Strain Localization and Stress Evolution Along the Yangsan Fault: A Geodetic Approach to Seismic Hazard Assessment
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
This study examines crustal deformation along the central Yangsan Fault in southeastern Korea using data from 23 geodetic control points, 10 CORS stations, GNSS baseline measurements, and elastic strain tensor analysis. Deformation fields from 2018–2020 and 2019–2020 show notable spatial variations in shear strain rates (up to 2.4 × 10⁻⁷ strain/year) and principal stress directions. Triangle 16, near the junction of the Yangsan and Ulsan faults, exhibits a high shear strain (2.325 µstrain/yr) and a shear stress orientation of 87.59°, indicating a NE–SW stress regime. Neighboring triangles, such as 17 and 8, show over 45° of angular deviation in shear direction, pointing to localized stress reorganization due to fault interactions. GNSS-derived strain tensors effectively distinguish between continuous and transient deformation signals, with persistent strain observed in networks 6, 10, and 11. These results emphasize the value of multi-epoch geodetic monitoring for understanding intraplate deformation and improving seismic hazard assessment in stable continental regions.
I have following suggestions to improve the mansucript
1) The background on the Yangsan Fault's tectonic history and seismogenic potential is thorough and well-referenced. However, a brief mention of the societal implications of seismic hazards in South Korea could further emphasize the study's relevance.
2) The description of Delaunay triangulation and its advantages is clear, but a short explanation of why this method was chosen over other interpolation techniques (e.g., kriging) would be helpful.
3) The section on strain tensor derivation is technically sound, but a brief example or schematic (e.g., in supplementary materials) could aid readers unfamiliar with tensor mathematics.
4) The high-strain regions (e.g., Triangles 6, 10, 11, 16) are well-identified, but the manuscript could further discuss why these specific triangles exhibit anomalous behavior. For instance, are there geological or structural features (e.g., fault intersections, lithological contrasts) that correlate with these strain patterns?
5) The temporal variability in strain rates (e.g., 2018–2020 vs. 2019–2020) is intriguing. A deeper discussion on potential causes (e.g., aseismic slip, fluid migration) would strengthen the interpretation.
6) The comparison of shear stress directions (e.g., Triangle 16 vs. neighboring triangles) is a highlight. The manuscript could expand on how these angular deviations might influence fault rupture scenarios (e.g., barrier vs. asperity behavior).
7) The link between geodetic strain and seismic hazard could be further developed. For example, how might the observed strain rates translate into probabilistic seismic hazard models?
8) The conclusion effectively summarizes the key findings but could briefly reiterate the study's broader implications for earthquake preparedness and infrastructure resilience in South Korea.
Minor Suggestions:
Some sentences are lengthy or complex (e.g., in the Methods section). Breaking these into shorter sentences would improve readability.
Ensure consistency in terminology (e.g., "GNSS" vs. "GPS" in some sections).
References:
The reference list is comprehensive, but a few recent studies on intraplate deformation (e.g., from other stable continental regions) could provide additional context.
Figures:
Consider adding a location map early in the manuscript to help readers visualize the study area relative to major tectonic features in East Asia.
For Figure 9, clarify whether the vectors represent absolute or relative shear strain magnitudes.
Comments on the Quality of English Language
English Can be improved
Author Response
Reviewer Comment 1:
The background on the Yangsan Fault's tectonic history and seismogenic potential is thorough and well-referenced. However, a brief mention of the societal implications of seismic hazards in South Korea could further emphasize the study's relevance.
Response 1:
We appreciate the reviewer’s insightful suggestion. In response, we have added a paragraph that addresses the societal and infrastructural implications of seismic hazards on the Korean Peninsula. Specifically, we highlight the occurrence of two recent moderate-magnitude earthquakes (Gyeongju 2016, Pohang 2017) and their proximity to major critical infrastructure, including nuclear power plants near Gyeongju and Ulsan. This addition underscores the urgent need for continued seismic vigilance, even within a traditionally considered intraplate-stable region.
The revised text appears in the Introduction section and reads as follows:(page3 lines 127-136)
“Although the Korean Peninsula is generally considered to be located within a tectonically stable intraplate region, recent moderate-magnitude earthquakes highlight the need for continued seismic vigilance. Notably, the Mw 5.8 Gyeongju earthquake in 2016 and the Mw 5.4–5.5 Pohang earthquake in 2017 occurred in the southeastern part of the peninsula, indicating active fault behavior in regions previously thought to be quiescent. This area also contains the majority of South Korea’s nuclear power plants, particularly near Gyeongju and Ulsan, making it a region of critical infrastructure concentration. Therefore, despite the intraplate setting, the potential for damaging seismic events necessitates high-resolution crustal deformation monitoring and comprehensive seismic hazard
Reviewer Comment 2:
The description of Delaunay triangulation and its advantages is clear, but a short explanation of why this method was chosen over other interpolation techniques (e.g., kriging) would be helpful.
