the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 May 2023
Offshore seismicity clusters in the West iberian Margin illustrated by two decades of events
Gabriela Fernandez-Viejo
Carlos Lopez-Fernandez
Patricia Cadenas
Abstract. An analysis of two decades (2003–2022) of seismicity recorded by the Spanish and Portuguese seismic networks along the West Iberian passive margin has resulted in a better understanding of the distribution of moderate seismic activity in this intraplate submarine area. The study provides a precise trend of specific alignments inferred from the density maps of seismicity, giving an accurate depiction of event distribution along two wide stripes that extend 700 km long through the ocean floor in a WNW-ESE direction. These bands are parallel to the Africa-Eurasia plate boundary but are distinctly separated from its related seismicity by approximately 300 km and 700 km, respectively. This is a sufficient distance to be considered as intraplate activity. When trying to relate this seismicity to structural and geophysical features, a conclusive picture doesn't emerge. The earthquakes occur indiscriminately across thinned continental, hyperextended, and exhumed mantle rift domains. They fade out in the proximity of undisputed oceanic crust, but some events extend beyond. The hypocentral depths signal a considerable amount of events nucleating in the upper mantle. The focal mechanisms, although scarce, are predominantly strike-slip. Considering these observations, hypothesis ranging from subduction initiation, development of strained corridors or local structures of the margin, have been discussed in order to explain this relatively anomalous seismicity. However, some of them do not portray convincing arguments, while others are too unspecific. None of them are flawless, suggesting that several factors may be at play. Despite being one of the most probed passive margins in the world, the present geodynamical status of the West Iberian Margin manifested in its modern seismicity remains unknown. Interpreting this data within a global tectonic plate framework, together with the potential addition of seafloor seismometers, may provide the key to understanding this activity along one of the most archetypical margins of the Atlantic Ocean.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Gabriela Fernandez-Viejo et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1013', Sara Martínez-Loriente, 13 Jun 2023
Dr. Fernandez-Viejo and co-authors present the analysis of two decades of seismicity recorded in the Western Iberian Margin. The authors analyze its distribution and discuss three possible scenarios that could cause such seismicity. My main criticism of this work is that they do not go beyond discussion. The authors just explain the pros and cons of each scenario without reaching any conclusions. They neither rule out nor opt for any of the scenarios disussed.
I attached a document with moderate corrections / suggestions / comments.-
AC1: 'Reply on RC1', Gabriela Fernández-Viejo, 30 Jun 2023
Thank you for your feedback and suggested revisions. We appreciate your time and effort in reviewing our work. We have considered the comments and have taken steps to address them accordingly.
We have made corrections to the majority of the mistakes pointed out by the reviewer. Additionally, we have contested some of the criticisms by providing further clarification and supporting evidence. We have also included additional references to strengthen the credibility of our work.
Regarding the critic about the lack of exploration beyond the discussion, we would like to emphasize that the primary aim of our research is to provide a comprehensive analysis and discussion of the seismicity in this particular margin. While we do lean towards the possibility of strike-slip faulting as an explanation for the current seismicity and its characteristics, we feel that the available data from the centra and northern parts of the margin do not provide enough diversity and/or quality to definitively rule out any hypotheses or fully support them.
In order to form a more conclusive hypothesis or even generate a new one, it would be crucial to obtain a substantial amount of data on the sea bottom of the margin, specifically related to seismology, as well as acquire high-resolution seismic 3D data in areas where seismicity is more prevalent. Once we have access to such data in the future, it may be possible to infer certain structures or parameters that could lend support to existing hypotheses or enable the formulation of new ones.
Until that time, we believe it would be premature to propose a final hypothesis regarding the origin of seismicity for the entire margin, as even the northern and central parts could exhibit distinct sources and dynamics contributing to the observed seismic activity.
We attach a document where comments are assessed one by one and corresponding changes have been made in the manuscript text and figures
Once again, we sincerely appreciate your input and the opportunity to address your concerns. If you have any further suggestions or inquiries, please do not hesitate to let us know.
