Tsunami Hazard Assessment in the South China Sea Based on Geodetic Locking of the Manila Subduction Zone

Zhao, Guangsheng; Niu, Xiaojing

doi:https://doi.org/10.5194/nhess-2023-227

Preprints

https://doi.org/10.5194/nhess-2023-227

Preprints

26 Jan 2024

| 26 Jan 2024

Status: a revised version of this preprint was accepted for the journal NHESS and is expected to appear here in due course.

Tsunami Hazard Assessment in the South China Sea Based on Geodetic Locking of the Manila Subduction Zone

Guangsheng Zhao and Xiaojing Niu

Abstract. This study provides a dataset and shows the spatial distribution of tsunami hazard in the South China Sea sourcing from the Manila subduction zone. The plate motion data around the Manila subduction zone is used to invert the geodetic locking of the Manila subduction zone, further used to estimate the maximum possible magnitude, and applied to tsunami hazard assessment. The spatial distribution of tsunami wave height with 1000 years return period is shown and several high hazard areas in the South China Sea are pointed out. Uncertainties of the seismic source are explored, including the slip heterogeneity, the upper limit of seismic magnitude and segmentation. The impact of locking distribution and randomness of slip on tsunami hazard assessment demonstrates that traditional uniform slip assumption significantly underestimates the tsunami hazard.

Received: 20 Dec 2023 – Discussion started: 26 Jan 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Guangsheng Zhao and Xiaojing Niu

Status: closed

RC1:
'Comment on nhess-2023-227', Anonymous Referee #1, 17 Feb 2024
This is an important study which estimate the spatial distribution of tsunami hazard in the South China Sea based on Geodetic locking of the Manila subduction zone. The study should be published because the influence of geodetic locking on the distribution of slip is rarely considered on tsunami hazard assessment in the South China Sea region among the current researches. It can help to understand the influence of uncertainties of the seismic source on tsunami hazard assessment. The article is well organized and well written. The present manuscript only needs minor revision for Natural Hazards and Earth System Science publication with the following comments.

Comments:
In general, the English of the text is good, but could be further improved. If you can, please ask a native speaker to polish the text to improve its readability.

At present, the abstract part does not give a good overview of the innovative points of the article. Please further summarize it.

Please check terminology consistency throughout the text. Such as “the maximum possible magnitude” and “the possible maximum magnitude”, as we all know, they represent different meanings.

Line 258: “in the current researchs” should be “in the current researches”; Please check out.

In the introduction part, it will be good that the quantitative tsunami hazard assessment results from other researchers should be addressed and cited.

The impact of source uncertainty on tsunami hazard assessment in Figure 3a, why does the authors use the 100-year return cycle as an example instead of 1000 years? In addition, we can find that the impacts did not have a consistent trend at different locations with the same heterogeneous slip scenarios. Please add possible reasons for the results.

It might be good to provide brief discussions on the limitation of the present method of based on geodetic locking especially for tsunami hazard assessment.
Citation: https://doi.org/10.5194/nhess-2023-227-RC1
- AC1: 'Reply on RC1', Guangsheng Zhao, 28 Mar 2024
  
  Dear Reviewer,
  
  We sincerely appreciate your interest in our research and the detailed comments provided, which have greatly contributed to improving the manuscript. We have implemented all the suggestions and have addressed all the discussion points. Our reply is in the attached document. We are happy to discuss any points and are grateful for the opportunity to further improve our manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-227-AC1
RC2:
'Comment on nhess-2023-227', Anonymous Referee #2, 03 Mar 2024
The manuscript “Tsunami Hazard Assessment in the South China Sea Based on Geodetic Locking of the Manila Subduction Zone” presents a new tsunami hazard assessment for the South China Sea due to earthquakes triggered in the Manila Subduction Zone. The new assessment incorporates geodetic information that is used to increase the likelihood of hosting larger slip within highly coupled regions and to determine an upper limit for the maximum earthquake moment magnitude. The study provides a significant advance to the efforts assessing tsunami hazards in the South China Sea. Here, I provide some comments to further improve the paper.
Major comments:
The paper needs a deeper analysis of previous geodetic studies in the Manila Subduction Zone. For example, I would include a deeper analysis of previous GNSS data used by Hsu et al. (Hsu, 2016, 2012) in the introduction. That study served as reference for several PTHA studies in the past (to define earthquake magnitude recurrences). It is also important to compare differences in the estimated coupling ratios of past studies to identify improvements in your new inversion.

