the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jun 2022
How do differences in interpreting seismic images affect estimates of geological slip rates? An example of a shear fault-bend fold
Abstract. Uncertainties of geological structural geometry constructed based on seismic reflections can stem from data acquisition, processing, analysis, or interpretation. Especially uncertainties arising from structural interpretations and subsequent estimates of geological slip have been little quantified and discussed. To illustrate the implications of interpretation uncertainties for seismic potential and structural evolution, I use an example of a shear fault-bend fold in the Central Himalaya. I apply a simple solution from the kinematic model of shear fault-bend folding to resolve the geological slip, and then compare the result with a previous study to show how differences in structural interpretations could impact dependent conclusions. The findings show that only a little variance in interpretations owing to subjectivity and an unclear seismic image could yield geological slip rates differing by up to ~10 mm/yr, resulting in significantly different scenarios of seismic potential. To reduce unavoidable subjectivity, this study also suggests that the epistemic uncertainty in raw data should be included in interpretations and conclusions.
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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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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.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-369', Filippo Carboni, 06 Jul 2022
The paper by Wan-Lin Hu is focussed on the very interesting, broader topic, of interpretation bias. The manuscript suggests that even small differences in the interpretation of the geometry of a fault, could lead to substantial differences in terms of displacement and slip rates. The novel approach introduced by the author is clearly outlined and supported by realistic assumptions, leading to interesting results and conclusions; the methodologies used are also well described. In addition, the author well explained the various limitation of the various methodologies and of his own work.
I am wondering if there might be further papers discussing the total amount of slip and slip rates in the areas studied and discussed by the author. This would improve the comparison and give more insights about the interpretation bias.
I think the title can be simplified with the removal of the last sentence “An example of a shear fault-bend fold” since the author also state the second case is not a pure fault-bend fold but also characterized by fault-propagation folding.
The manuscript is well organized and well written, with a fluent and precise language. I suggested small edits to few sentences which were difficult to read or uncomplete. The quantity and quality of the references is appropriated; however, I suggest adding a couple of references, which are strictly related to the balancing methodologies used through time. Pease refer to the manuscript by Wang et al., 2018 (https://doi.org/10.1016/j.jsg.2017.11.014), where the authors well describe the various methodologies used for cross section balancing and where they propose an improvement of the Area Depth Strain Method. However, there is a substantial lack of methodological limits and problematics in literature, which are discussed in Carboni et al., 2019 (https://doi.org/10.1016/j.jsg.2018.10.011).
Please check the attached pdf for minor technical suggestions of the text and figures.
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AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
I would like to thank Dr Carbonari for his detailed edits and comments. I agree that including more papers discussing the total amount of slip and slip rates in the studied area would provide additional insights into the manuscript’s main focus–interpretation bias. Unfortunately, to be best of my knowledge, no other studies have published seismic images in the Central Himalaya.
The references pointed out to me for inclusion are valuable suggestions, and I will add them and modify the title, the main text, and the figure accordingly.
Citation: https://doi.org/10.5194/egusphere-2022-369-AC1
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AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
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RC2: 'Comment on egusphere-2022-369', Filippo Carboni, 11 Jul 2022
Dear Wan-Lin Hu,
I would like to add a comment to my previous ones.
Usually we obtain the amount of shortening and shortening rates throughout a "manual" step-by-step restoration of a balanced cross-section. In my experience, in the case of complex and quite shortened belts, it is not straightforward to obtain the true displacement by applying the methods of Suppe or the ADS method, thus a manual step-by-step restoration is needed. In that case there would also be the classical problem of number approximation, plus the interpretation and restoration bias. I think it could be interesting also to explore this issue, and compare the results of the amount of shortening obtained by the manual restoration, with the one obtained by using the methods you already explored. I think that it would be enough to work with only the two case studies you have already analysed.
Citation: https://doi.org/10.5194/egusphere-2022-369-RC2 -
AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
I would like to thank Dr Carbonari for his detailed edits and comments. I agree that including more papers discussing the total amount of slip and slip rates in the studied area would provide additional insights into the manuscript’s main focus–interpretation bias. Unfortunately, to be best of my knowledge, no other studies have published seismic images in the Central Himalaya.
