Monitoring Megathrust-Earthquake-Cycle-Induced Relative Sea-Level Changes near Phuket, South Thailand, Using (Space) Geodetic Techniques

Round 1

Reviewer 1 Report

The authors analyzed the tide gauge data, GNSS data and satellite altimetry data to illustrate the effects of Megathrust Earthquake on local and regional sea level changes. In general, the manuscript focusses on the data processing and give the basic results in sea level trends et al. In the introduction, few related works were reviewed. Actually, the effects on sea level, such as annual sea level cycle, had been mentioned in other literatures (e.g., Cheng, 2016, Influence of ENSO on the variation of annual sea level cycle in the South China Sea, Ocean Eng., 126, 343-352, doi:10.1016/j.oceaneng.2016.09.019).  The frame of the manuscript could be improved based on the style of the journal. Particularly, it would be interesting if the data used in the work could be published online.

Author Response

Reviewer 1 indicates that on all accounts the manuscript can be improved. He/she is commenting as follows:

“The authors analyzed the tide gauge data, GNSS data and satellite altimetry data to illustrate the effects of Megathrust Earthquake on local and regional sea level changes. In general, the manuscript focusses on the data processing and give the basic results in sea level trends et al. In the introduction, few related works were reviewed. Actually, the effects on sea level, such as annual sea level cycle, had been mentioned in other literatures (e.g., Cheng, 2016, Influence of ENSO on the variation of annual sea level cycle in the South China Sea, Ocean Eng., 126, 343- 352, doi:10.1016/j.oceaneng.2016.09.019). The frame of the manuscript could be improved based on the style of the journal. Particularly, it would be interesting if the data used in the work could be published online.”

 

Which we interpret as follows: a few more related work should be referenced, the style of the paper can be more aligned with the journal’s style, and there is a preference of sharing data.

 

These issues have been taken into account in our revision: we added the following works/references:

 

Thepsiriamnuay, H., & Pumijumnong, N. Modelling Assessment of Sandy Beaches Erosion in Thailand: DOI: 10.32526/ennrj.17.2.2019.14. Environment and Natural Resources Journal. 2018, 17(2), 71–86. Retrieved from https://ph02.tci-thaijo.org/index.php/ennrj/article/view/160552

 

Pongpiachan, S., Settacharnwit, T., Chalangsut, P., Hirunyatrakul, P., & Kittikoon, I. Impacts And Preventative Measures Against Flooding And Coastal Erosion In Thailand: DOI: 10.2495/FRIAR120131. WIT Transactions on Ecology and the Environment. 2012, 159, 155 – 166. Retrieved from https:// www.witpress.com/elibrary/wit-transactions-on-ecology-and-the-environment/159/23361

 

Ritphring, S.; Nidhinarangkoon, P.; Udo, K.; Shirakawa, H. The Comparative Study of Adaptation Measure to Sea Level Rise in Thailand. J. Mar. Sci. Eng. 2021, 9, 588. https://doi.org/10.3390/jmse9060588

 

Kowalczyk, K.; Pajak, K.; Wieczorek, B.; Naumowicz, B. An Analysis of Vertical Crustal Movements along the European Coast from Satellite Altimetry, Tide Gauge, GNSS and Radar Interferometry. Remote Sens. 2021, 13, 2173. https://doi.org/10.3390/rs13112173

 

Harvey, T.C., Hamlington, B.D., Frederikse, T. et al. Ocean mass, sterodynamic effects, and vertical land motion largely explain US coast relative sea level rise. Commun Earth Environ. 2021, 2, 233. https://doi.org/10.1038/s43247-021-00300-w

 

Klos, A., Kusche, J., Fenoglio-Marc, L. et al. Introducing a vertical land motion model for improving estimates of sea level rates derived from tide gauge records affected by earthquakes. GPS Solut. 2019, 23, 102. https://doi.org/10.1007/s10291-019-0896-1

 

White, N. J., Church, J. A., & Gregory, J. M. Coastal and global averaged sea level rise for 1950 to 2000. Geophysical Research Letters, 2005, 32(4). https://doi.org/10.1029/2004GL021391.

 

Cheng, Y., Hamlington, B.D., Plag, H.-P., and Xu, Q. Influence of ENSO on the variation of annual sea level cycle in the South China Sea. Ocean Engineering 2016. 126. 343-352. https://doi.org/10.1016/j.oceaneng.2016.09.019.

