the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Review article: Deterministic seismic hazard assessment of the area comprised between west Gulf of Cádiz and east Alboran Sea
Adrián José Rosario Beltré
Carlos Paredes Bartolomé
Miguel Llorente Isidro
Abstract. The convergence zone of the NE-SW complex comprising the Gulf of Cádiz and the Alboran Sea, at the Eurasian-Nubian plate boundary, is frequently affected by seismic activity, caused by submarine long-strike strike-slip fault systems and arcuate fold-thrust systems found in the region. This has resulted in moderate to high magnitude earthquakes, including tsunamigenic earthquakes, and the area has also experienced tsunamis due to major earthquakes and gravitational landslides. This study carries out a Seismic Hazard Analysis, for bedrock conditions, of the marine area between the W of the Gulf of Cádiz and the E of the Alboran Sea in the Ibero-Maghrebian region, based on a deterministic approach, considering areal seismogenic sources. For the estimation of the seismic hazard, a Visual Basic script based on Excel has been used, which has been improved. The results obtained show that the most probable Peak Ground Acceleration (PGA) ranges from 0.2 to 0.4 g, although it can reach up to 1.0 g in certain areas. These results highlight the need for a detailed study of the distribution of seismic hazard in submarine areas, given the significant values of accelerations that can occur. This work is the first comprehensive deterministic seismic hazard assessment carried out in the Ibero-Maghrebian region and aims to take a first step to promote seismic hazard studies in marine areas, whose results can provide relevant information given the implications of earthquakes in the genesis of other natural hazards such as tsunamis and submarine landslides.
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Adrián José Rosario Beltré et al.
Status: final response (author comments only)
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RC1: 'Comment on nhess-2023-52', João Fonseca, 04 May 2023
Beltré et al. submitted a manuscript entitled “Review article: Deterministic seismic hazard assessment of the area comprised between west Gulf of Cádiz and east Alboran Sea” presenting the results of a deterministic seismic hazard assessment (DSHA) for the Cadiz Gulf / Alboran Sea region and benchmarking their results against previous studies for the emerged parts of their study area given the lack of previous studies for the submerged regions. The manuscript discusses at length the advantages and limitations of probabilistic (PSHA) versus deterministic assessments, and similarly for zoned versus non-zoned deterministic studies. Another aspect that deserves particular attention is the use of a Digital Terrain Model (DTM) to allow the correct estimate of the distance from source to site.
Although the manuscript presents itself as a review paper, it does not correspond in the opinion of this reviewer to the requirements of such qualification, namely “[to] summarize the status of knowledge and [to] outline future directions of research”. The remainder of this review will address the relevance of the manuscript as a potential research paper.
The manuscript adopts an in-depth description of the details of the work done that makes it read more like a technical report than a scientific paper, where the relevant aspects should be highlighted and additional details synthetized in tables or supplementary material. With the profusion of details, the reader looses the purpose of the manuscript: To present a new methodological approach? To advocate the need for hazard assessment of submerged regions? To put forward a product ready to be used for the design of infrastructures? All of the above? Perhaps because of the profusion of apparent goals, the manuscript fails, in the opinion of this reviewer, to be convincing for any of them.
When advocating for DSHA instead of PSHA, the authors seem to reinstate a dated controversy of the 1990s in the United States, after which vast clarifications of concepts and domains of applicability were obtained that were not discussed. While addressing a relatively untouched topic which is seabed hazard assessment, the authors fail to highlight the growing exposure of submarine structures and associated risk, which would be an interesting point in favour of the study. In what concerns the applicability of the results for practical purposes, the non-standard aspects of the approach would require much better justification for it to be considered fit for use.