The references pointed out to me for inclusion are valuable suggestions, and I will add them and modify the title, the main text, and the figure accordingly.
Citation: https://doi.org/10.5194/egusphere-2022-369-AC1 -
AC2: 'Reply on RC2', Wan-Lin Hu, 22 Jul 2022
Thank you for raising the point about exploring the issue of comparing the amounts of shortening obtained by manual restoration (line-length restoration) vs. by Suppe et al.’s or area-depth strain (ADS) methods.
It is true that Suppe et al.’s method would not be effective in more structurally complex cases. In some cases, restorations of line lengths or balanced cross sections could provide more detailed, exact estimates of displacements. However, not even restorations of line lengths are always possible, especially when data quality cannot provide sufficient details about features, such as well-defined paired horizons in hanging walls and foot walls.
Several good comparisons between ADS analysis and bed length restorations are presented in Groshong et al. (2012). One can notice that the structures studied by both methods are based on laboratory sand models, figures drawn based on theoretical models, or high-quality seismic images. All these three cases allowed better control of structures, which is not possible in my study.
In the case of the Bardibas thrust in Nepal (Almeida et al., 2018), the quality of the seismic image does not allow us to interpret in detail (as mentioned in the main text, lines 140-145). Lack of details would affect total slip estimation even if ADS analysis was applied. Conducting line lengths restorations, for which more details of given structures are required, would be more challenging, and highly likely lead to overinterpretation.
All of the above point to why I chose Suppe et al.’s method here, or to how to choose a suitable method, and why my result differs from Almeida et al.’s result.
The advantage of Suppe et al.’s method is the relations between geometric features are clear; thus it can be applied even when few variables are known. That is, Suppe et al.’s method can predict the total slip based on what the image in Almeida et al. can provide.
Of course, this method has inherited limitations (line 73): It is unreliable when there is significant sub-resolution deformation. To validate Suppe et al.’s method, here I demonstrate that the method is primarily applicable in an analogous case (from Le Béon et al. (2019). I assume that both Le Béon et al.’s and Almeida et al.’s geological structures studied are in their initial stages (i.e., younger and less complex structures) and are nearly model-like individual structures. Thus, in the case with a relatively simple structure located at the foremost part of two frontal fold-and-thrust belts, I suggest the simplification made in Suppe et al.’s study is acceptable.Le Béon et al. provide higher image quality, and apply ADS appropriately. Thus, ADS performs nearly as well as Suppe et al.’s kinematic model, and their result agrees with mine.
In contrast to Le Béon et al.’s caes, the main cause of the difference between mine and Almeida et al.’s result is the choice of method given the data quality and the application of the method.
Almeida et al., I believe, aimed to apply the ADS method. However, their application did not agree with how the method had been developed (line 142). In addition, their data quality could not to support ADS application (line 140). Also, Almeida et al. mentioned that they applied area-relief calculation following Lavé and Avouac (2000). However, the area-relief calculation used by Lavé and Avouac (2000) was not designed for estimating the total slip from seismic images: it was developed for calculations based on flights of fluvial terraces, which have different resolutions, scales, and considerations. What is more, the depth of décollement, required for this calculation, was subjectively determined by Almeida et al., and resulted in an unnoticed large bias.In short, the differences in interpretations shown in this study are not originated from applying different methods. This further implies that choosing a method compatible with data quality and applying it appropriately can help us avoid subjective uncertainty and thus minimize differences in interpretations among interpreters. In this short communication submission, I would like to focus on a more generalized but less discussed issue: effects on further application of minor differences in interpretations, instead of on how different interpretations could be when various models are selected (Line 38).
ReferencesAlmeida, R. V., Hubbard, J., Liberty, L., Foster, A., and Sapkota, S. N.: Seismic imaging of the Main Frontal Thrust in Nepal reveals a shallow décollement and blind thrusting, Earth Planet. Sci. Lett., 494, 216–225, https://doi.org/10.1016/j.epsl.2018.04.045, 2018.
Groshong, R. H., Withjack, M. O., Schlische, R. W., and Hidayah, T. N.: Bed length does not remain constant during deformation: Recognition and why it matters, J. Struct. Geol., 41, 86–97, https://doi.org/10.1016/j.jsg.2012.02.009, 2012.