 

We also added a few sentences to the end of the introduction that introduces the structure of the paper, more or less showing that we do adhere to the Remote Sensing’s flow of introduction, materials and methods, results, discussion, and conclusions, but with a bit of different naming. Regarding publishing the study’s underlying data, it is explained in the data availability section that the GPS data we processed for this paper (2018-2021) as well as the TG, SALT and Phuket time series will be made available in a repository at 4TU. The altimeter data can also be directly extracted from the RADS data base.

Reviewer 2 Report

The article is interesting.

 

The end result is not in line with the title. Combining Tide-Gauge, Satellite Altimetry and Vertical Land Motion Data not performed.

The article is too long, at times monotonous, saturated with unnecessary information.

The layout of the article is bad.

The executive summary should be rebuilt.

Chapter 2 definitely shortened and moved to introduction. Chapters 1 and 2 should be supplemented with literature.

The captions under the drawings should be short and understandable. Possible comment should be placed in the text.

Chapter 3.1 is not needed. The results (UP) in Table 1 are unreliable. The errors received exceed the values. Literature states that 3 years is the minimum to use GPS time series. If there are jumps in the time series (KTPH station - antenna shift), it can be eliminated mathematically. The GIPSy software has such a function.

Chapter 3.1 can be summarized in the introduction.

Do you need information about the horizontal movements of the earth's crust? Since the article is about VLM.

In Chapter 4, there is no formula for the relationship between TG, SA and GPS, on which the authors relied, that is the basis. See the literature https://www.researchgate.net/publication/352094586_An_Analysis_of_Vertical_Crustal_Movements_along_the_European_Coast_from_Satellite_Altimetry_Tide_Gauge_GNSS_and_Radar_InterferileContentlT. It would be good to include this literature in the Introduction.

Did the authors in chapter 4.1 make the map themselves (fig 5)? If so, this is redundant work. It was enough to limit oneself to the analyzed region.

The method used - interpolation is one of the appropriate methods.

Add a chapter: results and discussion

Present the results in a tabular form - clearly and legibly, and on a cartographic presentation. Describing in the text is vague.

 

I stress again, the article should consistently and clearly pursue the article's goal, which the authors should define at the beginning of the article.

 

Conclusion is too long.

Complete the literature.

Author Response

Reviewer 2 indicates that the document must be improved on all accounts, which includes improving the introduction, improving citations, improving description on research and methods, improving presentation of results and improving conclusions.

He/she further states: the article is interesting!

 

So, yes, we think that we improved the paper on all of these accounts, as said references have been added, and text has been improved, also processing has gotten a bit of more explanation (equations added to underline our reasoning).

 

And then pointwise comments follow, which we try to deal with here one by one:

