the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jul 2024
Mid-field tsunami hazards in greater Karachi from seven hypothetical ruptures of the Makran subduction thrust
Abstract. New Makran simulations imply two generalized zones of mid-field tsunami hazard in greater Karachi. The simulations presuppose seven megathrust ruptures that strike east-west, range in area from 100 × 150 km to 355 × 800 km, and lie west of the city by no less than 100 km. The assumed seismic slip is uniform across each rupture area. The smallest rupture approximates the 1945 Makran earthquake of magnitude 8.1, while the largest corresponds to a previously conjectured giant Makran earthquake of magnitude 9.2. None of the sources include a complication in 1945: late-arriving waves from submarine slides or splay faulting. Consequently, the first simulated wave is the largest in each of the seven scenarios. And because the sources are to the west, the simulated waves are higher, and arrive sooner, at Karachi Port (1.5 hr) than 30 km farther east at Port Qasim (nearly 3 hr). These combinations of height and arrival time can be generalized as properties of two hazard zones: a western one that includes Karachi Port, and an eastern one that includes Port Qasim. The simulated flooding extends farthest inland into low-lying residential areas of the western zone. Neither hazard zone is near-field or far-field. That is, neither is near enough to the fault ruptures for felt seismic shaking to dependably warn of a fast-arriving tsunami, yet neither is distant enough to receive more than three hours of advance notice through tsunami warning systems. Our simulations are intended to support emergency management in this mid-field setting.
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RC1: 'Comment on nhess-2024-110', Anonymous Referee #1, 22 Jul 2024
Comments
The article examines tsunami hazards in the greater Karachi area resulting from seven hypothetical ruptures of the Makran subduction thrust, identifying two primary hazard zones: one near Karachi Port and the other adjacent to Port Qasim. Through numerical modeling, it is demonstrated that larger ruptures produce higher waves and faster arrival times, thereby highlighting the central region's susceptibility due to its low elevation and critical infrastructure. The study emphasizes the necessity of early warning systems and preparedness strategies to mitigate tsunami risks in this region.
Overall, the paper is well-written, the methodology is robust, and I appreciated the way the results were presented. However, I have some comments and recommend a minor revision before publication:
- Discussion of Results: The paper lacks sufficient discussion regarding the results, particularly in comparing the actual Karachi marigram with the simulated results. Other studies have presented more robust simulations between the real data from the 1945 tsunami and their results. The authors should indicate that the discrepancies between their findings and the actual 1945 tsunami waveform may come from their simplistic representation of the tsunami source. The differences observed between the simulations and the two marigrams may not solely be attributed to tide gauge errors, but also to our limited understanding of the 1945 tsunami source.
- Title Adjustment: I recommend removing the word "seven" from the title.
- Abstract Corrections: In Abstract (line 13), "km" should be corrected to "km²." Additionally, in line 21, the repeated use of "neither" makes the text unnecessarily verbose.
- Typos and References: The manuscript contains several typographical errors, such as "Tohuku" (which should be "Tohoku") and "20011" (which should be "2011"). Moreover, in several instances, the authors have neglected to include the publication year of references in parentheses. A thorough review of the text for typos and potential grammatical errors is recommended.
- Figure Combination: It appears that Figures a&b could be combined into a single figure unless additional details (e.g., an earthquake catalog) are added to panel (a).
- Model Details: Additional information about the GeoClaw model should be included, such as inputs and outputs, the types of water equations used, the algorithms employed for inundation calculations, the nature of the friction effects (whether constant or variable), and whether structured or unstructured grids are used.
- Font Size in Figures: The font size in Figure 2 should be increased for better visibility.
- Scenario Simplifications: More details are needed regarding the simplifications made in selecting scenarios. Such simplifications (e.g., rectangular sources and uniform slip) could lead to potentially underestimated or less complex wave height distributions. The strike angle of 270 degrees appears inconsistent with the tectonics of the Makran thrust; any alterations in the strike direction could yield different wave directions and results. These simplifications require a more thorough explanation. Furthermore, these scenarios do not seem to offer new insights compared to those proposed in previous studies.
- Table A3 Enhancements: In Table A3, please add an additional column indicating the moment magnitude (Mw) for each scenario.