Lavé, J. and Avouac, J. P.: Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal, J. Geophys. Res. Solid Earth, 105, 5735–5770, https://doi.org/10.1029/1999jb900292, 2000.
Le Béon, M., Marc, O., Suppe, J., Huang, M. H., Huang, S. T., and Chen, W. S.: Structure and Deformation History of the Rapidly Growing Tainan Anticline at the Deformation Front of the Taiwan Mountain Belt, 38, 3311–3334, https://doi.org/10.1029/2019TC005510, 2019.
Suppe, J., Connors, C. D., and Zhang, Y.: Shear fault-bend folding, AAPG Mem., 303–323, https://doi.org/10.1306/m82813c17, 2004.
Citation: https://doi.org/10.5194/egusphere-2022-369-AC2
-
AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
-
RC3: 'Comment on egusphere-2022-369', Charlotte Botter, 12 Jul 2022
General comments:
Clearly written paper with a distinct main aim. To understand how the differences in interpreting seismic data can affect the estimation of geological slip rates, hence seismic hazards. The paper uses the solution developed by Suppe et al., (2004) on two seismic datasets.
The two case studies are clear representation of the method used and limits of defining quantitative and precise values based on seismic interpretation, especially when the data quality if overall low. I think this paper raises very well the fact that uncertainties should always be accounted for when presenting any type of results, especially when based on seismic interpretation.
Specific comments:
In section 3.11 l113 make sure on how you computed the standard deviation/uncertainty (is it based on few measurements or PDFs?)
Section 3.2 l139: can you precise what do you mean by ‘claimed to be decided based
Technical corrections
Well written and I could not find any typos.
Citation: https://doi.org/10.5194/egusphere-2022-369-RC3 -
AC3: 'Reply on RC3', Wan-Lin Hu, 22 Jul 2022
I appreciate Dr Botter’s encouraging comments on my manuscript.
In section 3.11, I have added the following explanation regarding what the computation of uncertainty is based on: The insertion is: distributed according to the assigned PDFs.
In section 3.2, I have modified the sentence in line 139 to: The décollement depth was stated by Almeida et al. (2018) to have been based on Suppe et al.’s (2004) shear fault-bend fold model together with a decided ramp location).
Citation: https://doi.org/10.5194/egusphere-2022-369-AC3
-
AC3: 'Reply on RC3', Wan-Lin Hu, 22 Jul 2022
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-369', Filippo Carboni, 06 Jul 2022
The paper by Wan-Lin Hu is focussed on the very interesting, broader topic, of interpretation bias. The manuscript suggests that even small differences in the interpretation of the geometry of a fault, could lead to substantial differences in terms of displacement and slip rates. The novel approach introduced by the author is clearly outlined and supported by realistic assumptions, leading to interesting results and conclusions; the methodologies used are also well described. In addition, the author well explained the various limitation of the various methodologies and of his own work.
I am wondering if there might be further papers discussing the total amount of slip and slip rates in the areas studied and discussed by the author. This would improve the comparison and give more insights about the interpretation bias.
I think the title can be simplified with the removal of the last sentence “An example of a shear fault-bend fold” since the author also state the second case is not a pure fault-bend fold but also characterized by fault-propagation folding.
The manuscript is well organized and well written, with a fluent and precise language. I suggested small edits to few sentences which were difficult to read or uncomplete. The quantity and quality of the references is appropriated; however, I suggest adding a couple of references, which are strictly related to the balancing methodologies used through time. Pease refer to the manuscript by Wang et al., 2018 (https://doi.org/10.1016/j.jsg.2017.11.014), where the authors well describe the various methodologies used for cross section balancing and where they propose an improvement of the Area Depth Strain Method. However, there is a substantial lack of methodological limits and problematics in literature, which are discussed in Carboni et al., 2019 (https://doi.org/10.1016/j.jsg.2018.10.011).
Please check the attached pdf for minor technical suggestions of the text and figures.
