Three-Dimensional Architecture of Foreland Basins from Seismic Noise Recording: Tectonic Implications for the Western End of the Guadalquivir Basin

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work constitutes an interesting contribution to the morphostructural study of sedimentary basins using passive methods. From this perspective, the configuration of the mechanical bedrock of the northern margin of the westernmost section of the Guadalquivir Basin, located in the southwest of the Iberian Peninsula, is obtained. The influence of fracturing, through normal faults, on the configuration of this bedrock is also demonstrated, including its influence on the present-day geomorphology of the basin surface. This would allow these fractures to be proposed as active, to a certain extent, during the Quaternary.

In my view, the authors should rigorously describe the uncertainties of the methodology used and separate the results and discussion sections when referring to mechanically bedding rock formations from the actual basement. This leads to some confusion for readers unfamiliar with these techniques.
Therefore, the contribution of the work would be enhanced if the results obtained were clearly defined, given that defining the basement using these techniques could correspond to different scenarios in different sectors of the basin.
Finally, I believe the work area should also be better localized. At the beginning of the reading, the reader gets the impression that this will be a cross-section of the basin from the north margin to the south margin, but in reality, the data are concentrated on the north margin. As other points for improvement in Figure 1, the upper left box is not read properly, likewise the broad picks are not clearly differentiated in Figure 7, the location of the soundings in Figure 10 and the basement depth isolines obtained from them are not clearly differentiated, therefore these three figures should be improved.

Author Response

We sincerely appreciate your comments, which we find highly relevant and constructive in improving the clarity and overall quality of our manuscript. Please find below our detailed responses to each of your observations:

1. Regarding the description of methodological uncertainties:
A paragraph has been added to the Discussion section addressing the uncertainty associated with the HVSR method and the empirical formula. Additionally, we have modified one of the figures to explicitly show the uncertainty in the calculations. The regression line was calibrated for values between 0.27 and 0.89 Hz, and it can be clearly seen that the error for frequencies above 1 Hz is negligible. This frequency range accounts for nearly 95% of the total data.

2. Distinction between mechanical bedrock and geological basement:
Thank you for pointing this out. We agree that this could cause confusion for readers unfamiliar with these techniques. We have thoroughly revised the Results and Discussion sections to explicitly differentiate between units acting as mechanical bedrock (recongised by the HVSR peaks) and the actual geological basement.

3. Clarification of the study area:
To avoid confusion, we have revised both the Introduction and the Study Area sections to clearly state that the research focuses exclusively on the northern margin of the basin.

4. Improvements to the figures:

Figure 1: The upper left inset has been simplified to enhance readability.

Figure 7: The “broad peak” layer has been moved to the top and its color modified to better highlight areas with broad peaks.

Figure 10: The figure has been revised to zoom in on the area of interest; in addition, line contrast has been increased and symbol sizes have been enlarged for improved clarity.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, authors present the results of a study conducted to discern the position of the seismic bedrock in the westernmost part of the Guadalquivir basin. For this purpose, ambient noise was measured at 334 sites and the resonance frequency of the sites was obtained. From data coming from boreholes and the frequency determined at the same sites where boreholes were drilled, authors stablish an empirical relationship that relates the frequency of basin fill with the corresponding thickness of sediments. Subsequent use of this relation allows transforming the frequencies into thicknesses. These results are then interpreted based on geological considerations.

Regarding the manuscript, I find several aspects that should be improved before considering the paper acceptable for publication.

The first issue is the lack of definition of seismic bedrock in the paper. This is important because I think that it represents the target of the geophysical study conducted. When I started to read the paper, I thought it was the Variscan formations, but this is not confirmed in the text. In fact, section 2 (Geographical and geological setting) is so confusing to this respect that at the moment of writing these lines, I am not sure if other units may also represent the bedrock. Please, revise the text and clearly state the bedrock considered in your study.

The second issue, and main one, is the relation proposed to link frequencies with basin fill thicknesses. Authors explain there are nine (5+2+2) sites with both variables known (only 8 sites in Fig. 4), with frequencies ranging from 0.27 to 0.89 Hz (thicknesses from, approx., 100 to 650 m) and fitted these data. If we want a rigorous use of this relation, it should only be used within this range (of frequencies/thicknesses) and any extrapolation may include important errors, not considered nor evaluated. It may induce to severe errors in the interpretations when using this relation for frequencies clearly outside these bounds (for instance, for frequencies above 1 Hz).