Citation: https://doi.org/10.5194/nhess-2024-110-RC1 -
AC1: 'Reply on RC1', Haider Hasan, 07 Aug 2024
Thank you for your comments and suggestions regarding our manuscript titled "Mid-field tsunami hazards in greater Karachi from seven hypothetical ruptures of the Makran subduction thrust." We appreciate the time and effort you have taken to review our work. In response to your comments, we have carefully addressed each point. A detailed reply to each comment is attached.
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RC2: 'Comment on nhess-2024-110', Anonymous Referee #2, 06 Sep 2024
The study by Hasan et al. identifies two mid-field tsunami hazard zones in greater Karachi, characterized by higher and faster-arriving waves at Karachi Port compared to Port Qasim. The simulations assume seven east-west megathrust ruptures with uniform seismic slip and magnitudes ranging from 8.1 to 9.2, highlighting that neither zone is near enough for reliable seismic warnings nor distant enough for more than three hours of advance notice. Although I acknowledge that their simulations could be helpful to identify potential tsunami hazards, the overall structure of the article is overly lengthy, and there are certain flaws in the research methodology. Overall, the article requires significant revisions. Regrettably, I have decided to reject the manuscript.
1) The article analyzes mid-field tsunami hazards. However, the concept of “mid-field” is presented rather vaguely. This concept is relative to “far-field” and “near-field”, but the authors lack a clear definition in terms of distance. Moreover, they do not explain the significance of using this concept for disaster prevention. Compared to near-field events, are there any specific characteristics of mid-field tsunami propagation that need attention? How would it differ in causing inundation?
2) The selection of the seven cases in the article appears somewhat arbitrary. This study considers a maximum magnitude of up to 9.2, but it lacks a discussion on the probability of such an earthquake occurring. The author should at least clarify the magnitude represented by each case (Figure 3). If the intention is to select randomly, I suggest the author consider using the PTHA method, rather than focusing on the worst-case scenario.
3) There is some lack of clarity in the method description, such as the absence of a clear definition for arrival time. I also hope the author can explain why the presence of splay faults would result in the run-up doubling (Line 155). What is the reason behind this?
4) Figure 7 compares the simulated values with the records of the 1945 tsunami, yet the whole figure is perplexing. The negative phase of the blue curve (simulation) shows a significant truncation, which is caused by the wet point depth of the bathymetry and can introduce substantial errors.
5) There are spelling issues throughout the entire article, with quite a few typos and grammatical errors. I suggest that the author thoroughly proofread and polish the text before resubmitting.
Line 32: “December 20011”?
Line 52: What do you mean “unseen”?
Line 102: What is the meaning of “dwarf”?
Line 181-185: This section seems to deviate from the main topic; I suggest deleting it.
Citation: https://doi.org/10.5194/nhess-2024-110-RC2 - AC2: 'Reply on RC2', Haider Hasan, 11 Sep 2024
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RC3: 'Comment on nhess-2024-110', Anonymous Referee #3, 19 Sep 2024
In the reviewed work, Hasan et al. propose an estimation of the hazard at Karachi as stemming from seven hypothetical subduction earthquakes. I did find the work to be mostly well written, and the methodology is carried out reasonably. However, there are two significant issues with it:
1) The argument that there is a problem for mid-field (should be regional?) feels like an artifice. It is stated early in the text (P1, L30), without any reference. But more importantly, it is rebuked by the article itself in section 2.3, specifically in Lines 200-205. If Pakistan has the same timeframes as Oman, and the “initial tsunami warning message in Pakistan should reach the public within 7-12 minutes after the earthquake”...then, what prevents Karachi to know a tsunami is coming? What kind ot Tsunami Early Warning System informs some and not others. If find it baffling.
2) However, the above is just a minor problem. The real issue is that the models are dated. To begin with, the use of uniform sources is known to underrepresent several Tsunami Intensity Metrics such as flow depth and runup. Next, the use of just seven scenarios is not properly justified. Finally, even though the authors recognise the possibility of secondary sources as relevant for the hazard (such as splay faults), they are all but dismissed. I think part of the problem is that the literature review omits relevant recent work that incorporated most of these topics while studying the MSZ:
-10.1016/j.tecto.2022.229462 Use of geodetic constraints to delimit plausible ruptures
-10.1080/21664250.2022.2117585 includes splay faults as an additional tsunami source
-10.1016/j.apor.2023.103784 use stochastic models for the seismic sources, although with smaller maximum magnitude.