Sincerely,
The authors
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AC1: 'Reply on RC1', Gabriela Fernández-Viejo, 30 Jun 2023
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RC2: 'Comment on egusphere-2023-1013', Chris Elders, 09 Aug 2023
This is an interesting manuscript that provides a compilation of recent seismicity on the Iberian Margin and critically evaluates four possible explanations for its distribution. The paper is through and well structured, and is largely acceptable for publication as it is. However, it would benefit from a more thorough editorial review to improve fluency – I have made a number of suggestions on the attached manuscript, but the authors might consider seeking further assistance. I apologise – my annotations are on the originally submitted manuscript, rather than the revised manuscript uploaded after the first review.
I concur with the author’s response to that original review – without more data, it is not possible to effectively distinguish between the different scenarios that the outline. The manuscript is useful as it stands – it provides a very clear summary of the current state of knowledge, and is hopefully a spur for further research. Given the very clear and significant intra-plate deformation that is documented in this manuscript, this area certainly has the potential to further our understanding of the way in which plates deform and the geodynamic processes that might operate.
My only substantial comment is what to me look more like a circular pattern illustrated on the magnetic anomaly map in Figure 4c, and the apparent distribution of earthquakes around its perimeter – or two thirds of it at least. Is this real, or just an illusion that results from plotting seismicity on such a map. Is there a different way of analysing the clustering of earthquakes that would test the validity of a radial, rather than a linear distribution of the data, and is there any significance to the patterns illustrated on the magnetic anomaly map?
Chris Elders
Curtin University
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AC2: 'Reply on RC2', Gabriela Fernández-Viejo, 31 Aug 2023
Response to Reviewer 2
We appreciate the time and effort invested on reviewing the work and thank you for the suggestions, comments and the editing. We have now made the pertinent changes included in the revised version. Most of them were accepted, except from paragraphs were other changes were included following the suggestions and comments of a previous revision.
The question aside from the text that the reviewer posts is if there would be a “radial” rather than linear pattern in the seismicity that seems to coincide with the pattern of a magnetic anomaly in the area (fig 4c).
The answer is: possibly. Although at large distance the striped NW-SE pattern seems quite clear, it is also acknowledgeable that in closer inspection the striped trend gets diffuse, adding difficulty on the interpretation and grouping events in smaller clusters. The magnetic anomaly that the reviewer refers to, results of the structural disposition of the basement rocks, the variscan formations that can be seen on land. Our understanding is that probably the pattern of seismicity presents more complication than the simplistic NW-SE bands we talk about, something that it is intrinsic to the study of this type of moderate, low seismicity in intraplate “quiet” settings. Therefore, any minor structure slightly moving, added to the uncertainty location, will give events that may be clustered in specific smaller scale structural features such as the one the magnetic anomaly evidences.
Regarding the text edits and comments in the manuscript, we proceed to deal with all of them individually.
Line 152 comment on “the map shows in this area”. It is an unfortunate phrasing. We have changed the text to serve the meaning we wanted to give to this sentence.
Line 198 “however the separation in two bands is not that simple”. Well, this is a poorly expressed explanation. Please refer to the introduction of this document. We have rephrased.
Line 218 “inverse fault” changed to reverse fault.
Line 226 “which relates to an unknown error”. This refers to the fact that the depth location of seismicity has always a higher error than in the XY coordinates. This is due to the method for getting the events depth, which uses as input a preliminary velocity model. In turn, this velocity model and its variations affect very much the depth of solutions and the more geologically restricted the better. In this area, where there is an abundant set of deep seismic reflection and refraction data, the local velocity model is as good as it can be, but it also has its own depth-velocity errors.
Line 232 “wihin it”. To the north of it.
Line 235 “the referred vertical alignment would be consistent with a volcanic origin for those particular events”. The reviewer says (rightly) that there is not vertical alignment in this location. The reviewer number one also pointed out this contradiction. We have amended those paragraphs, but still noting that some vertical alignments may have to do with volcanism as the area shows an abundance of these edifices in close areas, preferring this interpretation to a structural cause, due to the mantle depth of most of the events.