• The end result is not in line with the title. Combining Tide-Gauge, Satellite Altimetry and Vertical Land Motion Data not performed.
  We think the reviewer might have misunderstood what we mean by combining TG, SALT and VLM. We in fact do this for Phuket Island. In the revision we have made this clearer in the introduction. To aid in the understanding we now added a plot (Figure 7) where we do do the subtraction SALT – VLM to create a synthetic relative sea level change time series and compare that with the TG data. And also do SALT-VLM, and TG+VLM. Basically, we covered that in text but mainly for the linear estimates (prior to the earthquake). The new figure now also shows what happens after the earthquake.
• The article is too long, at times monotonous, saturated with unnecessary information.
  We moved part of the GPS work to the supplementary and summarize the main issues in the main text and also did a bit of rework on the main text to remove any duplication.
• The layout of the article is bad
  Unfortunately the reviewer is not indicating in what way it is bad so we cannot really respond to this. Suffice it to say that in line with the comments of Reviewer 1 we did change the layout a bit and also added a few lines to the introduction to explain the structure of the paper to the reader.
• The executive summary should be rebuilt.
  We think the reviewer meant the ‘abstract’; this will be adapted along with the changes to the manuscript.
• Section 2 definitely shortened and moved to introduction. Sections 1 and 2 should be supplemented with literature.
  We do not entirely agree with moving this section to the introduction. The GPS is needed to show the seismic cycle of the VLM, and that we still are in the post-seismic phase. As indicated earlier we did shorten the text, give extra attention to explaining the structure and we have in total added 8 references (listed under one of the reviewer 1 comments).
• The captions under the drawings should be short and understandable. Possible comment should be placed in the text.
  In principle this is a personal view. Extended captions can be useful as than all information is there and they can be interpreted on their own merit (without the ‘context’). But where appropriate and indeed very very long we have cut down it a bit.
• Chapter 3.1 is not needed. The results (UP) in Table 1 are unreliable. The errors received exceed the values. Literature states that 3 years is the minimum to use GPS time series. If there are jumps in the time series (KTPH station - antenna shift), it can be eliminated mathematically. The GIPSy software has such a function.
  We disagree on this point with the reviewer. We do need this basis laid down in 3.1 for our further analyses. So, we need to show that Phuket currently is still in post-seismic motion, especially as we use this non-linear part to fit both TG and VLM. Agreed that we have to emphasize this a bit more, we made an adjustment for this to 3.1. The comment on Table 1 is not substantiated. First of all, these are realistic 1-sigma 68% confidence level errors, using a simple but robust estimation scheme as now highlighted in the text. The table shows both horizontal and vertical motions, each serve their purpose. Horizontal shows a strong case for the still ongoing post-seismic motion, and VLM +/- error shows the vertical motion at present is near zero. We use also 3 years, and 3 years minimum is for vertical motions (Blewitt et al reference also mentions this). B.T.W. the jumps HAVE BEEN eliminated, and this was only for station SPKN in Bangkok, and not for any station in Phuket. So, we think that the reviewer did not entirely understand what was written and why we show the results as we show them. With a few adjustments to our text we hope to have clarified these things a bit more.
• Chapter 3.1 can be summarized in the introduction.
  Well, obviously not as we try to indicate in our previous remark that it is important as underlying base for the research. Reviewers 1, 3 and 4 also don’t require section 3.1 to be removed and/or summarized in the introduction.
  Do you need information about the horizontal movements of the earth's crust? Since the article is about VLM?
  Yes, because we want to show that Phuket is still in the post-seismic deformation phase. Our action here therefore is providing more clarity about this in the introduction and stating the importance of this again in Section 3.1. So that the reader understands better that this is a 3D phenomenon with (combined) horizontal and vertical effects and therefore we analyze GPS in 3D. After having showed this combined effect, we focus on the vertical part to analyze the impact on relative sea level.
• In Section 4, there is no formula for the relationship between TG, SA and GPS, on which the authors relied, that is the basis. See the literature https://www.researchgate.net/publication/352094586_An_Analysis_of_Vertical_Crustal_Movements_along_the_European_Coast_from_Satellite_Altimetry_Tide_Gauge_GNSS_and_Radar_InterferileContentlT. It would be good to include this literature in the Introduction.
  We think we do mention this relationship in the paper on a few occasions: relative sea level is SALT – VLM which at the same time could/should be identical to what the TG is reading (as long as other contributors can be excluded like for instance sediment compaction). Best we use this TG = SALT – VLM relation since our paper is about relative sea-level rise. We have now presented this equation/relation explicitly in the text (from which you can also deduce SALT-TG = VLM, for instance for areas where we do have SALT and TG but not directly VLM from GPS. And we have supplied an extra figure that shows the (combined) result (new figure 7).
• Did the authors in section 4.1 make the map themselves (fig 5)? If so, this is redundant work. It was enough to limit oneself to the analyzed region.
  Yes, we did, and it is certainly not redundant, we do that on purpose covering a larger area as to avoid any boundary effects. And yes, we also zoom in, and we have the map to check our results at the TG locations. So, we really want to keep Fig 5 as it is, it also shows absolute sea level in other areas of SE Asia. What we did with the two parts of the figure is now applying the same color bar to not to confuse the reader so that he/she understands that these results come from the same data/analysis.
• The method used - interpolation is one of the appropriate methods.
  Not clear whether this is a comment or just a remark. We cannot do anything with this. If this underlines we made the right choice for interpolation we thank the reviewer.
• Add a chapter: results and discussion
  Basically whole Section 4 is the results and discussion section as we now explain in the introduction. We do like to retain the original section names. Any other changes in the layout will to our belief not improve the readability of the paper.
• Present the results in a tabular form - clearly and legibly, and on a cartographic presentation. Describing in the text is vague.
  We have made an extra table for Phuket indicating all parts of our model fits to the SALt, the TG, and the VLM, it is Table 3 in the text.
• I stress again, the article should consistently and clearly pursue the article's goal, which the authors should define at the beginning of the article.
  We think we already did, but certainly it is already improved by all our changes and some extra explanation in the introduction. We now also state our research objective and explain the flow of the article by introducing the sections in the introduction.
• Conclusion is too long
  This is a bit of a vague comment, how long is too long, why is it too long. The only thing we can do to accommodate the reviewer a bit is to make the conclusion and recommendations section a bit more concise and to the point. So that we did.
• Complete the literature.
  We added the necessary references to the text. This has been mentioned before.

 

Reviewer 3 Report

The manuscript presents a study on the determination of relative sea-level change around Phuket Island in the South of Thailand using geodetic techniques that complement each other. The article is highly interesting, written transparently, and its presentation is technically correct.