Hsu, Y. J., Yu, S. B., Loveless, J. P., Bacolcol, T., Solidum, R., Luis, A., & Woessner, J. (2016). Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system. Journal of Geophysical Research: Solid Earth, 121, 7639–7665. https://doi.org/10.1002/2016JB013082

Hsu, Y. J., Yu, S. B., Song, T. R. A., & Bacolcol, T. (2012). Plate coupling along the Manila subduction zone between Taiwan and northern Luzon. Journal of Asian Earth Sciences, 51, 98–108.
The locking model is a very important input in the new tsunami assessment. Because of this, I recommend to add more information about the inversion method. For example, you need to provide information on how the Gaussian and Gamma distribution enter in the inversion method. Also, you shall need to provide some measure of the inversion constraints. For example, your slip seems to concentrate in shallow regions. Is this a bias due to the GPS station locations? This information will be very important for future efforts to improve the geodetic network or understand tsunami hazard uncertainties.

Due to computational limitations, the tsunami modeling of the study is based on linear superposition of unit sources. This approach has to assume linear tsunami waves. Though, tsunami waves are very non-linear in shallow waters. Because of this, the study would not be able to determine tsunami heights in coastal regions. If the linear superposition at the coast is used, I’m afraid the results will be very different from an approach using non-linear tsunami models. The authors, therefore, may be only allowed to determine wave heights in relatively deep waters (at some distance from the coast) and for waves that have not propagated through shallow waters before. This is an important limitation which can be only overcome by running non-linear models (where the unit source superposition is not valid). Furthermore, bottom friction will contribute with energy dissipation. This linear superposition limitation is discussed in several papers (Williamson et al., 2020; Sepulveda et al., 2019; Li eta al., 2017). An easy way to overcome this difficulty would be to analyze tsunami heights far from the coast. This would be easy as the authors already have the model results in all the domain. Finally, the innovative idea of using linear superposition (in the past) was designed before Zhang and Niu (2020). For example, Li et al. 2016. Please indicate if there is something different in the most recent cited paper or change the citation.

Williamson, A. L., Rim, D., Adams, L. M., LeVeque, R. J., Melgar, D., & González, F. I. (2020). A source clustering approach for efficient inundation modeling and regional scale probabilistic tsunami hazard assessment. Frontiers in Earth Science, 8, 591663.
Sepúlveda, I., Liu, P. L. F., & Grigoriu, M. (2019). Probabilistic tsunami hazard assessment in South China Sea with consideration of uncertain earthquake characteristics. Journal of Geophysical Research: Solid Earth, 124(1), 658-688.
Li, L., Switzer, A. D., Chan, C. H., Wang, Y., Weiss, R., & Qiu, Q. (2016). How heterogeneous coseismic slip affects regional probabilistic tsunami hazard assessment: A case study in the South China Sea. Journal of Geophysical Research: Solid Earth, 121(8), 6250-6272.
The abstract does not mention the very important innovations of the study. It rather focuses on ideas that are well-known from the past. For example, lines 12-14 compares the new slip model with the old uniform-slip model. Rather than this, I would tell how relevant is to include the locking distribution in the new slip model, compared to a slip model that only uses stochastic slip.

I could not see the details of the PTHA. What are the recurrences for every magnitude (i.e., p_i in Equation 1)? Are you able to determine new a and b value to create a Gutenberg-Richter Law? Please clarify as this is a very important input for future studies.

Line 121-125. Here I got confused. First, it says that the Okada (1985) model is used to get the tsunami initial condition unit (which assumes that water elevation mimics co-seismic deformation). Though, in line 124 you say “same Gaussian distribution initial water level is set on each point source”. Please clarify.

The introduction contains some ideas which may be true some years ago but not today: Line 44: “traditional tsunami research often assumes that earthquake rupture is uniform”. I think this is not true anymore. I rarely see tsunami assessments with uniform slip, even in the engineering industry. Line 48: “…unit source or sub-fault methods are usually used to convert tsunami simulation into linear superposition...”. I also think, the linear superposition is not used so often. Especially because tsunami hazards are commonly evaluated close to the coast.

To make the manuscript reproducible, the results need to be provided in accessible files. Are the coupling and locking rates included in a text file or repository in “Code/Data Availability”. These are essential to reproduce results.

Minor comments.
I think the writing needs some polishing. Here, I include a few examples (there are more):
Line 66: I would replace “increase the probability” by “correct the probability”

Line 72-73: I would remove the sentence “TDEFNODE is an inversion program developed by Professor McCaffrey of Portland State University in the United States” and rather talk about the method and formulations (which are missing in the text).

Line 88: what constitutes ‘”great uncertainty” for the study? Please provide quantification or justification.

Line 147: “…1.26x10^20 N*m/a, respectively.”.

Line 153: maybe use “the aforementioned studies” instead of “the latest research results”, to emphasize that you are talking about the papers you described in the previous sentence.

