volcanoes in the CVZ lack historical eruptions, having good radiometric ages is key to evaluate how active or potentially active a volcano might be. However, there are few young ages, in part because of the arid climate that makes it difficult to carry out C14 dating. But there are however available ages, not only Holocene ages (for example Lastarria has some C14 ages, Socompa has a Holocene zircon double-dating age) but also ages < 100 ka which are of interest (i.e., a ‘Pleistocene’ volcano with ages < 100 ka I think should be considered more potentially active than a ‘Pleistocene’ volcano lacking ages or with ages of, say, 500 ka or 1 Ma). I think that some of this should be presented and discussed in the MS. Also, in Sup. Mat. 1 it would be good to add the available radiometric ages for each volcano.
-It is not clear why 1000 years is chosen as the limit to consider volcanoes for the VRR. Also, why 3 signs of unrest and not only one or two? As stated in section 5.2, you are excluding possibly highly impactful volcanoes with these constraints. Maybe it would be worth stating that in the future it is desirable to carry out the VRR on all 59 volcanoes.
-Although I understand that details have been given in Reyes-Hardy et al. (2023), I think that some more information should be provided as to how and from what sources the data given in Sup. Mat. 1 was obtained. Maybe a doc file with this information could be added as a supplement. Some questions concerning Sup. Mat 1: why aren’t radiometric ages given? What are the sources of volumes and areas? A footnote says that the number in parenthesis is number of eruptions confirmed by GVP, then what is the other number, eruptions according to who? What are the sources for the Holocene products lists of ‘yes’ or ‘no’? How was sector collapse potential evaluated? Also, shouldn’t columns of unrest signals be added to this table?
-Tables 1 to 3 are not really necessary in the main text, they could go as supplementary material.
-Table 4 is rather poor, it would be much better to give a more complete table for the main text, with several columns, summarizing the Sup. Mat. 1 table, i.e.: lat-lon, country, type, age, if had eruptions in last 1000 years, signs of unrest, number of Holocene eruptions, maximum VEI.
-Most of Section 5.1 is not really a discussion but rather more for intro, methods or results. It is not clear what is the point or goal of this section, what is the take-home message? I think the issue of how to identify active and potentially active volcanoes in this region is important and warrants a discussion, but it should be re-written, keeping in mind the problems of arid climate (difficulty in determining how young volcanoes are based on morphology, difficulty in C14 dating), un-even intensity in glaciation (more intense towards the north, almost non-existent towards the south of the CVZ), scarce young ages, the fact that many/most volcanoes in the region have not been historically active and in turn volcanoes in this region can have very long repose periods. Something that could be highlighted is that, given the challenges, the list of 59 volcanoes (and also the list of 19 considered as ‘more active’) is not final or definitive, but can change depending on future studies, in particular new ages, that can push volcanoes in or out of the lists.
-Section 5.2 is nice, but in parts a bit repetitive of the results, it could be shortened a bit and in turn some further interpretations/discussions could be added (see comments in PDF).
-Section 5.3 is nice, see a few comments in PDF.
-The first four bullet points of the conclusions should be re-written (see comments in PDF).

Citation: https://doi.org/10.5194/nhess-2023-225-RC2
- AC2: 'Reply on RC2', Maria-Paz Reyes Hardy, 20 May 2024
  