The referee finds the manuscript potentially appropriate for publication in the RS journal though some explanations may need further discussion. Some comments, questions and recommendations for improving the manuscript are provided below:

- The title reflects the content and purpose of the research. But the referee suggests the authors edit it, taking into account the following considerations: relative sea-level rise is not "in" the island and using ground-based measurements other than space (GNSS/GPS and altimetry) geodetic measurements.

- The abstract summarises the manuscript’s content but is more descriptive. It should present the primary purpose, results and conclusions from the study.

- The introduction should clearly explain the state of the art of research, the novelty and the aim and objectives of the work. Why do the authors use these three techniques to determine relative sea-level change, namely TG, GNSS (GPS) and altimetry measurements? Authors should highlight the strengths of their study. Is there any similar research in this study area to confirm the study’s findings regarding the increased erosion activity after the 2004 107 Mw 9.2 Sumatra-Andaman earthquake?

- To distinguish the values of the change of the KTPH-PMBC baseline length shown in fig. 4, which is determined by campaign and permanent GPS measurements, it would be better to graphically represent them with an offset along the y-axis in the individual components - east, north and vertical.

- The explanations for the seasonal variations (in the last paragraph of item 3) in the vertical component of the measured baseline have not been confirmed by specific studies using data from meteorological measurements. Therefore, they should be regarded as assumptions that objective measurements and comparisons have not proved.

- The colour scale for the regional sea-level rise trend from altimetric observations in fig.5 is misleading because it shows different values (positive and negative) of the regional map and that of the local area shown in the inset.

- The authors could move some texts from one place to another in the individual parts to better justify the main purpose of the study, the results obtained and what they can refer to in future research.

 

- Some spelling/typos mistakes have been made and should be corrected. Also, the term “platelet” is not appropriately used instead of “plate”.

Author Response

Reviewer 3 indicates that on all accounts the paper can be improved and that on the account of sufficient background the paper should be improved. He/she further writes: the manuscript presents a study on the determination of relative sea-level change around Phuket Island in the South of Thailand using geodetic techniques that complement each other. The article is highly interesting, written transparently, and its presentation is technically correct. The reviewer finds the manuscript potentially appropriate for publication in the RS journal though some explanations may need further discussion. Some comments, questions and recommendations for improving the manuscript are provided below:

We will deal with all comments point by point but have as general remark to the suggestion of improving introduction and adding a few more references to acknowledge work by others that with our current revision we believe we did. In total 8 references were added to the text.

 

The title reflects the content and purpose of the research. But the referee suggests the authors edit it, taking into account the following considerations: relative sea-level rise is not "in" the island and using ground-based measurements other than space (GNSS/GPS and altimetry) geodetic measurements.

The ‘in’ issue is a good point, and then to avoid that it should be on or at we leave that out and combine to: …. Level changes near Phuket, south ….” The remainder is okay we think as with the brackets around space we indeed mean combining in situ measurements with space measurements …

 

- The abstract summarizes the manuscript’s content but is more descriptive. It should present the primary purpose, results and conclusions from the study.

In the current state we think the abstract does comply with your suggestion.

 

- The introduction should clearly explain the state of the art of research, the novelty and the aim and objectives of the work. Why do the authors use these three techniques to determine relative sea-level change, namely TG, GNSS (GPS) and altimetry measurements? Authors should highlight the strengths of their study. Is there any similar research in this study area to confirm the study’s findings regarding the increased erosion activity after the 2004 107 Mw 9.2 Sumatra-Andaman earthquake?

We made an effort to make this clearer in the Introduction and we also added references. There is definitely increased erosion observed in these papers and reality but never directly linked to the seismic cycle, which of course is an interesting study and has our attention.

 

- To distinguish the values of the change of the KTPH-PMBC baseline length shown in fig. 4, which is determined by campaign and permanent GPS measurements, it would be better to graphically represent them with an offset along the y-axis in the individual components - east, north and vertical.

We agree with the reviewer his/her suggestion and have included a second Y-axis on the right of the figure 4 for the north and east baseline changes, so just a single Y-axis is now used on the left for the vertical changes which are also the main result we present and discuss. 

 

- The explanations for the seasonal variations (in the last paragraph of item 3) in the vertical component of the measured baseline have not been confirmed by specific studies using data from meteorological measurements. Therefore, they should be regarded as assumptions that objective measurements and comparisons have not proved.

We think you are absolutely right with this observation, we therefore has phrased it as “ might be related” and “a plausible explanation could be”. We have not yet proved this and it certainly is something worth investigating (but outside the scope of this paper).