Response 2:
We appreciate the reviewer’s constructive suggestion. To clarify the rationale for selecting Delaunay triangulation, we have elaborated on its advantages relative to alternative interpolation techniques in the revised manuscript. Specifically, we explain that since strain tensors are computed within triangular elements, mesh-independent methods such as kriging, spline interpolation, inverse distance weighting (IDW), and nearest neighbour interpolation are generally less appropriate for this application. These methods may oversimplify spatial variability, disregard geometric relationships, or introduce discontinuities in the strain field. In contrast, Delaunay triangulation preserves the geometric integrity of the observation network and supports more stable and physically meaningful strain estimation.
This clarification appears in the revised manuscript as follows: (page12 lines 458-465)
“Moreover, since strain tensors are computed within triangular elements, interpolation techniques that do not incorporate mesh geometry—such as kriging, spline interpolation, inverse distance weighting (IDW), and nearest neighbour methods—are generally less appropriate for this application. These approaches tend to either oversimplify spatial variability, ignore underlying geometric constraints, or introduce discontinuities in strain fields. In contrast, Delaunay-based triangulation respects the spatial configuration of the observation network, enabling more physically consistent, locally responsive, and geometrically constrained strain estimation.”
We hope this addition resolves the reviewer’s concern.
Reviewer Comment 3:
The section on strain tensor derivation is technically sound, but a brief example or schematic (e.g., in supplementary materials) could aid readers unfamiliar with tensor mathematics.
Response 3:
Thank you for your thoughtful suggestion. To enhance the accessibility of the strain tensor derivation for readers less familiar with tensor mathematics, we have added a schematic illustration corresponding to Equations A2–A6 in the Appendix. This visual aid demonstrates the geometric interpretation of strain components and their relation to GNSS displacement data.
For this purpose, the original Figure 2 has been relocated to the Supplementary Appendix as Figure A1, where it now serves as a conceptual diagram supporting the strain tensor formulation. We believe this addition will help bridge the gap between theoretical expressions and their physical meaning, without interrupting the technical flow of the main manuscript.
Reviewer Comment 4:
The high-strain regions (e.g., Triangles 6, 10, 11, 16) are well-identified, but the manuscript could further discuss why these specific triangles exhibit anomalous behavior. For instance, are there geological or structural features (e.g., fault intersections, lithological contrasts) that correlate with these strain patterns?
Response 4:
We thank the reviewer for pointing out the need to further contextualize the anomalous strain behavior observed in specific triangular regions. We have expanded the discussion by examining geological and structural factors that may underlie these anomalies. In particular, we highlight that Triangles 6, 10, 11, and 16 exhibit elevated first principal strain (ε₁) magnitudes, suggestive of localized stress concentration. Triangle 16 is emphasized as a key example due to its location at the intersection of the Yangsan and Ulsan Faults, where oblique fault geometries are likely to induce distributed deformation. Additionally, this region aligns with a distinct lithological boundary between Mesozoic basement and Cretaceous sedimentary rocks, which introduces significant mechanical heterogeneity. These geological features are interpreted to enhance localized strain accumulation and directional deviation, providing a plausible geodynamic explanation for the observed patterns. We hope this addition meets the reviewer’s request for a more thorough interpretation of the strain anomalies. (page28 lines 983-990))
Reviewer Comment 5:
The temporal variability in strain rates (e.g., 2018–2020 vs. 2019–2020) is intriguing. A deeper discussion on potential causes (e.g., aseismic slip, fluid migration) would strengthen the interpretation.
Response 5:
We thank the reviewer for this insightful comment. In response, we have expanded the discussion in the manuscript to address the potential causes of the observed temporal variability in strain rates. Specifically, we now include a paragraph discussing plausible mechanisms such as aseismic slip and episodic fluid migration, both of which could modulate local strain accumulation without producing detectable seismicity. This addition appears on of the revised manuscript and helps to contextualize the differences observed between the 2018–2020 and 2019–2020 strain fields within a broader framework of transient tectonic processes. We believe this enhancement strengthens the interpretation of our results and aligns well with the reviewer’s recommendation. (page28 lines 1023-1035)
Reviewer Comment 6:
The comparison of shear stress directions (e.g., Triangle 16 vs. neighboring triangles) is a highlight. The manuscript could expand on how these angular deviations might influence fault rupture scenarios (e.g., barrier vs. asperity behavior).