  We are grateful to the reviewer for insightful comments on our paper. We integrated changes to reflect all suggestions provided.
  Regarding absolute ages, we agree it would have been interesting to include this aspect in our database. However, as explained in the Methods section, we compared six catalogues that had already classified volcanoes by type and activity, and we used their best agreement to compile our list of active and potentially active volcanoes. Based on these volcanoes, we applied the existent Volcanic Risk Ranking methodology that considers the recurrence rate as being more informative of the volcanic hazard than absolute ages. For this reason, we have not included absolute ages but recurrence rate and records of the last eruption in our study.
  The criteria of selection of volcanoes to apply the VRR methodology has been expanded, as it was suggested. Nieto-Torres et al., 2021 found that results for Mexican volcanoes were independent on the analyzed time window of eruption occurrence (i.e., <1 and <10 ka). Additionally, Guimaraes et al. (2021), who first applied this methodology on Latin American volcanoes, used the threshold of 1ka as these eruptions are the best constrained and described in the eruptive records. The grouping of volcanoes based on the age, most recent eruptions and eruption periodicities, has also been previously used to rank volcanoes in a general order of “decreasing concern” (e.g., Bailey et al., 1983) and currently the occurrence of eruptions within the last 1,000 years represents one of the controlling factors in developing strategies to increase resilience (Nieto-Torres et al., 2021). In this work, we applied the same methodology to the CVZA. However, we appreciate that focusing on the VRR analysis of the last 1,000 years of volcanic activity might exclude potentially impactful volcanoes. Therefore, we constrained this aspect by integrating into the VRR analysis all the volcanoes presenting records of 3 signs of unrest (i.e., Uturuncu, Lastarria, and Cerro Blanco). Unrest signs include seismic, ground deformation, and degassing (details are available in the Supplementary Material 1). Due to lack of information for many of these volcanoes, the analysis of all 59 volcanoes remains a task to be addressed in the future as our knowledge of the CVZA volcanoes and its surroundings improves (we state this limitation in the discussion).
  Following your suggestion, the sources for the number of eruptions are specified in Table 4 (now Table 2). All other sources considered in Supplementary material 1 are specified in the open access report of Reyes-Hardy et al. (2023) (“Report – Version 2” in https://archive-ouverte.unige.ch/unige:172413). This report includes a brief one-sheet description of each of the 59 identified volcanoes, describing physical characteristics, eruption frequency and hazard types (with associated references).
  Tables 1 and 3 were moved to Supplementary Material 2. Table 2 (now Table 1) was kept in the main text in order to homogenize the different datasets. Table 4 (now Table 2) has been modified to show all suggested characteristics.
  We have also restructured some sections as suggested. In particular, most of Section 5.1 was moved to the introduction, methods and results. We modified parts of Section 5.2 to avoid repetition and expanded on the explanations/interpretations of our main results, as well as on the criteria of volcano selection for the VRR method.
  Finally, we modified the conclusions and better discussed the criteria behind volcanic risk rankings.
  
  Citation: https://doi.org/10.5194/nhess-2023-225-AC2
RC3:
'Comment on nhess-2023-225', Francisca Vergara-Pinto, 03 Apr 2024