Line 275 “main N-S disposition of the magnetic anomalies”. The reviewer points out the circular pattern in the magnetic anomaly, which seems to cluster also the events. And yes, the reviewer is right about that, we have partially answered or commented this fact at the beginning of the document and also in the text. The phrase refers more to the N-S alignment of the oceanic magnetic anomalies, which are parallel to the ridge. Of course, in the continental platform and margin, the complexity of the magnetic anomalies has to do with the continental basement. The seismicity seems to cluster around several structural features within the margin but we did not find a clear relation to known structures or outcropping features in the sea floor.
Line 299 “the text highlights the limitations of seismotectonic interpretations due to inconclusive mapping of structures in the sea floor of the WIM”. We have rewritten this paragraph and tried not to repeat the things, moving them to section 2.
Line 227 “may have been bouncing from its perpendicular to extension direction”. This means that the Iberian Peninsula, specifically its N-S Margin along Spain and Portugal, which now is parallel to the ridge, may have been oblique to the ridge extension direction in several periods while the Atlantic opening took place. This means the micro Iberian plate included some type of rotation adding complexity to the margin structure. The presence of the triple point to the north also supports the evidences of rotational movement of Iberia while extension along the ridge maintained its direction.
The revised manuscript with the changes is uploaded in the system for its reassessment.
Thank you, sincerely,
The Authors
Citation: https://doi.org/10.5194/egusphere-2023-1013-AC2
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AC2: 'Reply on RC2', Gabriela Fernández-Viejo, 31 Aug 2023
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EC1: 'Comment on egusphere-2023-1013', Tiago Alves, 31 Aug 2023
Dear authors,
I checked and validated all the comments you provided and I happy to recommend the submission of your final manuscript to Solid Earth. I believe you will be given instruction on how to procede as such.
The only remark I have is about my own Alves (2023) - https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1671/, which is in review at the moment, a paper that seems to complement your work insomuch as it proves the fact that large fault zones in SW Iberia demonstrate coherenece in their geometry and displacement records. In other words, the faults mapped by Alves (2023) are prone to interact to form long fault zones when considering their geological records of (seismogenic) movement. Reviewer #2 made a comment alog those lines when referring to a 'curved/arched' distribution of epicentres along the coontinental-oceani boundary of West Iberia. I suggest you ask the main editor of the Special Volume (F. Zwann) about this particular point, and how the reference to Alves (2023) work in review is justified. I cannot reach the decision myself as the handling editor.
Thanks for completing all the necessary amendements to your work.
Tiago
Citation: https://doi.org/10.5194/egusphere-2023-1013-EC1 -
AC3: 'Reply on EC1', Gabriela Fernández-Viejo, 04 Sep 2023
Dear Editor:
Thank you for the recommendation for publication and we are pleased that the revisions were pertinent. Regarding your comment about Alves (in review), I have read the manuscript and although it deals with the southernmost segment of the margin, it may be adequate to cite. I will write to the Editor of the volume and ask if this can be done now or later.
Thank you for your work as handling Editor,
Sincerely
The Authors
Citation: https://doi.org/10.5194/egusphere-2023-1013-AC3
-
AC3: 'Reply on EC1', Gabriela Fernández-Viejo, 04 Sep 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1013', Sara Martínez-Loriente, 13 Jun 2023
Dr. Fernandez-Viejo and co-authors present the analysis of two decades of seismicity recorded in the Western Iberian Margin. The authors analyze its distribution and discuss three possible scenarios that could cause such seismicity. My main criticism of this work is that they do not go beyond discussion. The authors just explain the pros and cons of each scenario without reaching any conclusions. They neither rule out nor opt for any of the scenarios disussed.
I attached a document with moderate corrections / suggestions / comments.-
AC1: 'Reply on RC1', Gabriela Fernández-Viejo, 30 Jun 2023
Thank you for your feedback and suggested revisions. We appreciate your time and effort in reviewing our work. We have considered the comments and have taken steps to address them accordingly.
We have made corrections to the majority of the mistakes pointed out by the reviewer. Additionally, we have contested some of the criticisms by providing further clarification and supporting evidence. We have also included additional references to strengthen the credibility of our work.
Regarding the critic about the lack of exploration beyond the discussion, we would like to emphasize that the primary aim of our research is to provide a comprehensive analysis and discussion of the seismicity in this particular margin. While we do lean towards the possibility of strike-slip faulting as an explanation for the current seismicity and its characteristics, we feel that the available data from the centra and northern parts of the margin do not provide enough diversity and/or quality to definitively rule out any hypotheses or fully support them.
