the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Apr 2024
Lesser Antilles Seismotectonic Zoning Model for Seismic Hazard Assessment
Abstract. Subduction zones pose a considerable challenge within the realm of seismotectonics, owing to their faults and structures interactions. The Lesser Antilles arc is a good example of how these complexities impact seismic hazard studies with a strong along-strike variations in tectonic, seismic, and volcanic activities. While they have generated significant damages, the 1839 and 1843 event characteristics (locations, depths, mechanisms, magnitudes) remain a subject of debate along with their potential implications in the megathrust seismicity, in particular in the frame of low interseismic coupling. This study is grounded in the compilation of instrumental and historical seismicity, and fault catalogs, completed by analyses of focal mechanisms and rupture types as well as geodetic velocities and strain rates. The seismotectonic model and zoning of the Lesser Antilles encompass the upper plate, subducting oceanic plate, subduction interface, mantle wedge, and volcanoes. We propose a better depth resolution, resulting from recent studies on slab top and upper plate bottom geometries, a specific area source for the Marie-Galante graben, new propositions for mantle wedge and volcanic zoning, and fully revised area sources for the subduction interface. Our study highlights specific needs for a better seismic hazard assessment in this region.
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Status: final response (author comments only)
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RC1: 'Comment on nhess-2024-53', Anonymous Referee #1, 06 Jun 2024
Foix et al. proposed an updated seismotectonic zoning model for the Lesser Antilles. This study benefits from a variety of recent findings by others on such as seismicity, focal mechanisms, and geodetic observations. While summarizing these findings and using them as a basis to define seismotectonic zones is beneficial, the article itself does not clearly illustrate how these data are used, or how the zoning is defined quantitatively. Section 3.1 states that three principles are followed, but it is not clear in the data and methods section how seismicity distribution, for instance, is used for zoning in different parts of the subduction zone. Is it based on depth, or spatial clustering? These aspects need to be introduced in the methods section.
A new seismotectonic zoning model would be useful for seismic hazard assessment. In the current manuscript, it is not clear whether there was a previous zoning model for the Lesser Antilles. If so, a comparison with the previous version would help to identify the contributions from the recent findings. It would be even better if a preliminary hazard model could be provided and compared with the 2002 version, as this would enrich the discussion.
The modeling work to explore interseismic coupling on the plate interface is interesting and could be included in the main text. The observation and modeling support the claim that more data covering the plate interface, potentially from seafloor geodesy, are needed to resolve the coupling issue.
The current writing is more like a report than an article. Line 590 and 595 mention a report and dataset that were already published by the authors. How does the current submitted article differ from that report?
Line 120-125, did you include focal mechanisms from Lindner et al., 2022?
Line 130, plate interface geometry plays an important role in your work. How are the two slab models unified? This needs to be detailed in the main text or the appendix.
Line 365, Bie et al., 2022 presented a seismic velocity model that supports the cold mantle wedge nose.
Line 395, a circular source area with 10-km radius is defined for all active volcanoes. What is the logic behind selecting 10 km? Is it arbitrary? Should some physical properties be considered in deciding this number? This part warrants a discussion in the final section.
The authors may consider depositing their homogenized seismic catalogue for open access.
Citation: https://doi.org/10.5194/nhess-2024-53-RC1 -
AC1: 'Reply on RC1', Océane Foix, 02 Aug 2024
Dear RC1, editor,
You will find in attached the answer to the RC1's comments.
Best regards,
Océane Foix.
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AC2: 'Reply on AC1', Océane Foix, 28 Aug 2024
The seismic catalog is now available online - Bertil et al. (2024) - https://doi.org/10.18144/7a1f2980-4440-4d4f-a78b-326acdb5e873
Citation: https://doi.org/10.5194/nhess-2024-53-AC2
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AC2: 'Reply on AC1', Océane Foix, 28 Aug 2024
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AC1: 'Reply on RC1', Océane Foix, 02 Aug 2024
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RC2: 'Comment on nhess-2024-53', Anonymous Referee #2, 05 Sep 2024
It took me a while to realize that the manuscript is a condensed report that was previously submitted to a funding source (Foix et al., 2023a). It would be helpful to the reader to clearly define the assignment and parameters of this report.