 

- The colour scale for the regional sea-level rise trend from altimetric observations in fig.5 is misleading because it shows different values (positive and negative) of the regional map and that of the local area shown in the inset.

This we already noticed and changed that so that the colorscale is now the same for the larger map and the zoomed in  inset.

 

- The authors could move some texts from one place to another in the individual parts to better justify the main purpose of the study, the results obtained and what they can refer to in future research.

We did quite a revision on the text, removed parts, moved parts and added extra explanation, so we do hope that those changes contributed to a better understanding of our research, our findings, and the relevance for future research.

 

- Some spelling/typos mistakes have been made and should be corrected. Also, the term “platelet” is not appropriately used instead of “plate”.

All dealt with now.

Reviewer 4 Report

Excellent study. Authors might consider some minor comments itemized below:

1/ Move Legend of Table 1 to page 6, the same for Fig. 4

 2/ Fig2 could be enlarged

3/ l. 254 write: “the same (tectonic) plate”

 4/ l.271: “(1994-present) motion”, it would be more appropriate to write: “(1994-2022) motion”

5/ l.507: “posterior period” is not commonly used,

 6/ l.564: “but significant coastal erosion may have occurred in the previous 16 years (as reported by Thai news media)”, this statement is scientifically vague.

 7/ l.621: “Given that in Thailand Phuket is located he closest to the Mw 9.2 earthquake epicentre” ???

 

Author Response

Reviewer 4 indicates his/her approval on all accounts. He/she is commenting as follows:

“Excellent study. Authors might consider some minor comments itemized below:

 

1/ Move Legend of Table 1 to page 6, the same for Fig. 4

We checked the original submitted word document and the captions are at the same page as the table and the figure. It could be that this was split up on different pages in the pdf version for the reviewer; we will monitor this when the manuscript will be published. We also noticed that the line numbering in the word document is different from the line numbering in the PDF … we take the pdf as guidance for your line referrals.

 

2/ Fig2 could be enlarged

We removed the SPKN results (being about Bangkok) from the figure, and at the same time enlarged the remaining figure. The SPKN bit, still interesting as additional info we moved to the supplement.

 

3/ l. 254 write: “the same (tectonic) plate”

Done (changed platelet into plate).

 

4/ l.271: “(1994-present) motion”, it would be more appropriate to write: “(1994-2022) motion”

Done.

 

5/ l.507: “posterior period” is not commonly used,

It is definitely an appropriate word but we agree with the reviewer that it might not be used as much, so we chose to replace this word by “following”. On a few occasions in the text also the word “after”’ is used which we think is also fine.

 

6/ l.564: “but significant coastal erosion may have occurred in the previous 16 years (as reported by Thai news media)”, this statement is scientifically vague.

Agreed, so we remove “as reported by Thai news media” and put a few actual references in the text to support the statement, of which

 

Thepsiriamnuay, H., & Pumijumnong, N. Modelling Assessment of Sandy Beaches Erosion in Thailand: DOI: 10.32526/ennrj.17.2.2019.14. Environment and Natural Resources Journal. 2018, 17(2), 71–86. Retrieved from https://ph02.tci-thaijo.org/index.php/ennrj/article/view/160552

 

Pongpiachan, S., Settacharnwit, T., Chalangsut, P., Hirunyatrakul, P., & Kittikoon, I. Impacts And Preventative Measures Against Flooding And Coastal Erosion In Thailand: DOI: 10.2495/FRIAR120131. WIT Transactions on Ecology and the Environment. 2012, 159, 155 – 166. Retrieved from https:// www.witpress.com/elibrary/wit-transactions-on-ecology-and-the-environment/159/23361

 

Ritphring, S.; Nidhinarangkoon, P.; Udo, K.; Shirakawa, H. The Comparative Study of Adaptation Measure to Sea Level Rise in Thailand. J. Mar. Sci. Eng. 2021, 9, 588. https://doi.org/10.3390/jmse9060588

 

7/ l.621: “Given that in Thailand Phuket is located he closest to the Mw 9.2 earthquake epicentre” ???”

The reviewer likely hints to the word ‘he’ that obviously must be ‘the’, but we also chose to alter this sentence a bit, it reads now: “Given that in Thailand Phuket is located the closest to the Mw 9.2 earthquake epicentre, it also underwent most co- and post-seismic motion in Thailand.

Round 2

Reviewer 2 Report

Thank you for the detailed answers.

Reviewer 3 Report

The reviewer considers that all notes and comments for improving the text are reflected in the edited manuscript. Therefore, I recommend that the manuscript be accepted for publication in its current form.