To illustrate the latter aspect: the authors advocate the adoption of a zoned DSHA method according to Wang and Huang (2014), relying heavily on that single reference – in fact, from which the EXCEL code to perform the computations was obtained - to support that not-so-standard choice. To differentiate zoned from non-zoned DSHA, the authors state that “the application of the DSHA is not zoned, if it is done on a homogeneous seismicity region, with a single seismic source of global influence” (line 394). However, it is well known that one of the strongest arguments in favour of DSHA, and one that makes it adopted for the design of critical structures, is that it is reality-based, taking into account the geological structures surrounding the site and their seismogenic characteristics. DSHA has at its core the concept of maximum credible earthquake (MCE) at a particular geological structure or set of well identified geological structures, and usually avoids the use of area sources altogether. In the manuscript, the authors describe in details the relevant geologic structures of the region of interest, but at the time of computing the hazard these structures do not seem to be taken in consideration, the MCE being considered equal to the MMax of each area source in previous (probabilistic!) studies.
Even more concerning, it was not possible for this reviewer to understand how the distance from source to site was computed prior to its insertion in the GMPEs: after describing several second-order corrections to the distance, the authors mention that those corrections were applied to “the minimum distance calculated with the original script” (line 618, referring to a pre-existing EXCEL script that was adapted for the calculations). Were the authors assuming that a fault capable of producing the MCE for each area zone existed at the minimum distance from the site to each area source? This would certainly increase the hazard unwarrantedly.
Another non-standard approach that the authors take, still following Wang and Huang (2014), is the consideration of “non-control area sources” in addition to the “control area source” (i.e., the source producing the maximum maximum credible earthquake). In this case, the authors remark that the hazard is higher at sites near the “confluence” of source zones, an observation that in and by itself should raise concerns: if the pattern of the hazard is driven by the source zone geometry, something must be wrong with the methodology (actually, this limitation is even clearer in Wang and Huang (2014)’s hazard assessment for Taiwan, which shows a “checkered” pattern when “non-control sources” are considered, with no similarity whatever to the results obtained when non-control sources are not used (their figures 5 and 6); it should be pointed out that Wang and Huang (2014) was published in a journal which is very remote from mainstream seismological research, which may account for a less rigorous review process for that paper; it is a tell-tale detail that no mainstream hazard publication has refered Wang and Huang (2014)). The authors report a 445% (!) increase of the hazard at Cadiz (84% percentile) when non-control area sources are added to the control area source, and fail to identify that discrepancy as anomalous.
Because fundamental aspects like these were not documented to a point that this reviewer could understand, I am forced to consider the results unfit for use.
In addition to the serious reservations listed about, it must be mentioned that the manuscript contains inaccuracies easily corrected had the manuscript bee n previously subjected to any level of review prior to submission. Just to mention a few:
Line 93: “As shown by geodetic data and geodynamic studies that have been carried out for decades (e.g., Montessus de Ballore, 1894; Pastor, 1927; Munuera, 1963) tectonic models of NW-SE to WNW-ESE oblique convergence between the plates (Reilly et al., 1992; Herraiz et al., 2000) present displacements of 2 to 5 mm/yr”
Pre-plate tectonics studies can hardly have any relevance to the discussion rates of convergence between plates.
Line 212: “Seismic hazard is the probability that an earthquake occurs in a given geographic area, within a given time period, and with a ground motion intensity exceeding a given threshold (McGuire, 2004)”
This passage indicates a lack of familiarity with the concept of seismic hazard that is not adequate in a manuscript on seismic hazard assessment. Seismic hazard concerns probability of exceedance of a level of ground motion at a site over a period of time, not probability of occurrence of an earthquake in a geographic area.
line 387: “… the probability density of the 50th percentile, or average value, …”
The 50% percentile of a pdf corresponds to the median, not the “average” (in other passages. e.g., line 423, it is called “mean”, also wrongly), and even without that imprecision the sentence is devoid of meaning (the median is a value of the random variable, not of the probability density).
etc, etc…
In view of the above remarks, I regret that I have to recommend that the manuscript should not be accepted for publication in NHESS. Should the authors consider rewriting it to try a subsequent submission, the attached pdf includes extensive comments on the passages that need improvement.
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AC1: 'Reply on RC1', Adrián José Rosario Beltré, 11 May 2023
Dear reviewer,
We would like to express our gratitude for the time and effort invested in reviewing our manuscript. We thank you for your constructive and valuable suggestions. We believe that the quality of this manuscript will improve considerably after your review.
