the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 May 2023
Seismotectonic and Probabilistic Seismic Hazard of Sunda Strait Region
Abstract. The Sunda Strait region is the riskiest area between Java and Sumatra. The impact of seismic activities must be considered to reduce disaster risk. This study has answered the lack of information about detailed seismotectonic conditions in the Sunda Strait. It confirms geotectonic actions controlled by the Asymmetry Subduction of the Sunda Arc between Oblique Subduction (Sumatra) and Frontal Subduction (Java). These activities produce five main seismotectonic zones based on their respective sources: Megathrust Seismotectonic Zones, Benioff Wadati Seismotectonic Zones, Sumatra Active Fault Seismotectonic Zones, Lampung Block Seismotectonic Zone and Center Part of Sunda Strait Seismotectonic Zone. Based on these seismic source zones, the Probabilistic Seismic Hazard Analyses (PSHA) of Sunda Strait have maximum Peak Ground Acceleration (PGA)=0.465 g, Pseudo Seismic Acceleration (PSA) Ss=0.2 second=1.114 g, PSA S1=1 second=0.465 g in Site Class SB at 7 % probability in 75 years (equivalent to a mean return of 1000 years), maximum Peak Ground Acceleration (PGA)=0.484 g, Pseudo Seismic Acceleration (PSA) Ss=0.2 second=1.159 g, PSA S1=1 second=0.484 g in Site Class SB at 2 % probability in 50 years (equivalent to a mean return of 2500 years), Peak Ground Acceleration (PGA)=0.499 g, Pseudo Seismic Acceleration (PSA) Ss=0.2 second=1.193 g, PSA S1=1 second=0.499 g, in Site Class SB at 2 % probability in 100 years (equivalent to a mean return of 5000 years). The results of this effort are expected to be used as the primary data for seismic risk assessment in the Sunda Strait and its surrounding area.
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-423', Domenico Giardini, 15 Nov 2023
I reviewed the PSHA study of the Sunda Strait region by A. Soehaimi and coworkers, and I find it severely insufficient and to be rejected. The team is using the most advanced PSHA code available today – the OpenQuake code developed at GEM – but it appears that they are not sufficiently knowledgeable to use it properly.
I list here the most important elements of the paper which I deem insufficient, missing or wrong, at least on the basis of what is written in the paper.
Seismic sources
- An overview of the regional geological history is given, with the sequence of the major geological units, but this has little relevance for the definition of the seismogenic sources.
- A list of active faults is also given, but the geometry of the seismic sources does not follow the geology nor the orientation of the major faults, which cut across the borders of the chosen seismic sources.
- The region chosen is much too small for an area which has potential for earthquakes of 8.5 and more, which would have dimensions reaching or even exceeding the chosen region.
- No indication is provided to explain how extended sources will be treated, and what happens when they go beyond the chosen borders.
- This is a major subduction region, and subduction earthquakes need to be modelled on an inclined interface, separated from the overlaying crustal sources.
- No indication is given of how depth is treated in the seismic sources.
Seismic catalogue and statistical analysis
- The USGS an CMT catalogues starting in 1997 are used; this is not more acceptable in modern PHSA, especially for regions of high-magnitude seismicity where the longest possible catalogues should be used, both historical and instrumental, considering also that the study covers up to a 2500 yr period.
- The Gutenberg-Richter fits are shown, but the method used is not specified.
- The type of magnitude used and possible conversions are not specified.
- The completeness magnitude and how it has been computed are not given.
- The procedure used for declustering is not specified.
- The maximum magnitude is not given.
- The b-values have unrealistically low values in the 0.3-0.6 range, whereas it is universally observed that tectonic seismicity is characterized by b-values around 1.
Uncertainties
- No indication is given on how statistical uncertainties are propagated across the whole PSHA study.
- The OpenQuake code has been written to take into proper account epistemic uncertainties through the use of an appropriate logic tree; the logic-tree branches should cover as a minimum: (i) different approaches to model seismicity including seismogenic sources and faults+background seismicity; (ii) Mmax; (iii) declustering and Mc; (iv) different GMPEs.