This manuscript is well documented and provides an important contribution to the Central Volcanic Zone of the Andes, which is approached from a disaster risk perspective that is relevant to the aims and scope of the Journal. The regional mapping of active (and potentially active) volcanoes and applying a ranking that considers the interaction of hazard, exposure, vulnerability and resilience to assess volcanic risk contribute to broadening the understanding of disaster risk reduction in volcanic areas. In this sense, this work offers an opportunity to rethink the ways of assessing risk regarding the broad environmental, economic and social contexts with which a volcanic eruption interacts. This understanding deserves the general attention of the volcanology audience.
Therefore, I recommend its publication with moderate revision. To further enhance this work, I suggest some moderate changes that could be considered. The suggestions and comments (see PDF) are organised according to the structure of the paper to make the audience more familiar with the topic:
ABSTRACT: The summary concludes with a result. I recommend adding a sentence suggesting, as volcanologists, some ways to mitigate the risk in the identified areas, bearing in mind that such mitigation actions will typically fall on decision-makers and development processes. Therefore, the novelty of this ranking and methodology could also be translated into implemented recommendations. The same applies to the last sentence of the conclusion.
INTRODUCTION: In addition to the transboundary nature of this volcanic area, the socio-cultural and geographical composition that existed (and exists) prior to the states' formation and the area's transboundary nature is also important. The volcanic areas of this zone are characterised by being ancestrally and historically inhabited (but not only) by indigenous populations (Aymara, Quechua, Licanantay, etc.), and some authors have already studied this aspect in relation to volcanic risk (Melcher 2004; Romero & Albornoz, 2013; Petit-Breuilh, 2016; Ramos & Tironi, 2022; among others). Therefore, I suggest starting the introduction by mentioning that the central volcanic zone has been inhabited for centuries, even before the division of borders. Both qualities today involve "significant socio-cultural and cross-border challenges". Including this kind of background will bring this work on volcanic risk to the Earth and social science audience as well.
It is perhaps inappropriate to begin the paper by stating that it is "one of the most active areas in the Andes" compared to other volcanic zones, as its main characteristic is instead that it is an area that encompasses "a number of previously unrecognized, geologically young, potentially active giant ignimbrite centers and caldera systems, both in the Central Andes" (Stern, 2004), along the border of these countries. Moreover, the importance of the CVZA can be underlined by the fact that it has the "long record of voluminous silicic pyroclastic activity' associated with these giant systems 'has important implications for the safety of several major cities near the mouths of Andean canyons" (Stern, 2004, p.164).
The objective is missing in the introduction. I suggest stating the main objective of this work, what VRR's most relevant findings were, and why it is considered necessary to use it as an approach or method for the CVZA. This will give the reader an idea of how this new ranking differs from others.
METHODOLOGY: While the reader is asked to refer to the supplementary material for information on these concepts, the definition of Hazard and Resilience must be mentioned at least once in this part of the Methodology. They are the central elements of this new approach to risk ranking that the authors are trying to promote and should be present.
RESULT: Regarding using the 4-Factor VRR formula, could the authors explain the relevance of assessing each factor for each volcano? For example, it is not very clear that Misti tops the ranking of Normalized Exposure and Resilience simultaneously. On the other hand, it also needs to be clarified to determine a value (of H, V, E, R) for a volcano as a unit because depending on the distribution of the population in different flanks or sectors, it will vary. Finally, the authors need to mention how resilience is associated with a specific volcano, as resilience is the human or social characteristic within the formula, so there would be more or less resilient groups or villages around volcanoes. Moreover, resilience alone does not reduce risk; it must be associated with vulnerability reduction.
DISCUSSION: I suggest moving part of the discussion to the introduction for presenting conceptual definitions of active and potentially active volcanoes.
Here, it is mentioned that the regional mapping carried out can contribute to further risk analysis. Do the authors consider that they are already providing a risk analysis with these results and the method applied? If yes, it should be mentioned. On the other hand, "the VRR method provides an in-depth risk analysis", but some interpretations by the authors on the combination of the 4 factors are missing, apart from those done individually. I recommend then to improve this part of the discussion to mention "What is the risk for the different segments of the central volcanic zone?" or "Why does the identification of active or potentially active volcanoes not considered in other classifications contribute to a better understanding of CVZA risks?".
It is also mentioned that each country evaluates specific volcanoes. This idea is central to the paper. The authors should emphasise that they are interested in demonstrating the value of assessing risk on a regional scale and that, in the event of an eruption of CVZA volcanoes in a border zone, this type of "regional" risk analysis can significantly aid collaboration between countries in managing a volcanic emergency that may affect their populations (in different ways probably, e.g., tephra fall or PDCs).
CONCLUSION: What are the riskiest volcanoes in the CVZA? The authors could mention the 5 or 10 most risky volcanoes (or a number they consider appropriate) in the conclusion without breaking them down again by factor or element.
Finally, it would be ideal to conclude why it is important to rethink volcanic risk classifications (in this case, at the regional level), and how this could reflect DRR and mitigation measures tailored to the regional reality of the area: for instance, it is an area with very explosive volcanoes, with volcanoes whose history is partially unknown and may be potentially active, and with diverse populations around them that indeed share some socio-cultural and economic characteristics because they have been there since before the division of borders. Finally, because some of these volcanoes are located on the border, it is necessary to rethink the volcanic risk of the Central Andes at a regional level.