COMMENTS --->
Beltré et al. submitted a manuscript entitled “Review article: Deterministic seismic hazard assessment of the area comprised between west Gulf of Cádiz and east Alboran Sea” presenting the results of a deterministic seismic hazard assessment (DSHA) for the Cadiz Gulf / Alboran Sea region and benchmarking their results against previous studies for the emerged parts of their study area given the lack of previous studies for the submerged regions. The manuscript discusses at length the advantages and limitations of probabilistic (PSHA) versus deterministic assessments, and similarly for zoned versus non-zoned deterministic studies. Another aspect that deserves particular attention is the use of a Digital Terrain Model (DTM) to allow the correct estimate of the distance from source to site.
Reply (R): We sincerely thank the reviewer for these valuable comments.
Although the manuscript presents itself as a review paper, it does not correspond in the opinion of this reviewer to the requirements of such qualification, namely “[to] summarize the status of knowledge and [to] outline future directions of research”. The remainder of this review will address the relevance of the manuscript as a potential research paper.
R: We agree with this comment. Our work was conceived and carried out as a research paper. It was the editor's recomendation to change our manuscript type to review.
The manuscript adopts an in-depth description of the details of the work done that makes it read more like a technical report than a scientific paper, where the relevant aspects should be highlighted and additional details synthetized in tables or supplementary material. With the profusion of details, the reader looses the purpose of the manuscript: To present a new methodological approach? To advocate the need for hazard assessment of submerged regions? To put forward a product ready to be used for the design of infrastructures? All of the above? Perhaps because of the profusion of apparent goals, the manuscript fails, in the opinion of this reviewer, to be convincing for any of them.
R: We agree with this observation. Consequently, we will work on reducing and synthesizing the additional details. Thanks to the selection of elements suggested in the written text on the website as well as in the interesting and detailed comments included in the attached pdf, our effort to reduce it will benefit the paper with a more concrete content.
When advocating for DSHA instead of PSHA, the authors seem to reinstate a dated controversy of the 1990s in the United States, after which vast clarifications of concepts and domains of applicability were obtained that were not discussed. While addressing a relatively untouched topic which is seabed hazard assessment, the authors fail to highlight the growing exposure of submarine structures and associated risk, which would be an interesting point in favour of the study. In what concerns the applicability of the results for practical purposes, the non-standard aspects of the approach would require much better justification for it to be considered fit for use.
R: We will clarify that our paper is one step in a path towards SHA by probabilistic methods, splitting the work into several articles (tasks or phase steps) to provide as much info on our work as possible. As you have seen, this first work is approached from a deterministic perspective, providing preliminary results, which will be refined in successive articles using probabilistic methods. Without doubt, one of the priority uses of these more precise results, before the determination of the effects on the instability of submarine slopes and the possible origin of tsunamis, could be for the anti-seismic dimensioning of infrastructures (oil and gas pipelines, communications cables, exploration structures, wind generators, etc.), including the design of tunnels or bridges located on the seabed, including the design of tunnels or bridges) which are settled on the seabed.
To illustrate the latter aspect: the authors advocate the adoption of a zoned DSHA method according to Wang and Huang (2014), relying heavily on that single reference – in fact, from which the EXCEL code to perform the computations was obtained - to support that not-so-standard choice. To differentiate zoned from non-zoned DSHA, the authors state that “the application of the DSHA is not zoned, if it is done on a homogeneous seismicity region, with a single seismic source of global influence” (line 394). However, it is well known that one of the strongest arguments in favour of DSHA, and one that makes it adopted for the design of critical structures, is that it is reality-based, taking into account the geological structures surrounding the site and their seismogenic characteristics. DSHA has at its core the concept of maximum credible earthquake (MCE) at a particular geological structure or set of well identified geological structures, and usually avoids the use of area sources altogether. In the manuscript, the authors describe in details the relevant geologic structures of the region of interest, but at the time of computing the hazard these structures do not seem to be taken in consideration, the MCE being considered equal to the MMax of each area source in previous (probabilistic!) studies.