To this respect, I miss a plot showing real vs. estimated thicknesses and a table showing these data and the percent of error in estimations. This would be more illustrative of the correctness of the relation proposed and the uncertainties in the results. Notice that this is partially done in Fig. 10, but obtaining an opinion of the goodness of the fit when analyzing this figure is complicated to impossible.

In the text, lines 516 to 531, some data are provided, but I find notable differences in the values reported and those shown in Figure 8. For instance, for borehole Huelva-1, basement is located at -603 m (below sea level) and, according to Figure 8, this depth should be of the order of -450 to -470 m, not the -500 m reported in the text (this last value minimizes the error). Conclusions of the manuscript report discrepancies of 30 to 100 m, but this is not informative as it is not the same 100 m discrepancy with respect to a total depth of 300 m (33% error) than with respect to 600 m (16%). I consider that a more exhaustive analysis should be provided in the text.

Authors report that the relation obtained differs from another previously published by some of co-authors for the same area (line 333). Please explain why are they different if they were set for the same study zone, the same materials and, probably, same input data. How different are predictions of thicknesses? It would be convenient including this relation in Figure 4 to show differences.

 

Other minor issues found:

 

1. Geographical names.

Paper includes too many geographical names, many of them shown in several figures, but not all names are shown (Doñana National Park, Villamanrique de la Condesa, Huelva province, Marismas del Odiel, and so on). Please remove from text or add them in Figures (at the risk of making figures unreadable).

 

2. Geological setting.

This section is too long and repetitive. Description of geological formations is done from a geological point of view, but sometimes the lithological nature of materials is not provided. Notice that potential readers will be interested in the possible contrast existing between the seismic bedrock (hard rock) and basin fill (soft rocks); for this, the lithological nature of rocks is more important than other tectonic or stratigraphic characteristics of the units.

In other cases, name of units does not correspond to units showed in figures (for instance, where is Niebla formation located in Figure 1?, and Gibraleon clays?, blue marls formations? …). Please, revise and correct. If Figure represents units on a chronological basis, describe them with the same criterion.

Description of the structure and distribution of geological units is a bit confusing. I miss a synthetic cross-section of the study zone, showing the distribution of units. This will be more illustrative than text (lines 162 to 242).

 

3. Map of basement.

Although authors have made a great effort to measure as much sites as possible, the isobath map presented in Figure 8 has several areas based exclusively on extrapolation of data, without information. For instance, the area where a borehole with known depth of -621 m is located in an area with no data, approximately 30 x 30 km wide. This would explain the large discrepancies observed between observed (-621m) and estimated (range -350 to -450 m). Perhaps, two separate maps (those where measurements were done) would be more confident and representative of the position of seismic bedrock.

 

4. Conclusions

They are too long and not valid conclusions. They mix real conclusions with interpretations. Please, limit conclusions to those aspects relevant, not affected by personal interpretations.

 

5. Interpretation of results

This item refers to interpretation made of results in Figure 9. Cross-sections have been prepared for profiles more than 50 km long. Each profile includes (more or less) about 20 sites of measurement. Faults proposed are based on surface data but they are not supported by geophysical data because linear density of data is very low. For instance, fault proposed in ‘Huelva South’ profile (at about 6 km position) has not representation in the geophysical results. Something similar occurs for other faults in other profiles. To recognize the possible effect of fault on the position of seismic bedrock, it would require having measurements every 300 to 500 m along the profile.

Because this plot does not include the uncertainty in thickness estimated for each site, it is unknown if displacements attributed to faults may be within the range of error of the method and may not have any geological origin. I would suggest including bar errors for each site and limit the possible faults to only those sites where data density is higher (for instance, eastern part of profile ‘Huelva north’).

 

6. Figures

Figure 3 is not a good location map for showing the sites of measurement. Figure 6 is better for this purpose. I would recommend removing Figure 3. Legend of frequencies in Figure 6 should be reversed (from low to high).

Colors of some symbols in Figure 7 are very similar to the fill used for some geological units, making difficult to locate the position of these symbols.

Figure 10 should be changed by a plot showing real vs. estimated thicknesses of sites with available data. Current figure does not allow any comparison.

Figure 11: Is satellite image necessary?