-10.1080/21664250.2024.2324520 introduce uncertainty by means of logic treesto mention just a few.
Hence, though the work is a case study for Karachi, I can not help to feel that is using a dated approach, and the results are not a relevant scientific addition.
I have several other specific comments, but since these two objections suffice to sustain my recommendation, I will not include them here.
Citation: https://doi.org/10.5194/nhess-2024-110-RC3 - AC3: 'Reply on RC3', Haider Hasan, 11 Oct 2024
Status: closed
-
RC1: 'Comment on nhess-2024-110', Anonymous Referee #1, 22 Jul 2024
Comments
The article examines tsunami hazards in the greater Karachi area resulting from seven hypothetical ruptures of the Makran subduction thrust, identifying two primary hazard zones: one near Karachi Port and the other adjacent to Port Qasim. Through numerical modeling, it is demonstrated that larger ruptures produce higher waves and faster arrival times, thereby highlighting the central region's susceptibility due to its low elevation and critical infrastructure. The study emphasizes the necessity of early warning systems and preparedness strategies to mitigate tsunami risks in this region.
Overall, the paper is well-written, the methodology is robust, and I appreciated the way the results were presented. However, I have some comments and recommend a minor revision before publication:
- Discussion of Results: The paper lacks sufficient discussion regarding the results, particularly in comparing the actual Karachi marigram with the simulated results. Other studies have presented more robust simulations between the real data from the 1945 tsunami and their results. The authors should indicate that the discrepancies between their findings and the actual 1945 tsunami waveform may come from their simplistic representation of the tsunami source. The differences observed between the simulations and the two marigrams may not solely be attributed to tide gauge errors, but also to our limited understanding of the 1945 tsunami source.
- Title Adjustment: I recommend removing the word "seven" from the title.
- Abstract Corrections: In Abstract (line 13), "km" should be corrected to "km²." Additionally, in line 21, the repeated use of "neither" makes the text unnecessarily verbose.
- Typos and References: The manuscript contains several typographical errors, such as "Tohuku" (which should be "Tohoku") and "20011" (which should be "2011"). Moreover, in several instances, the authors have neglected to include the publication year of references in parentheses. A thorough review of the text for typos and potential grammatical errors is recommended.
- Figure Combination: It appears that Figures a&b could be combined into a single figure unless additional details (e.g., an earthquake catalog) are added to panel (a).
- Model Details: Additional information about the GeoClaw model should be included, such as inputs and outputs, the types of water equations used, the algorithms employed for inundation calculations, the nature of the friction effects (whether constant or variable), and whether structured or unstructured grids are used.
- Font Size in Figures: The font size in Figure 2 should be increased for better visibility.
- Scenario Simplifications: More details are needed regarding the simplifications made in selecting scenarios. Such simplifications (e.g., rectangular sources and uniform slip) could lead to potentially underestimated or less complex wave height distributions. The strike angle of 270 degrees appears inconsistent with the tectonics of the Makran thrust; any alterations in the strike direction could yield different wave directions and results. These simplifications require a more thorough explanation. Furthermore, these scenarios do not seem to offer new insights compared to those proposed in previous studies.
- Table A3 Enhancements: In Table A3, please add an additional column indicating the moment magnitude (Mw) for each scenario.
Citation: https://doi.org/10.5194/nhess-2024-110-RC1 -
AC1: 'Reply on RC1', Haider Hasan, 07 Aug 2024
Thank you for your comments and suggestions regarding our manuscript titled "Mid-field tsunami hazards in greater Karachi from seven hypothetical ruptures of the Makran subduction thrust." We appreciate the time and effort you have taken to review our work. In response to your comments, we have carefully addressed each point. A detailed reply to each comment is attached.