R: We agree with this observation. Accordingly, we will review that line. It could be said that we actually use the DSHA using a zoned approach. It is true that the aforementioned paragraph (line 394) is incorrect and misleading. It even indicates that a zoned DSHA method is adopted, justifying it by the work of Wang and Huang (2014), which is neither exclusive nor correct. Likewise, for the reviewer's clarification, we will detail how some of the seismically relevant geological structures that are taken into account in the seismic hazard calculation process have been described.
Even more concerning, it was not possible for this reviewer to understand how the distance from source to site was computed prior to its insertion in the GMPEs: after describing several second-order corrections to the distance, the authors mention that those corrections were applied to “the minimum distance calculated with the original script” (line 618, referring to a pre-existing EXCEL script that was adapted for the calculations). Were the authors assuming that a fault capable of producing the MCE for each area zone existed at the minimum distance from the site to each area source? This would certainly increase the hazard unwarrantedly.
R: Thanks for this comment. We will recheck to further clarify how the distance is being considered.
Another non-standard approach that the authors take, still following Wang and Huang (2014), is the consideration of “non-control area sources” in addition to the “control area source” (i.e., the source producing the maximum maximum credible earthquake). In this case, the authors remark that the hazard is higher at sites near the “confluence” of source zones, an observation that in and by itself should raise concerns: if the pattern of the hazard is driven by the source zone geometry, something must be wrong with the methodology (actually, this limitation is even clearer in Wang and Huang (2014)’s hazard assessment for Taiwan, which shows a “checkered” pattern when “non-control sources” are considered, with no similarity whatever to the results obtained when non-control sources are not used (their figures 5 and 6); it should be pointed out that Wang and Huang (2014) was published in a journal which is very remote from mainstream seismological research, which may account for a less rigorous review process for that paper; it is a tell-tale detail that no mainstream hazard publication has refered Wang and Huang (2014)). The authors report a 445% (!) increase of the hazard at Cadiz (84% percentile) when non-control area sources are added to the control area source, and fail to identify that discrepancy as anomalous.
R: Thank you for this comment. We will review and try to clarify the statistical justification for adding uncontrollable seismic sources to the hazard assessment, beyond the fact that the cited authors use them in a publication of little impact, and improve the explanation of the limited real effect they have. We will adjust the definition of anomalous cases.
Because fundamental aspects like these were not documented to a point that this reviewer could understand, I am forced to consider the results unfit for use.
In addition to the serious reservations listed about, it must be mentioned that the manuscript contains inaccuracies easily corrected had the manuscript bee n previously subjected to any level of review prior to submission. Just to mention a few:
Line 93: “As shown by geodetic data and geodynamic studies that have been carried out for decades (e.g., Montessus de Ballore, 1894; Pastor, 1927; Munuera, 1963) tectonic models of NW-SE to WNW-ESE oblique convergence between the plates (Reilly et al., 1992; Herraiz et al., 2000) present displacements of 2 to 5 mm/yr”
Pre-plate tectonics studies can hardly have any relevance to the discussion rates of convergence between plates.
R: The reviewer is right. We recheck and consider updated references.
Line 212: “Seismic hazard is the probability that an earthquake occurs in a given geographic area, within a given time period, and with a ground motion intensity exceeding a given threshold (McGuire, 2004)”
This passage indicates a lack of familiarity with the concept of seismic hazard that is not adequate in a manuscript on seismic hazard assessment. Seismic hazard concerns probability of exceedance of a level of ground motion at a site over a period of time, not probability of occurrence of an earthquake in a geographic area.
R: The reviewer is right. We will review and clarify this passage.
line 387: “… the probability density of the 50th percentile, or average value, …”
The 50% percentile of a pdf corresponds to the median, not the “average” (in other passages. e.g., line 423, it is called “mean”, also wrongly), and even without that imprecision the sentence is devoid of meaning (the median is a value of the random variable, not of the probability density).
R: The reviewer is right. We will revise the passages to improve the explanation and clarify why the coincidence of the 50th percentile, mean and median that is being used interchangeably and is causing confusion to the reviewer and future readers.
etc, etc…
In view of the above remarks, I regret that I have to recommend that the manuscript should not be accepted for publication in NHESS. Should the authors consider rewriting it to try a subsequent submission, the attached pdf includes extensive comments on the passages that need improvement.