-
AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
I would like to thank Dr Carbonari for his detailed edits and comments. I agree that including more papers discussing the total amount of slip and slip rates in the studied area would provide additional insights into the manuscript’s main focus–interpretation bias. Unfortunately, to be best of my knowledge, no other studies have published seismic images in the Central Himalaya.
The references pointed out to me for inclusion are valuable suggestions, and I will add them and modify the title, the main text, and the figure accordingly.
Citation: https://doi.org/10.5194/egusphere-2022-369-AC1
-
AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
-
RC2: 'Comment on egusphere-2022-369', Filippo Carboni, 11 Jul 2022
Dear Wan-Lin Hu,
I would like to add a comment to my previous ones.
Usually we obtain the amount of shortening and shortening rates throughout a "manual" step-by-step restoration of a balanced cross-section. In my experience, in the case of complex and quite shortened belts, it is not straightforward to obtain the true displacement by applying the methods of Suppe or the ADS method, thus a manual step-by-step restoration is needed. In that case there would also be the classical problem of number approximation, plus the interpretation and restoration bias. I think it could be interesting also to explore this issue, and compare the results of the amount of shortening obtained by the manual restoration, with the one obtained by using the methods you already explored. I think that it would be enough to work with only the two case studies you have already analysed.
Citation: https://doi.org/10.5194/egusphere-2022-369-RC2 -
AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
I would like to thank Dr Carbonari for his detailed edits and comments. I agree that including more papers discussing the total amount of slip and slip rates in the studied area would provide additional insights into the manuscript’s main focus–interpretation bias. Unfortunately, to be best of my knowledge, no other studies have published seismic images in the Central Himalaya.
The references pointed out to me for inclusion are valuable suggestions, and I will add them and modify the title, the main text, and the figure accordingly.
Citation: https://doi.org/10.5194/egusphere-2022-369-AC1 -
AC2: 'Reply on RC2', Wan-Lin Hu, 22 Jul 2022
Thank you for raising the point about exploring the issue of comparing the amounts of shortening obtained by manual restoration (line-length restoration) vs. by Suppe et al.’s or area-depth strain (ADS) methods.
It is true that Suppe et al.’s method would not be effective in more structurally complex cases. In some cases, restorations of line lengths or balanced cross sections could provide more detailed, exact estimates of displacements. However, not even restorations of line lengths are always possible, especially when data quality cannot provide sufficient details about features, such as well-defined paired horizons in hanging walls and foot walls.
Several good comparisons between ADS analysis and bed length restorations are presented in Groshong et al. (2012). One can notice that the structures studied by both methods are based on laboratory sand models, figures drawn based on theoretical models, or high-quality seismic images. All these three cases allowed better control of structures, which is not possible in my study.
In the case of the Bardibas thrust in Nepal (Almeida et al., 2018), the quality of the seismic image does not allow us to interpret in detail (as mentioned in the main text, lines 140-145). Lack of details would affect total slip estimation even if ADS analysis was applied. Conducting line lengths restorations, for which more details of given structures are required, would be more challenging, and highly likely lead to overinterpretation.
All of the above point to why I chose Suppe et al.’s method here, or to how to choose a suitable method, and why my result differs from Almeida et al.’s result.
The advantage of Suppe et al.’s method is the relations between geometric features are clear; thus it can be applied even when few variables are known. That is, Suppe et al.’s method can predict the total slip based on what the image in Almeida et al. can provide.
Of course, this method has inherited limitations (line 73): It is unreliable when there is significant sub-resolution deformation. To validate Suppe et al.’s method, here I demonstrate that the method is primarily applicable in an analogous case (from Le Béon et al. (2019). I assume that both Le Béon et al.’s and Almeida et al.’s geological structures studied are in their initial stages (i.e., younger and less complex structures) and are nearly model-like individual structures. Thus, in the case with a relatively simple structure located at the foremost part of two frontal fold-and-thrust belts, I suggest the simplification made in Suppe et al.’s study is acceptable.Le Béon et al. provide higher image quality, and apply ADS appropriately. Thus, ADS performs nearly as well as Suppe et al.’s kinematic model, and their result agrees with mine.
In contrast to Le Béon et al.’s caes, the main cause of the difference between mine and Almeida et al.’s result is the choice of method given the data quality and the application of the method.