In order to form a more conclusive hypothesis or even generate a new one, it would be crucial to obtain a substantial amount of data on the sea bottom of the margin, specifically related to seismology, as well as acquire high-resolution seismic 3D data in areas where seismicity is more prevalent. Once we have access to such data in the future, it may be possible to infer certain structures or parameters that could lend support to existing hypotheses or enable the formulation of new ones.
Until that time, we believe it would be premature to propose a final hypothesis regarding the origin of seismicity for the entire margin, as even the northern and central parts could exhibit distinct sources and dynamics contributing to the observed seismic activity.
We attach a document where comments are assessed one by one and corresponding changes have been made in the manuscript text and figures
Once again, we sincerely appreciate your input and the opportunity to address your concerns. If you have any further suggestions or inquiries, please do not hesitate to let us know.
Sincerely,
The authors
-
AC1: 'Reply on RC1', Gabriela Fernández-Viejo, 30 Jun 2023
-
RC2: 'Comment on egusphere-2023-1013', Chris Elders, 09 Aug 2023
This is an interesting manuscript that provides a compilation of recent seismicity on the Iberian Margin and critically evaluates four possible explanations for its distribution. The paper is through and well structured, and is largely acceptable for publication as it is. However, it would benefit from a more thorough editorial review to improve fluency – I have made a number of suggestions on the attached manuscript, but the authors might consider seeking further assistance. I apologise – my annotations are on the originally submitted manuscript, rather than the revised manuscript uploaded after the first review.
I concur with the author’s response to that original review – without more data, it is not possible to effectively distinguish between the different scenarios that the outline. The manuscript is useful as it stands – it provides a very clear summary of the current state of knowledge, and is hopefully a spur for further research. Given the very clear and significant intra-plate deformation that is documented in this manuscript, this area certainly has the potential to further our understanding of the way in which plates deform and the geodynamic processes that might operate.
My only substantial comment is what to me look more like a circular pattern illustrated on the magnetic anomaly map in Figure 4c, and the apparent distribution of earthquakes around its perimeter – or two thirds of it at least. Is this real, or just an illusion that results from plotting seismicity on such a map. Is there a different way of analysing the clustering of earthquakes that would test the validity of a radial, rather than a linear distribution of the data, and is there any significance to the patterns illustrated on the magnetic anomaly map?
Chris Elders
Curtin University
-
AC2: 'Reply on RC2', Gabriela Fernández-Viejo, 31 Aug 2023
Response to Reviewer 2
We appreciate the time and effort invested on reviewing the work and thank you for the suggestions, comments and the editing. We have now made the pertinent changes included in the revised version. Most of them were accepted, except from paragraphs were other changes were included following the suggestions and comments of a previous revision.
The question aside from the text that the reviewer posts is if there would be a “radial” rather than linear pattern in the seismicity that seems to coincide with the pattern of a magnetic anomaly in the area (fig 4c).
The answer is: possibly. Although at large distance the striped NW-SE pattern seems quite clear, it is also acknowledgeable that in closer inspection the striped trend gets diffuse, adding difficulty on the interpretation and grouping events in smaller clusters. The magnetic anomaly that the reviewer refers to, results of the structural disposition of the basement rocks, the variscan formations that can be seen on land. Our understanding is that probably the pattern of seismicity presents more complication than the simplistic NW-SE bands we talk about, something that it is intrinsic to the study of this type of moderate, low seismicity in intraplate “quiet” settings. Therefore, any minor structure slightly moving, added to the uncertainty location, will give events that may be clustered in specific smaller scale structural features such as the one the magnetic anomaly evidences.
Regarding the text edits and comments in the manuscript, we proceed to deal with all of them individually.
Line 152 comment on “the map shows in this area”. It is an unfortunate phrasing. We have changed the text to serve the meaning we wanted to give to this sentence.
Line 198 “however the separation in two bands is not that simple”. Well, this is a poorly expressed explanation. Please refer to the introduction of this document. We have rephrased.
Line 218 “inverse fault” changed to reverse fault.