Line 156: “locking may only be released” instead of “locking can only be released”

Line 258: Sentence starting with “In the current researchs…” I would suggest “Existing studies rarely consider the…”
Citation: https://doi.org/10.5194/nhess-2023-227-RC2
- AC2: 'Reply on RC2', Guangsheng Zhao, 28 Mar 2024
  
  Dear Reviewer,
  
  We sincerely appreciate your interest in our research and the detailed comments provided, which have greatly contributed to improving the manuscript. We have implemented all the suggestions and have addressed all the discussion points. Our reply is in the attached document. We are happy to discuss any points and are grateful for the opportunity to further improve our manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-227-AC2

Status: closed

RC1:
'Comment on nhess-2023-227', Anonymous Referee #1, 17 Feb 2024
This is an important study which estimate the spatial distribution of tsunami hazard in the South China Sea based on Geodetic locking of the Manila subduction zone. The study should be published because the influence of geodetic locking on the distribution of slip is rarely considered on tsunami hazard assessment in the South China Sea region among the current researches. It can help to understand the influence of uncertainties of the seismic source on tsunami hazard assessment. The article is well organized and well written. The present manuscript only needs minor revision for Natural Hazards and Earth System Science publication with the following comments.

Comments:
In general, the English of the text is good, but could be further improved. If you can, please ask a native speaker to polish the text to improve its readability.

At present, the abstract part does not give a good overview of the innovative points of the article. Please further summarize it.

Please check terminology consistency throughout the text. Such as “the maximum possible magnitude” and “the possible maximum magnitude”, as we all know, they represent different meanings.

Line 258: “in the current researchs” should be “in the current researches”; Please check out.

In the introduction part, it will be good that the quantitative tsunami hazard assessment results from other researchers should be addressed and cited.

The impact of source uncertainty on tsunami hazard assessment in Figure 3a, why does the authors use the 100-year return cycle as an example instead of 1000 years? In addition, we can find that the impacts did not have a consistent trend at different locations with the same heterogeneous slip scenarios. Please add possible reasons for the results.

It might be good to provide brief discussions on the limitation of the present method of based on geodetic locking especially for tsunami hazard assessment.
Citation: https://doi.org/10.5194/nhess-2023-227-RC1
- AC1: 'Reply on RC1', Guangsheng Zhao, 28 Mar 2024
  
  Dear Reviewer,
  
  We sincerely appreciate your interest in our research and the detailed comments provided, which have greatly contributed to improving the manuscript. We have implemented all the suggestions and have addressed all the discussion points. Our reply is in the attached document. We are happy to discuss any points and are grateful for the opportunity to further improve our manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-227-AC1
RC2:
'Comment on nhess-2023-227', Anonymous Referee #2, 03 Mar 2024
The manuscript “Tsunami Hazard Assessment in the South China Sea Based on Geodetic Locking of the Manila Subduction Zone” presents a new tsunami hazard assessment for the South China Sea due to earthquakes triggered in the Manila Subduction Zone. The new assessment incorporates geodetic information that is used to increase the likelihood of hosting larger slip within highly coupled regions and to determine an upper limit for the maximum earthquake moment magnitude. The study provides a significant advance to the efforts assessing tsunami hazards in the South China Sea. Here, I provide some comments to further improve the paper.
Major comments:
The paper needs a deeper analysis of previous geodetic studies in the Manila Subduction Zone. For example, I would include a deeper analysis of previous GNSS data used by Hsu et al. (Hsu, 2016, 2012) in the introduction. That study served as reference for several PTHA studies in the past (to define earthquake magnitude recurrences). It is also important to compare differences in the estimated coupling ratios of past studies to identify improvements in your new inversion.

Hsu, Y. J., Yu, S. B., Loveless, J. P., Bacolcol, T., Solidum, R., Luis, A., & Woessner, J. (2016). Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system. Journal of Geophysical Research: Solid Earth, 121, 7639–7665. https://doi.org/10.1002/2016JB013082

Hsu, Y. J., Yu, S. B., Song, T. R. A., & Bacolcol, T. (2012). Plate coupling along the Manila subduction zone between Taiwan and northern Luzon. Journal of Asian Earth Sciences, 51, 98–108.
The locking model is a very important input in the new tsunami assessment. Because of this, I recommend to add more information about the inversion method. For example, you need to provide information on how the Gaussian and Gamma distribution enter in the inversion method. Also, you shall need to provide some measure of the inversion constraints. For example, your slip seems to concentrate in shallow regions. Is this a bias due to the GPS station locations? This information will be very important for future efforts to improve the geodetic network or understand tsunami hazard uncertainties.