PSHA results
- The PGA maps shown in Figure 7 cannot be obtained by the simple sources shown in Figure 5, unless site corrections were applied; the authors should display hazard for uniform site conditions as well as the map of the site corrections, before combining them in a site-specific map.
Further editorial comments:
- A long explanation and list of equations are provided for the GR statistics and PSHA; these are well known and established since many decades; such a repetition is not needed.
- It makes no sense to provide PGA values with 3 decimal figures, considering the associated uncertainties.
- The references provided are sometimes not the right ones, some refer to obscure studies and some key references are missing.
- The English language requires in any case a professional editing.
Normally I am not so negative in my reviews, but here I need to be clear in pointing out that PSHA studies are not merely scientific investigations, they are done to provide values of relevance for the protection of our society against seismic risk. They must be performed at the highest possible standard available today, and absolutely not in the way done in this paper. It is not a question if the results shown are right or wrong, it’s more that the study lacks the basic fundaments of a modern seismic hazard assessment, and the results cannot be defended and should not be used in any possible application.
From the language used in the text, it appears that the team is composed by geologists with a good knowledge of the regional geology, but little experience in seismic hazard assessment. I would advise the team to seek the cooperation of teams or institutes with an established tradition and proven experience in carrying out PSHA assessments in areas of high seismic risk.
Citation: https://doi.org/10.5194/egusphere-2023-423-RC1 -
RC2: 'Comment on egusphere-2023-423', Anonymous Referee #2, 29 Dec 2023
The manuscript titled 'Assessment of Seismotectonic and Probabilistic Seismic Hazard in the Sunda Strait Region' examines the probabilistic seismic hazard within the Sunda Strait and its surrounding areas. The findings of this document could prove valuable for future seismic risk assessments in the broader Sunda Strait region, encompassing the islands of Java and Sumatra. Nevertheless, I would like to express some concerns and offer suggestions regarding the content of the manuscript. Here are my detailed comments:
Major concerns:
1. Lack of innovation:
Prior to this study, several publications have focused on seismic hazard assessment in Indonesia, such as the work by the National Center for Earthquake Studies in Indonesia (commonly referred to as 'PuSGeN,' as demonstrated by Irsyam et al., 2020), and the research conducted by Petersen et al. in 2004. It is noteworthy that this manuscript does not appear to provide a comprehensive review of these prior studies. Furthermore, it remains unclear whether this study introduces innovative methodologies or incorporates updated databases to enhance the seismic hazard assessment.
References:
Irsyam, M., Cummins, P. R., Asrurifak, M., Faizal, L., Natawidjaja, D. H., Widiyantoro, S., Meilano, I., Triyoso, W., Rudiyanto, A., Hidayati, S., Ridwan, M., Hanifa, N. R., & Syahbana, A. J. (2020). Development of the 2017 National Seismic Hazard Maps of Indonesia. Earthquake Spectra, 36(1_suppl), 112–136. https://doi.org/10.1177/8755293020951206.
Petersen, M. D., Dewey, J., Hartzell, S., Mueller, C., Harmsen, S., Frankel, A. D., & Rukstales, K. (2004). Probabilistic Seismic Hazard Analysis for Sumatra, Indonesia and Across the Southern Malaysian Peninsula. Tectonophysics, 390(1–4), 141–158. https://doi.org/10.1016/j.tecto.2004.03.026.
2. Catalogs integration:
This study has introduced multiple earthquake catalogs, specifically the GCMT, USGS, and ISC catalogs (as indicated in Lines 44-45). However, the procedures for integrating these catalogs are not clearly articulated within the manuscript. It is crucial to clarify how various magnitude scales have been harmonized, how duplicated events have been addressed, and whether the catalog has undergone declustering (i.e., the removal of foreshocks and aftershocks). Providing a detailed description of these processes would be highly beneficial for the audience, as it would enable them to assess the credibility of the seismic hazard assessment with greater confidence.