The manuscript promises to be an improvement over past assessments in the area, although nowhere it is specified what the past assessments were and whether their aims and methodologies were the same. The only past assessments listed are Pagani (global assessment), Zimmerman (Caribbean-wide) and Geoter (France, but hard to tell because of the partial reference).
If I am not mistaken, the focus of the paper is on defining a seismotectonic zoning model for the Lesser Antilles (although the abstract says “seismotectonic model and zoning” (L16). How is “model” defined? Is it simply the authors’ interpreting the geophysical evidence as belonging to different tectonic regimes?
Most of the work appears to be compilations of catalogs and of past studies. The only original work includes the magnitude frequency distributions, the calculations of b-values and the 2-D geodetic modeling, all of which are inexplicably hidden in the Supplement (S2-S4).
Given the sparse data on which the zonation is based, their interpretation is likely not definitive, yet there is little discussion about alternatives and implications of the proposed zonation.
A more fundamental question is whether it is useful to divide the study area into so many zones given the sparse instrumental and historical seismicity, and the geodetic data available for analysis. Wouldn’t it be simpler to divide the north and south parts of the Lesser Antilles arc into Outer Rise, intraslab, slab interface, sub-crustal and crustal zones? This particularly true because the zonation exercise is meant as a first step for hazard, ground shaking, and damage assessments, each of which, will likely have large uncertainties. The effort should therefore be to minimize the propagation of uncertainties even at the expense of lumping together several “zones” to achieve a more robust characterization of each “mega-zone”.
To turn the manuscript into a paper suitable for global readership, I would recommend:
- Clarifying what past assessments were and what your improvements are.
- Moving the Methods and data into its own heading before the Seismotectonic zoning model section
- Clearly stating what you mean by “seismotectonic zoning model” or simply call it “proposed seismotectonic zonation for the Lesser Antilles”.
- Including the original work, shown in Supplement S2-S4 in the main part of the paper.
- The discussion chapter is weak. Please strengthen it with a discussion of alternative zonations and their implications to seismic hazard.
- Please give a brief explanation how the zonation helps in the calculation of seismic hazard, ground shaking, etc.
Other comments:
Line 320 – The slab interface is divided into sections one of them spanning depths of 35-65 km, which according to Figure 7, is the maximum depth of slab interface earthquakes (although this is never being clearly stated anywhere in the manuscript). On the other hand, the geodetic model defines the high coupling zone to be between 40-80 km (Line 295), where 80 km is close to a temperature of 450°C according to Ezenwaka’s model, not 350°. An upper temperature limit of 450° for subduction interface earthquakes was also postulated in Cascadia (e.g., Hyndman and Wang, 1995).
There is very little information about seismic hazard from volcanic activity (Section 3.6), so it can be safely removed. The lack of information can be mentioned in a single sentence in either the Introduction or the Discussion sections.
It will be helpful to add a generic cross-section of the subduction zone at the beginning of Section 3 showing the locations of the different zones discussed in this section.
Outer rise or subduction interface sources proposed from tsunami models (Cordrie et al., 2022, Wei et al. 2024) are not mentioned or considered here. Why?
Minor comments:
Table 1 – For Anegada and Muertos – Either cite specific references for these regions not block models or general regional compilations, or delete.
Fig. 4 – What is “AW Death”? Do you mean Inactive accretionary wedge?
Citation: https://doi.org/10.5194/nhess-2024-53-RC2
Data sets
A New Seismic Source Zone Model for Lesser Antilles Seismic Hazard Assessment - Data Océane Foix, Stéphane Mazzotti, and Hervé Jomard https://doi.org/10.15148/dd520135-7656-4e80-91fe-f0284accbc76
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