Response 6:
We appreciate the reviewer’s insightful comment highlighting the importance of angular deviations in shear stress direction. In response, we have added a detailed discussion to the revised manuscript (see Section 4, paragraph 6) elaborating on the possible implications of the observed shear orientation discrepancies for fault rupture dynamics. Specifically, we discuss how the substantial deviation in θ_γₘₐₓ at Triangle 16 relative to neighboring regions could reflect a transition zone with complex fault geometry and lithological heterogeneity. This structural setting may function either as a rupture barrier or an asperity depending on the stress alignment and rupture conditions. Such scenarios are now more explicitly linked to the potential for rupture nucleation, propagation, and arrest. We believe this addition improves the manuscript’s interpretation of geodetic strain results in the context of seismic hazard modeling.(page28 lines 1013- 1022)
Reviewer Comment 7:
The link between geodetic strain and seismic hazard could be further developed. For example, how might the observed strain rates translate into probabilistic seismic hazard models?
Response 7:
We appreciate the reviewer’s suggestion and have expanded the discussion to explicitly address the integration of geodetic strain rates into probabilistic seismic hazard models (PSHM). As now described in the revised manuscript, GNSS-derived strain rates can serve as crucial inputs to PSHM, especially in regions with sparse seismic records. Specifically, we explain how strain accumulation can be related to long-term seismic moment release to estimate earthquake recurrence rates. The revised text also cites recent studies (e.g., Stevens and Avouac, 2021; Rollins et al., 2023; Magistrale et al., 2020; Materna and Maurer, 2023) that demonstrate the utility of strain rates in calibrating seismic source models and constraining magnitude–frequency distributions. These additions clarify the relevance of our strain results for seismic hazard analysis in southeastern Korea.(Page.30 line 1086-1101)
Reviewer Comment 8:
The conclusion effectively summarizes the key findings but could briefly reiterate the study's broader implications for earthquake preparedness and infrastructure resilience in South Korea. Minor Suggestions: Some sentences are lengthy or complex (e.g., in the Methods section). Breaking these into shorter sentences would improve readability. Ensure consistency in terminology (e.g., "GNSS" vs. "GPS" in some sections)
Response 8:
We appreciate the reviewer’s insightful suggestions. In response, we have revised the manuscript to improve readability by breaking down long or complex sentences, particularly in the Methods section. Additionally, we ensured consistency in technical terminology by unifying the usage of “GNSS” throughout the manuscript, replacing previously mixed references to “GPS.” We also acknowledge the importance of broader contextualization and have expanded the Conclusion section to briefly emphasize the study’s implications for earthquake preparedness and infrastructure resilience in South Korea.
Reviewer Comment 9:
References: The reference list is comprehensive, but a few recent studies on intraplate deformation (e.g., from other stable continental regions) could provide additional context.
Response 9:
We thank the reviewer for the insightful suggestion regarding the inclusion of recent studies on intraplate deformation in other stable continental regions. In response, we have expanded the Discussion section to incorporate relevant findings from recent literature, including case studies from northwestern Iberia (Cesca et al., 2023), southeastern France (Barbarand et al., 2022), and central Australia (Dyksterhuis and Müller, 2008). These studies collectively emphasize that even tectonically stable regions can experience localized deformation driven by inherited fault structures, lithological heterogeneity, and the transmission of far-field stresses.
By highlighting parallels between these intraplate environments and the geological setting of the Korean Peninsula, we underscore the necessity of integrating geodetic and geological data to assess long-term strain accumulation and seismic potential, even in regions traditionally considered stable. This addition strengthens the broader relevance of our findings and aligns with the reviewer’s recommendation to enhance the contextual framing of intraplate deformation processes.
These updates are now reflected in the revised manuscript (Page2 lines 92-100)
Reviewer Comment 10:
Figures: Consider adding a location map early in the manuscript to help readers visualize the study area relative to major tectonic features in East Asia. For Figure 9, clarify whether the vectors represent absolute or relative shear strain magnitudes.
Response 10:
In response to the reviewer’s suggestion, we have added a location map in Figure 3 to help readers visualize the study area in relation to major tectonic features in East Asia. Additionally, we have revised the captions for Figures 8 and 9 to clearly indicate that the vectors represent absolute shear strain magnitudes, thereby improving interpretability. (Figure 9 caption)
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsBased on intensive GNSS baseline measurements and elastic tensor analysis, this paper quantitatively assesses the crustal deformation evolution in the southeastern region of South Korea. It uses geophysical data to reveal the deep fault activities and their influence process on the occurrence of earthquakes. The research has important theoretical significance and practical value.
The introduction section lacks an introduction to the relationship between active tectonic zones and earthquakes as well as faults. Please add this content.
How do geophysical methods reveal deep geological structures? Is the structure obtained from the single-source data adopted in this study reliable?
Please appropriately optimize the font layout in Figure 3. Additionally, it is suggested to appropriately add relevant geological information.