Almeida et al., I believe, aimed to apply the ADS method. However, their application did not agree with how the method had been developed (line 142). In addition, their data quality could not to support ADS application (line 140). Also, Almeida et al. mentioned that they applied area-relief calculation following Lavé and Avouac (2000). However, the area-relief calculation used by Lavé and Avouac (2000) was not designed for estimating the total slip from seismic images: it was developed for calculations based on flights of fluvial terraces, which have different resolutions, scales, and considerations. What is more, the depth of décollement, required for this calculation, was subjectively determined by Almeida et al., and resulted in an unnoticed large bias.In short, the differences in interpretations shown in this study are not originated from applying different methods. This further implies that choosing a method compatible with data quality and applying it appropriately can help us avoid subjective uncertainty and thus minimize differences in interpretations among interpreters. In this short communication submission, I would like to focus on a more generalized but less discussed issue: effects on further application of minor differences in interpretations, instead of on how different interpretations could be when various models are selected (Line 38).
ReferencesAlmeida, R. V., Hubbard, J., Liberty, L., Foster, A., and Sapkota, S. N.: Seismic imaging of the Main Frontal Thrust in Nepal reveals a shallow décollement and blind thrusting, Earth Planet. Sci. Lett., 494, 216–225, https://doi.org/10.1016/j.epsl.2018.04.045, 2018.
Groshong, R. H., Withjack, M. O., Schlische, R. W., and Hidayah, T. N.: Bed length does not remain constant during deformation: Recognition and why it matters, J. Struct. Geol., 41, 86–97, https://doi.org/10.1016/j.jsg.2012.02.009, 2012.
Lavé, J. and Avouac, J. P.: Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal, J. Geophys. Res. Solid Earth, 105, 5735–5770, https://doi.org/10.1029/1999jb900292, 2000.
Le Béon, M., Marc, O., Suppe, J., Huang, M. H., Huang, S. T., and Chen, W. S.: Structure and Deformation History of the Rapidly Growing Tainan Anticline at the Deformation Front of the Taiwan Mountain Belt, 38, 3311–3334, https://doi.org/10.1029/2019TC005510, 2019.
Suppe, J., Connors, C. D., and Zhang, Y.: Shear fault-bend folding, AAPG Mem., 303–323, https://doi.org/10.1306/m82813c17, 2004.
Citation: https://doi.org/10.5194/egusphere-2022-369-AC2
-
AC1: 'Reply on RC1', Wan-Lin Hu, 22 Jul 2022
-
RC3: 'Comment on egusphere-2022-369', Charlotte Botter, 12 Jul 2022
General comments:
Clearly written paper with a distinct main aim. To understand how the differences in interpreting seismic data can affect the estimation of geological slip rates, hence seismic hazards. The paper uses the solution developed by Suppe et al., (2004) on two seismic datasets.
The two case studies are clear representation of the method used and limits of defining quantitative and precise values based on seismic interpretation, especially when the data quality if overall low. I think this paper raises very well the fact that uncertainties should always be accounted for when presenting any type of results, especially when based on seismic interpretation.
Specific comments:
In section 3.11 l113 make sure on how you computed the standard deviation/uncertainty (is it based on few measurements or PDFs?)
Section 3.2 l139: can you precise what do you mean by ‘claimed to be decided based
Technical corrections
Well written and I could not find any typos.
Citation: https://doi.org/10.5194/egusphere-2022-369-RC3 -
AC3: 'Reply on RC3', Wan-Lin Hu, 22 Jul 2022
I appreciate Dr Botter’s encouraging comments on my manuscript.
In section 3.11, I have added the following explanation regarding what the computation of uncertainty is based on: The insertion is: distributed according to the assigned PDFs.
In section 3.2, I have modified the sentence in line 139 to: The décollement depth was stated by Almeida et al. (2018) to have been based on Suppe et al.’s (2004) shear fault-bend fold model together with a decided ramp location).
Citation: https://doi.org/10.5194/egusphere-2022-369-AC3
-
AC3: 'Reply on RC3', Wan-Lin Hu, 22 Jul 2022
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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
(3056 KB) - Metadata XML