Citation: https://doi.org/10.5194/nhess-2023-225-RC3
- AC3: 'Reply on RC3', Maria-Paz Reyes Hardy, 20 May 2024
  
  Thanks for the insightful comments. Following your suggestions, we have modified the last sentence of the abstract and added some recommendations to mitigate the risk in the discussion (e.g., volcanic hazard and risk/impact assessments, monitoring systems, educational activities, and implementation of early warning systems).
  We modified the introduction to clarify the objectives and underline the importance of our study in the context of CVZA volcanoes.
  In the Results, we better explain the importance of assessing each risk factor separately for each volcano. In fact, each risk factor has different implications, and, therefore, it is important to assess their specific contribution to the final risk ranking. Unlike previous rankings that only assess hazard and exposure parameters, this VRR strategy integrates hazard, exposure, vulnerability, and resilience for each volcano. Whilst the reviewer is correct in stating that the distribution of elements in the flanks of the volcano may not be homogeneous and that resilience can strongly change from one community to another, this VRR strategy accounts for the first appearance within a certain radius around the volcano. It is therefore independent on the distribution.
  Parts of section 5.1 of the Discussion were moved to the introduction to provide conceptual definitions of active and potentially active volcanoes as suggested, and some parts were moved to methods and results.
  We clarified that our regional map of the CVZA provides a first step to quickly identify target areas that require a more detailed risk analysis and could be a helpful approach for stakeholders. We also better explained some key assumptions of the VRR strategy. For example, we explained that the analyzed elements at risk consider population, residential buildings and critical infrastructures potentially exposed within four distance thresholds (i.e., 5, 10, 30 and 100 km). These radii have been considered as they cover the most susceptible area to the impact of the different types of hazards (e.g. tephra, PDCs). In addition, the vulnerability factor help to differentiate volcanic systems with equal or similar threat, while the resilience factor allows to highlight volcanoes that require the implementation of risk reduction strategies. Finally, we better explain that active and potentially active volcanoes had already been identified in the CVZA in previous catalogues.
  Conclusions were expanded with a final paragraph summarizing why it is important to rethink volcanic risk classifications at the regional level. Indeed, we encourage the use of volcanic risk ranking at a regional level to highlight critical aspects. As an example, most CVZA volcanoes are located within less than 25 km from an international border and at least 20 of them are sharing borders. Consequently, a wide diversity of elements at risk might be affected in case of an eruption, which need to be treated at multiple scale. Finally, regional volcanic risk assessments are needed to analyze the level of preparedness especially in the case of cross-border volcanoes. We, therefore, hope that our work motivates further studies at different scales and promotes cooperation amongst CVZA countries.
  
  Citation: https://doi.org/10.5194/nhess-2023-225-AC3

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https://doi.org/10.5194/nhess-2023-225-supplement

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Active and potentially active volcanoes of the Central Volcanic Zone of the Andes (CVZA) Maria-Paz Reyes-Hardy, Luigia Sara Di Maio, Lucia Dominguez, Corine Frischknecht, Sébastien Biass, Leticia Guimarães, Amiel Nieto-Torres, Manuela Elissondo, Gabriela Pedreros, Rigoberto Aguilar, Álvaro Amigo, Sebastián García, Pablo Forte, and Costanza Bonadonna https://archive-ouverte.unige.ch//unige:172413

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The Central Volcanic Zone of the Andes is shared by four countries and groups 59 volcanoes. We identified the ones with the most intense and frequent eruptions (e.g., El Misti and Ubinas), the cities with the highest density of elements at risk (e.g., Arequipa and Mequegua), and the volcanoes with the highest potential impact (e.g., Cerro Blanco and Yucamane). Our study contributes into the prioritization of risk reduction resources, which is crucial for surrounding communities.


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