 

Author Response

We want to thank you for your comments. We respectfully ask the reviewer to check the revised manuscript and the attached response document, where each of the raised concerns has been carefully addressed.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors (1) The author explores the subsurface of its westernmost sector using the HVSR method, recording seismic noise at 334 stations between the mouths of the Guadiana and the Guadalquivir rivers, near Doñana National Park. It presents the first three-dimensional model of the basin basement over an area exceeding 2,300 km², revealing a horst-and-graben system formed by foreland deformation linked to the westward advance of the Rif-Betic orogenic front. However, the research content seems inconsistent with the theme of the journal Remote Sensing, and the data used by the author are not remote sensing data. (2) Has there been any historical earthquake in the study area? How reliable is the first three-dimensional model of the basin basement presented by the author? It is suggested that corresponding indicators be provided in the paper. (3) Are all the data used in the paper collected by the author's team themselves? It is recommended to put the data on a sharing platform. (4) The author mentions the advantages of the HVSR method and that it has achieved good results in the study area. Has the author compared it with other methods? If there is no comparison, how to judge that the results obtained by the HVSR method are the most ideal? (5) It is suggested that the author strengthen the analysis of the data part. From the current paper, this part of the content is relatively insufficient. The key of this paper lies in the data collected in the study area. Only when the data are well processed can the final conclusion drawn by the author be possibly correct. Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

First of all, I would like to thank the reviewer for the detailed and thoughtful comments, which have been very helpful in improving the manuscript. Below, I provide responses to each of the concerns raised:

On historical seismicity in the study area:

Yes, the region has experienced historical seismic events. For example, the 1755 Lisbon earthquake had documented effects across southwestern Iberia, including the study area. However, it is important to note that while the broader region is considered to have a significant seismic hazard, the main source of this hazard is the Azores-Gibraltar fault zone, located offshore. The faults identified and discussed in our study area, though structurally relevant for the basin architecture, are currently considered aseismic. It's true that the instrumental seismicity data recorded by the Spanish National Geographic Institute (IGN) in the study area may correlate with some of the fractures documented in this work, based on the location of the hypocenter of certain events. However, our research does not address regional seismicity or the analysis of active faulting. It would nevertheless be of great interest to relate seismicity data to the active fractures identified in the investigated area. At this stage, in our view, we can present a set of faults that, in the absence of further seismic evidence, would be classified as very slow-moving or aseismic in most cases. Our current investigation—and those planned in the coming years—could help improve the understanding of the tectonic framework associated with low- to moderate-magnitude earthquakes. One such event is the earthquake recorded on December 20, 1989, with its epicenter in Ayamonte and a magnitude of 5.1, according to the IGN. A more detailed study of the seismic source area and its seismotectonic characteristics would be required to confirm such interpretations, but that lies beyond the scope of the present study. We have clarified this distinction in the revised introduction and discussion sections, emphasizing the tectonic but non-seismogenic role of the mapped structures.

On the reliability of the 3D model and need for indicators:

The 3D model is based on a dense network of 334 HVSR measurements across an area of more than 2,300 km². In addition to this, we validated the HVSR-derived depth estimates using deep boreholes provided by mining companies after the campaign was completed. These boreholes were not used in the construction of the model and thus serve as independent validation points. Discrepancies between the model and borehole data are generally within acceptable limits, especially considering the presence of intermediate calcarenite layers that act as mechanical basement but not geological basement. We have clarified this distinction in the revised manuscript and included a new figure showing the associated uncertainties in the HVSR-derived depths.

On data ownership and sharing:

All HVSR measurements were carried out by the authors during several field campaigns. The data presented in this study have not been previously published or uploaded to any open-access repository. This manuscript represents the first time these results are being shared. However, for transparency and reproducibility, all relevant data are fully included in the manuscript itself. Specifically, Table 1 provides a complete list of the measurements and results obtained at the 334 stations, including coordinates, fundamental frequencies, and inferred depths to the seismic bedrock. We believe this presentation ensures the accessibility and traceability of the dataset for interested readers and researchers.

On comparison with other methods:

We acknowledge that no other geophysical methods (such as MASW, seismic reflection, or resistivity) were applied in parallel in this study. However, the HVSR technique was chosen due to its low cost, minimal environmental impact (especially important in protected areas like Doñana), and its proven effectiveness in similar sedimentary basins. Where possible, we compared the results with independent data (e.g., boreholes) and with previous regional studies. These comparisons demonstrate that the HVSR method yields reliable and consistent results. We have now discussed these aspects more explicitly in the revised version of the manuscript.

On strengthening the data analysis:

We have now included more detail on the processing of the HVSR curves.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The author has answered all the questions I raised.

Update via email: The paper can be accepted once it undergoes some linguistic polishing. What I meant by "minor revisions" primarily refers to linguistic polishing. I'm very sorry for not making it clear. Thank you!