R: Dear reviewer, we greatly appreciate the work you have done and your dedication to reviewing this manuscript. We are reviewing the text considering your interesting and appropriate appreciations, comments, recommendations and corrections. Likewise, we will integrate your comments with those we may obtain from other reviewers to prepare a revised version of our manuscript for submission in the near future.
Thank you very much.
The authors
Citation: https://doi.org/10.5194/nhess-2023-52-AC1 - AC3: 'Reply on RC1', Adrián José Rosario Beltré, 17 Jul 2023
- AC4: 'Reply on RC1', Adrián José Rosario Beltré, 17 Jul 2023
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AC1: 'Reply on RC1', Adrián José Rosario Beltré, 11 May 2023
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RC2: 'Comment on nhess-2023-52', Alexandra Carvalho, 21 Jun 2023
Beltré et al. present a seismic hazard analysis, for bedrock conditions, of the marine area comprised between west Gulf of Cádiz and east Alboran Sea, based on a deterministic approach (DSHA), considering areal seismogenic sources.
The article is classified as a review paper, even though presents as stated “…the first comprehensive deterministic seismic hazard assessment carried out in the Ibero-Maghrebian region”, implying that the paper should be reviewed as a research paper.
The manuscript is very exhaustive and difficult to read because of so many detailed issues, some of them not deserving such a consideration as they are very technical and can be found elsewhere, and even so, some fundamental questions and debating are lacking. The state of the art is not truly addressed. If it might be considered a research paper, more than half of the manuscript is useless and should be deleted.
The study uses a mid-resolution digital terrain model, which seems a positive innovation. Nonetheless, I want to express a deep concern about this Seismic hazard assessment:
DSHA lies on the selection of the controlling earthquake (the earthquake that is expected to produce the strongest level of shaking) or a maximum credible earthquake for a specific fault. It depends a lot on the quantity and quality of the data collected, detailed active fault data and estimation of the dimensions of the fault, so that both maximum magnitude potential and source to site distance calculations result in realistic values. In the study presented, the geometry and characterization of the geological structures do not seem to have been under consideration either in terms of defining the maximum credible earthquake or in terms of the distance to the structure. In fact, considering arial seismogenic sourcs, a not very well understood distance (the distance to the area plus the depth of the area?) and the maximum magnitude, Mmax , of zones, I wonder why the use of a DSHA approach.
A few More comments:
line 36 …a Mw 7.7 and a unique reference for the 1755 Lisbon earthquake. Then, in line 119, it is mentioned a magnitude 8.5, with no reference. The same for the 1969 earthquake.
Line 43 “ …and mitigate seismic hazard” ?. Risk is mitigated, not seismic hazard.
Line 221: hazard IS NOT the probability that an earthquake occurs in a given geographic area.!!!
Line 285 : it is not true the statement “ … obtaining higher Y because they are closer to seismogenic sources”, as, as an example, directivity changes profoundly results.
And many mistakes easily corrected.
I would recommend a deep level of review, by the authors, before trying to submit it again.
Citation: https://doi.org/10.5194/nhess-2023-52-RC2 -
AC2: 'Reply on RC2', Adrián José Rosario Beltré, 27 Jun 2023
Dear Alexandra,
Thank you for your constructive comments. We have undertaken a major revision of our work following your suggestions and those of the other reviewers. We truly appreciate your effort in providing us feedback which has been utterly useful to improve our paper.
Kind regards,
The authors
Citation: https://doi.org/10.5194/nhess-2023-52-AC2 - AC5: 'Reply on RC2', Adrián José Rosario Beltré, 17 Jul 2023
- AC6: 'Reply on RC2', Adrián José Rosario Beltré, 17 Jul 2023
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AC2: 'Reply on RC2', Adrián José Rosario Beltré, 27 Jun 2023
Adrián José Rosario Beltré et al.
Adrián José Rosario Beltré et al.
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