3. Proper seismic model:
The seismic model proposed in this study is primarily based on regression analysis of earthquake data collected between 1976 and 2022. It is worth noting that relying solely on this relatively short observational period may introduce potential biases to the seismic model. To enhance the model's accuracy and reliability, I would recommend considering additional parameters from geological, geomorphological, or paleo-seismological evidence. Incorporating factors such as slip rate data and paleo-seismic catalogs can provide valuable constraints and improve the robustness of the seismic model.
Furthermore, it's important to address concerns related to the maximum magnitudes proposed for the probability density function (PDF) and probabilities of occurrence (6.5 and 7.0, respectively). Given the Sunda subduction system's potential for generating significantly large earthquakes, it might be beneficial to explore the inclusion of larger magnitude values in the model to better account for the full range of seismic events in the region.
Regarding the b-values observed in the five regions (Figure 6), it's notable that these values appear unusually low, as b-values are typically close to 1.0. There could be concerns about the methodology or data analysis, possibly including the potential inclusion of incomplete or biased parts of the earthquake catalog. It's essential to thoroughly investigate and validate the reasons behind these low b-values to ensure the accuracy of the seismic hazard assessment.
4. Definition of area source geometry:
The definition of area sources in seismic hazard assessment is indeed crucial, and it is typically based on the tectonic regime and seismic activity. This study does not define area source for the Sumatran Fault, a significant fault system in Sumatra. Given the importance of the Sumatran Fault in the region's seismic activity, it may be beneficial to consider including it as a separate area source in the seismic hazard assessment.
Additionally, I suggest to define the southwest boundary of Zones 4 and 5, taking into account the alignment of the Sunda Trench. Ensuring that the boundaries of these zones align with the tectonic features in the region can contribute to a more accurate and representative seismic hazard assessment.
5. Selecting proper GMPEs:
Selecting appropriate Ground Motion Prediction Equations (GMPEs) is indeed a critical aspect of PSHA. In cases where specific GMPEs derived from strong-motion data within the study region (Sumatra or Java) are not available, it is crucial to provide a clear rationale for the selection of alternative GMPEs.
In this study, where GMPEs by Chiou and Youngs (2014) have been chosen, it's essential to explain the reasons behind this choice. The explanation should include any relevant features of the GMPEs, such as their performance in regions with similar tectonic settings or geological conditions, and any considerations that led to their selection as the most suitable option for the study area.
It is important to implement proper GMPEs for various tectonic regimes within the study region, including both shallow crustal and subduction system sources. Given the diversity of tectonic settings in the area, it would be advisable to consider a set of GMPEs that can adequately represent the range of potential earthquake sources. These GMPEs should be selected based on their applicability to the specific tectonic regimes present in the study region.
Minor Comments:
- Lines51-52: The earthquake ‘m’agnitude ‘d’istribution.
- Line 52-53: I don’t quite get the meaning of ‘Earthquakes of varying magnitudes may be caused by tectonic faults (i.e., magnitudes).’
- Figure 7: I don’t understand why the hazard is spatially heterogeneous, since only 5 area sources are considered and a fixed Vs30 is assumed.
Citation: https://doi.org/10.5194/egusphere-2023-423-RC2
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-423', Domenico Giardini, 15 Nov 2023
I reviewed the PSHA study of the Sunda Strait region by A. Soehaimi and coworkers, and I find it severely insufficient and to be rejected. The team is using the most advanced PSHA code available today – the OpenQuake code developed at GEM – but it appears that they are not sufficiently knowledgeable to use it properly.
I list here the most important elements of the paper which I deem insufficient, missing or wrong, at least on the basis of what is written in the paper.
Seismic sources
- An overview of the regional geological history is given, with the sequence of the major geological units, but this has little relevance for the definition of the seismogenic sources.
- A list of active faults is also given, but the geometry of the seismic sources does not follow the geology nor the orientation of the major faults, which cut across the borders of the chosen seismic sources.
- The region chosen is much too small for an area which has potential for earthquakes of 8.5 and more, which would have dimensions reaching or even exceeding the chosen region.
- No indication is provided to explain how extended sources will be treated, and what happens when they go beyond the chosen borders.
- This is a major subduction region, and subduction earthquakes need to be modelled on an inclined interface, separated from the overlaying crustal sources.
- No indication is given of how depth is treated in the seismic sources.