-
RC2: 'Comment on nhess-2024-110', Anonymous Referee #2, 06 Sep 2024
The study by Hasan et al. identifies two mid-field tsunami hazard zones in greater Karachi, characterized by higher and faster-arriving waves at Karachi Port compared to Port Qasim. The simulations assume seven east-west megathrust ruptures with uniform seismic slip and magnitudes ranging from 8.1 to 9.2, highlighting that neither zone is near enough for reliable seismic warnings nor distant enough for more than three hours of advance notice. Although I acknowledge that their simulations could be helpful to identify potential tsunami hazards, the overall structure of the article is overly lengthy, and there are certain flaws in the research methodology. Overall, the article requires significant revisions. Regrettably, I have decided to reject the manuscript.
1) The article analyzes mid-field tsunami hazards. However, the concept of “mid-field” is presented rather vaguely. This concept is relative to “far-field” and “near-field”, but the authors lack a clear definition in terms of distance. Moreover, they do not explain the significance of using this concept for disaster prevention. Compared to near-field events, are there any specific characteristics of mid-field tsunami propagation that need attention? How would it differ in causing inundation?
2) The selection of the seven cases in the article appears somewhat arbitrary. This study considers a maximum magnitude of up to 9.2, but it lacks a discussion on the probability of such an earthquake occurring. The author should at least clarify the magnitude represented by each case (Figure 3). If the intention is to select randomly, I suggest the author consider using the PTHA method, rather than focusing on the worst-case scenario.
3) There is some lack of clarity in the method description, such as the absence of a clear definition for arrival time. I also hope the author can explain why the presence of splay faults would result in the run-up doubling (Line 155). What is the reason behind this?
4) Figure 7 compares the simulated values with the records of the 1945 tsunami, yet the whole figure is perplexing. The negative phase of the blue curve (simulation) shows a significant truncation, which is caused by the wet point depth of the bathymetry and can introduce substantial errors.
5) There are spelling issues throughout the entire article, with quite a few typos and grammatical errors. I suggest that the author thoroughly proofread and polish the text before resubmitting.
Line 32: “December 20011”?
Line 52: What do you mean “unseen”?
Line 102: What is the meaning of “dwarf”?
Line 181-185: This section seems to deviate from the main topic; I suggest deleting it.
Citation: https://doi.org/10.5194/nhess-2024-110-RC2 - AC2: 'Reply on RC2', Haider Hasan, 11 Sep 2024
-
RC3: 'Comment on nhess-2024-110', Anonymous Referee #3, 19 Sep 2024
In the reviewed work, Hasan et al. propose an estimation of the hazard at Karachi as stemming from seven hypothetical subduction earthquakes. I did find the work to be mostly well written, and the methodology is carried out reasonably. However, there are two significant issues with it:
1) The argument that there is a problem for mid-field (should be regional?) feels like an artifice. It is stated early in the text (P1, L30), without any reference. But more importantly, it is rebuked by the article itself in section 2.3, specifically in Lines 200-205. If Pakistan has the same timeframes as Oman, and the “initial tsunami warning message in Pakistan should reach the public within 7-12 minutes after the earthquake”...then, what prevents Karachi to know a tsunami is coming? What kind ot Tsunami Early Warning System informs some and not others. If find it baffling.
2) However, the above is just a minor problem. The real issue is that the models are dated. To begin with, the use of uniform sources is known to underrepresent several Tsunami Intensity Metrics such as flow depth and runup. Next, the use of just seven scenarios is not properly justified. Finally, even though the authors recognise the possibility of secondary sources as relevant for the hazard (such as splay faults), they are all but dismissed. I think part of the problem is that the literature review omits relevant recent work that incorporated most of these topics while studying the MSZ:
-10.1016/j.tecto.2022.229462 Use of geodetic constraints to delimit plausible ruptures
-10.1080/21664250.2022.2117585 includes splay faults as an additional tsunami source
-10.1016/j.apor.2023.103784 use stochastic models for the seismic sources, although with smaller maximum magnitude.
-10.1080/21664250.2024.2324520 introduce uncertainty by means of logic treesto mention just a few.
Hence, though the work is a case study for Karachi, I can not help to feel that is using a dated approach, and the results are not a relevant scientific addition.
I have several other specific comments, but since these two objections suffice to sustain my recommendation, I will not include them here.
Citation: https://doi.org/10.5194/nhess-2024-110-RC3 - AC3: 'Reply on RC3', Haider Hasan, 11 Oct 2024
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