Due to computational limitations, the tsunami modeling of the study is based on linear superposition of unit sources. This approach has to assume linear tsunami waves. Though, tsunami waves are very non-linear in shallow waters. Because of this, the study would not be able to determine tsunami heights in coastal regions. If the linear superposition at the coast is used, I’m afraid the results will be very different from an approach using non-linear tsunami models. The authors, therefore, may be only allowed to determine wave heights in relatively deep waters (at some distance from the coast) and for waves that have not propagated through shallow waters before. This is an important limitation which can be only overcome by running non-linear models (where the unit source superposition is not valid). Furthermore, bottom friction will contribute with energy dissipation. This linear superposition limitation is discussed in several papers (Williamson et al., 2020; Sepulveda et al., 2019; Li eta al., 2017). An easy way to overcome this difficulty would be to analyze tsunami heights far from the coast. This would be easy as the authors already have the model results in all the domain. Finally, the innovative idea of using linear superposition (in the past) was designed before Zhang and Niu (2020). For example, Li et al. 2016. Please indicate if there is something different in the most recent cited paper or change the citation.

Williamson, A. L., Rim, D., Adams, L. M., LeVeque, R. J., Melgar, D., & González, F. I. (2020). A source clustering approach for efficient inundation modeling and regional scale probabilistic tsunami hazard assessment. Frontiers in Earth Science, 8, 591663.
Sepúlveda, I., Liu, P. L. F., & Grigoriu, M. (2019). Probabilistic tsunami hazard assessment in South China Sea with consideration of uncertain earthquake characteristics. Journal of Geophysical Research: Solid Earth, 124(1), 658-688.
Li, L., Switzer, A. D., Chan, C. H., Wang, Y., Weiss, R., & Qiu, Q. (2016). How heterogeneous coseismic slip affects regional probabilistic tsunami hazard assessment: A case study in the South China Sea. Journal of Geophysical Research: Solid Earth, 121(8), 6250-6272.
The abstract does not mention the very important innovations of the study. It rather focuses on ideas that are well-known from the past. For example, lines 12-14 compares the new slip model with the old uniform-slip model. Rather than this, I would tell how relevant is to include the locking distribution in the new slip model, compared to a slip model that only uses stochastic slip.

I could not see the details of the PTHA. What are the recurrences for every magnitude (i.e., p_i in Equation 1)? Are you able to determine new a and b value to create a Gutenberg-Richter Law? Please clarify as this is a very important input for future studies.

Line 121-125. Here I got confused. First, it says that the Okada (1985) model is used to get the tsunami initial condition unit (which assumes that water elevation mimics co-seismic deformation). Though, in line 124 you say “same Gaussian distribution initial water level is set on each point source”. Please clarify.

The introduction contains some ideas which may be true some years ago but not today: Line 44: “traditional tsunami research often assumes that earthquake rupture is uniform”. I think this is not true anymore. I rarely see tsunami assessments with uniform slip, even in the engineering industry. Line 48: “…unit source or sub-fault methods are usually used to convert tsunami simulation into linear superposition...”. I also think, the linear superposition is not used so often. Especially because tsunami hazards are commonly evaluated close to the coast.

To make the manuscript reproducible, the results need to be provided in accessible files. Are the coupling and locking rates included in a text file or repository in “Code/Data Availability”. These are essential to reproduce results.

Minor comments.
I think the writing needs some polishing. Here, I include a few examples (there are more):
Line 66: I would replace “increase the probability” by “correct the probability”

Line 72-73: I would remove the sentence “TDEFNODE is an inversion program developed by Professor McCaffrey of Portland State University in the United States” and rather talk about the method and formulations (which are missing in the text).

Line 88: what constitutes ‘”great uncertainty” for the study? Please provide quantification or justification.

Line 147: “…1.26x10^20 N*m/a, respectively.”.

Line 153: maybe use “the aforementioned studies” instead of “the latest research results”, to emphasize that you are talking about the papers you described in the previous sentence.

Line 156: “locking may only be released” instead of “locking can only be released”

Line 258: Sentence starting with “In the current researchs…” I would suggest “Existing studies rarely consider the…”
Citation: https://doi.org/10.5194/nhess-2023-227-RC2
- AC2: 'Reply on RC2', Guangsheng Zhao, 28 Mar 2024
  
  Dear Reviewer,
  
  We sincerely appreciate your interest in our research and the detailed comments provided, which have greatly contributed to improving the manuscript. We have implemented all the suggestions and have addressed all the discussion points. Our reply is in the attached document. We are happy to discuss any points and are grateful for the opportunity to further improve our manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-227-AC2

Guangsheng Zhao and Xiaojing Niu

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Short summary

The purpose of this study is to estimate the spatial distribution of the tsunami hazard in the South China Sea from the Manila subduction zone. The plate motion data is used to invert the degree of locking on the fault plane. The degree of locking is used to estimate the maximum possible magnitude of earthquakes and describe the slip distribution. A spatial distribution map of the 1000-year return period tsunami wave height in the South China Sea was obtained by tsunami hazard assessment.


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