Seismic catalogue and statistical analysis
- The USGS an CMT catalogues starting in 1997 are used; this is not more acceptable in modern PHSA, especially for regions of high-magnitude seismicity where the longest possible catalogues should be used, both historical and instrumental, considering also that the study covers up to a 2500 yr period.
- The Gutenberg-Richter fits are shown, but the method used is not specified.
- The type of magnitude used and possible conversions are not specified.
- The completeness magnitude and how it has been computed are not given.
- The procedure used for declustering is not specified.
- The maximum magnitude is not given.
- The b-values have unrealistically low values in the 0.3-0.6 range, whereas it is universally observed that tectonic seismicity is characterized by b-values around 1.
Uncertainties
- No indication is given on how statistical uncertainties are propagated across the whole PSHA study.
- The OpenQuake code has been written to take into proper account epistemic uncertainties through the use of an appropriate logic tree; the logic-tree branches should cover as a minimum: (i) different approaches to model seismicity including seismogenic sources and faults+background seismicity; (ii) Mmax; (iii) declustering and Mc; (iv) different GMPEs.
PSHA results
- The PGA maps shown in Figure 7 cannot be obtained by the simple sources shown in Figure 5, unless site corrections were applied; the authors should display hazard for uniform site conditions as well as the map of the site corrections, before combining them in a site-specific map.
Further editorial comments:
- A long explanation and list of equations are provided for the GR statistics and PSHA; these are well known and established since many decades; such a repetition is not needed.
- It makes no sense to provide PGA values with 3 decimal figures, considering the associated uncertainties.
- The references provided are sometimes not the right ones, some refer to obscure studies and some key references are missing.
- The English language requires in any case a professional editing.
Normally I am not so negative in my reviews, but here I need to be clear in pointing out that PSHA studies are not merely scientific investigations, they are done to provide values of relevance for the protection of our society against seismic risk. They must be performed at the highest possible standard available today, and absolutely not in the way done in this paper. It is not a question if the results shown are right or wrong, it’s more that the study lacks the basic fundaments of a modern seismic hazard assessment, and the results cannot be defended and should not be used in any possible application.
From the language used in the text, it appears that the team is composed by geologists with a good knowledge of the regional geology, but little experience in seismic hazard assessment. I would advise the team to seek the cooperation of teams or institutes with an established tradition and proven experience in carrying out PSHA assessments in areas of high seismic risk.
Citation: https://doi.org/10.5194/egusphere-2023-423-RC1 -
RC2: 'Comment on egusphere-2023-423', Anonymous Referee #2, 29 Dec 2023
The manuscript titled 'Assessment of Seismotectonic and Probabilistic Seismic Hazard in the Sunda Strait Region' examines the probabilistic seismic hazard within the Sunda Strait and its surrounding areas. The findings of this document could prove valuable for future seismic risk assessments in the broader Sunda Strait region, encompassing the islands of Java and Sumatra. Nevertheless, I would like to express some concerns and offer suggestions regarding the content of the manuscript. Here are my detailed comments:
Major concerns:
1. Lack of innovation:
Prior to this study, several publications have focused on seismic hazard assessment in Indonesia, such as the work by the National Center for Earthquake Studies in Indonesia (commonly referred to as 'PuSGeN,' as demonstrated by Irsyam et al., 2020), and the research conducted by Petersen et al. in 2004. It is noteworthy that this manuscript does not appear to provide a comprehensive review of these prior studies. Furthermore, it remains unclear whether this study introduces innovative methodologies or incorporates updated databases to enhance the seismic hazard assessment.
References:
Irsyam, M., Cummins, P. R., Asrurifak, M., Faizal, L., Natawidjaja, D. H., Widiyantoro, S., Meilano, I., Triyoso, W., Rudiyanto, A., Hidayati, S., Ridwan, M., Hanifa, N. R., & Syahbana, A. J. (2020). Development of the 2017 National Seismic Hazard Maps of Indonesia. Earthquake Spectra, 36(1_suppl), 112–136. https://doi.org/10.1177/8755293020951206.