Line 226 “which relates to an unknown error”. This refers to the fact that the depth location of seismicity has always a higher error than in the XY coordinates. This is due to the method for getting the events depth, which uses as input a preliminary velocity model. In turn, this velocity model and its variations affect very much the depth of solutions and the more geologically restricted the better. In this area, where there is an abundant set of deep seismic reflection and refraction data, the local velocity model is as good as it can be, but it also has its own depth-velocity errors.
Line 232 “wihin it”. To the north of it.
Line 235 “the referred vertical alignment would be consistent with a volcanic origin for those particular events”. The reviewer says (rightly) that there is not vertical alignment in this location. The reviewer number one also pointed out this contradiction. We have amended those paragraphs, but still noting that some vertical alignments may have to do with volcanism as the area shows an abundance of these edifices in close areas, preferring this interpretation to a structural cause, due to the mantle depth of most of the events.
Line 275 “main N-S disposition of the magnetic anomalies”. The reviewer points out the circular pattern in the magnetic anomaly, which seems to cluster also the events. And yes, the reviewer is right about that, we have partially answered or commented this fact at the beginning of the document and also in the text. The phrase refers more to the N-S alignment of the oceanic magnetic anomalies, which are parallel to the ridge. Of course, in the continental platform and margin, the complexity of the magnetic anomalies has to do with the continental basement. The seismicity seems to cluster around several structural features within the margin but we did not find a clear relation to known structures or outcropping features in the sea floor.
Line 299 “the text highlights the limitations of seismotectonic interpretations due to inconclusive mapping of structures in the sea floor of the WIM”. We have rewritten this paragraph and tried not to repeat the things, moving them to section 2.
Line 227 “may have been bouncing from its perpendicular to extension direction”. This means that the Iberian Peninsula, specifically its N-S Margin along Spain and Portugal, which now is parallel to the ridge, may have been oblique to the ridge extension direction in several periods while the Atlantic opening took place. This means the micro Iberian plate included some type of rotation adding complexity to the margin structure. The presence of the triple point to the north also supports the evidences of rotational movement of Iberia while extension along the ridge maintained its direction.
The revised manuscript with the changes is uploaded in the system for its reassessment.
Thank you, sincerely,
The Authors
Citation: https://doi.org/10.5194/egusphere-2023-1013-AC2
-
AC2: 'Reply on RC2', Gabriela Fernández-Viejo, 31 Aug 2023
-
EC1: 'Comment on egusphere-2023-1013', Tiago Alves, 31 Aug 2023
Dear authors,
I checked and validated all the comments you provided and I happy to recommend the submission of your final manuscript to Solid Earth. I believe you will be given instruction on how to procede as such.
The only remark I have is about my own Alves (2023) - https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1671/, which is in review at the moment, a paper that seems to complement your work insomuch as it proves the fact that large fault zones in SW Iberia demonstrate coherenece in their geometry and displacement records. In other words, the faults mapped by Alves (2023) are prone to interact to form long fault zones when considering their geological records of (seismogenic) movement. Reviewer #2 made a comment alog those lines when referring to a 'curved/arched' distribution of epicentres along the coontinental-oceani boundary of West Iberia. I suggest you ask the main editor of the Special Volume (F. Zwann) about this particular point, and how the reference to Alves (2023) work in review is justified. I cannot reach the decision myself as the handling editor.
Thanks for completing all the necessary amendements to your work.
Tiago
Citation: https://doi.org/10.5194/egusphere-2023-1013-EC1 -
AC3: 'Reply on EC1', Gabriela Fernández-Viejo, 04 Sep 2023
Dear Editor:
Thank you for the recommendation for publication and we are pleased that the revisions were pertinent. Regarding your comment about Alves (in review), I have read the manuscript and although it deals with the southernmost segment of the margin, it may be adequate to cite. I will write to the Editor of the volume and ask if this can be done now or later.
Thank you for your work as handling Editor,
Sincerely
The Authors
Citation: https://doi.org/10.5194/egusphere-2023-1013-AC3
-
AC3: 'Reply on EC1', Gabriela Fernández-Viejo, 04 Sep 2023
Peer review completion
Journal article(s) based on this preprint
Gabriela Fernandez-Viejo et al.
Gabriela Fernandez-Viejo et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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