Petersen, M. D., Dewey, J., Hartzell, S., Mueller, C., Harmsen, S., Frankel, A. D., & Rukstales, K. (2004). Probabilistic Seismic Hazard Analysis for Sumatra, Indonesia and Across the Southern Malaysian Peninsula. Tectonophysics, 390(1–4), 141–158. https://doi.org/10.1016/j.tecto.2004.03.026.
2. Catalogs integration:
This study has introduced multiple earthquake catalogs, specifically the GCMT, USGS, and ISC catalogs (as indicated in Lines 44-45). However, the procedures for integrating these catalogs are not clearly articulated within the manuscript. It is crucial to clarify how various magnitude scales have been harmonized, how duplicated events have been addressed, and whether the catalog has undergone declustering (i.e., the removal of foreshocks and aftershocks). Providing a detailed description of these processes would be highly beneficial for the audience, as it would enable them to assess the credibility of the seismic hazard assessment with greater confidence.
3. Proper seismic model:
The seismic model proposed in this study is primarily based on regression analysis of earthquake data collected between 1976 and 2022. It is worth noting that relying solely on this relatively short observational period may introduce potential biases to the seismic model. To enhance the model's accuracy and reliability, I would recommend considering additional parameters from geological, geomorphological, or paleo-seismological evidence. Incorporating factors such as slip rate data and paleo-seismic catalogs can provide valuable constraints and improve the robustness of the seismic model.
Furthermore, it's important to address concerns related to the maximum magnitudes proposed for the probability density function (PDF) and probabilities of occurrence (6.5 and 7.0, respectively). Given the Sunda subduction system's potential for generating significantly large earthquakes, it might be beneficial to explore the inclusion of larger magnitude values in the model to better account for the full range of seismic events in the region.
Regarding the b-values observed in the five regions (Figure 6), it's notable that these values appear unusually low, as b-values are typically close to 1.0. There could be concerns about the methodology or data analysis, possibly including the potential inclusion of incomplete or biased parts of the earthquake catalog. It's essential to thoroughly investigate and validate the reasons behind these low b-values to ensure the accuracy of the seismic hazard assessment.
4. Definition of area source geometry:
The definition of area sources in seismic hazard assessment is indeed crucial, and it is typically based on the tectonic regime and seismic activity. This study does not define area source for the Sumatran Fault, a significant fault system in Sumatra. Given the importance of the Sumatran Fault in the region's seismic activity, it may be beneficial to consider including it as a separate area source in the seismic hazard assessment.
Additionally, I suggest to define the southwest boundary of Zones 4 and 5, taking into account the alignment of the Sunda Trench. Ensuring that the boundaries of these zones align with the tectonic features in the region can contribute to a more accurate and representative seismic hazard assessment.
5. Selecting proper GMPEs:
Selecting appropriate Ground Motion Prediction Equations (GMPEs) is indeed a critical aspect of PSHA. In cases where specific GMPEs derived from strong-motion data within the study region (Sumatra or Java) are not available, it is crucial to provide a clear rationale for the selection of alternative GMPEs.
In this study, where GMPEs by Chiou and Youngs (2014) have been chosen, it's essential to explain the reasons behind this choice. The explanation should include any relevant features of the GMPEs, such as their performance in regions with similar tectonic settings or geological conditions, and any considerations that led to their selection as the most suitable option for the study area.
It is important to implement proper GMPEs for various tectonic regimes within the study region, including both shallow crustal and subduction system sources. Given the diversity of tectonic settings in the area, it would be advisable to consider a set of GMPEs that can adequately represent the range of potential earthquake sources. These GMPEs should be selected based on their applicability to the specific tectonic regimes present in the study region.
Minor Comments:
- Lines51-52: The earthquake ‘m’agnitude ‘d’istribution.
- Line 52-53: I don’t quite get the meaning of ‘Earthquakes of varying magnitudes may be caused by tectonic faults (i.e., magnitudes).’
- Figure 7: I don’t understand why the hazard is spatially heterogeneous, since only 5 area sources are considered and a fixed Vs30 is assumed.
Citation: https://doi.org/10.5194/egusphere-2023-423-RC2
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