Whether relevant geological parameters were referred to in the workflow of crustal deformation analysis to correct the calculation model
The main objective is to gain a more accurate understanding of the crustal deformation and seismic potential in this area. The main conclusion in the text is too long and needs to be appropriately concise to highlight the main objective of the research.
Author Response
Reviewer Comment 1:
The introduction section lacks an introduction to the relationship between active tectonic zones and earthquakes as well as faults. Please add this content.
Response 1:
Thank you for your valuable comment. In response, we have added a new paragraph in the Introduction section that explicitly describes the relationship between active tectonic zones, earthquakes, and fault systems. This addition enhances the contextual understanding of how crustal deformation leads to seismic hazards in tectonically active regions.
Revised Text:( page2 lines 82-91)
Active tectonic zones are regions of intense crustal deformation driven by plate interactions, typically characterized by active faults, folds, and fault-bounded basins. Earthquake generation in these zones results from the gradual accumulation of tectonic stress along faults, with seismic rupture occurring once fault strength is exceeded. Recurrence intervals and rupture patterns are influenced by fault slip rates, regional stress regimes, and the presence of overpressured fluids, which can promote nucleation. Modern geodetic and remote sensing techniques have significantly advanced our ability to map surface ruptures, estimate slip distributions, and monitor crustal deformation in near-real time. These technological developments play a crucial role in assessing seismic hazards and mitigating risk in infrastructure-critical regions (Yu et al., 2021; Sibson, 2020; Elliott et al., 2016; Duong et al., 2024; Ren et al., 2018; Liang et al., 2023).
Reviewer Comment 2:
How do geophysical methods reveal deep geological structures? Is the structure obtained from the single-source data adopted in this study reliable?
Response 2:
Thank you for your insightful question. Geophysical methods such as gravity, magnetics, seismic reflection/refraction, and GNSS-based geodesy play an essential role in imaging deep geological structures. These techniques indirectly infer subsurface features by measuring variations in physical properties—such as density, velocity, or displacement—that are linked to geological discontinuities, fault zones, and lithological contrasts. In particular, the combination of high-resolution digital elevation models (DEMs), LiDAR-based surface mapping, and subsurface trenching (as demonstrated in Ha et al., 2022; 2025) has proven effective in delineating both surface and near-surface fault architectures.
In the present study, while GNSS-based strain analysis serves as the primary method, we acknowledge the limitations of relying on a single data source. Therefore, our methodology was strengthened by incorporating multiple GNSS campaign surveys in addition to permanent CORS data to enhance spatial resolution. Moreover, we emphasize in the manuscript that long-term reliability and interpretive depth require integrative approaches that combine GNSS strain data with geological cross-sections, fault segmentation maps, and regional elastic property models. Future studies are planned to incorporate InSAR, seismic tomography, and 3-D structural modeling to further improve the resolution and reliability of deep structure interpretations along the Yangsan Fault zone.
This integrated approach ensures that the structural inferences made from GNSS data are cross-validated and scientifically robust, particularly in regions of critical infrastructure such as the Gyeongju–Pohang corridor. Page 3 lines 137-143 and page 29 lines 1057-1067
Reviewer Comment 3:
Please appropriately optimize the font layout in Figure 3. Additionally, it is suggested to appropriately add relevant geological information.
Response 3:
We appreciate the reviewer’s insightful comment. In response, the font layout in Figure 3 has been carefully revised to improve visual clarity and consistency. Specifically, the font size for station labels, velocity vectors, and legend entries has been standardized, and the overall layout has been adjusted to enhance readability at publication scale.
Additionally, relevant geological information has been added to the map. Major fault lines, including the Yangsan Fault and Ulsan Fault, are now more clearly indicated with labeled red lines. We have also included key tectonic features in the inset map, such as surrounding tectonic plates and known microplates (e.g., Okhotsk Plate, Amurian Plate), to provide a clearer regional geodynamic context. These enhancements are intended to support the interpretation of crustal motion patterns and improve the map’s utility for readers unfamiliar with the regional tectonic framework. (Page 11 Figure 3)
Reviewer Comment 4:
Whether relevant geological parameters were referred to in the workflow of crustal deformation analysis to correct the calculation model
Response 4:
Thank you for your important observation. In the present study, we did not directly incorporate geological parameters (e.g., lithological heterogeneity, fault zone material properties, structural boundary conditions) into the numerical strain calculation model. However, we fully recognize the limitations of this assumption and have accordingly discussed the importance of integrating geological parameters in the Discussion section (see page 28, lines 1023-1035).
Specifically, we emphasize that the interpretation of GNSS-derived elastic strain fields should be conducted in conjunction with complementary datasets such as paleoseismic records, seismicity catalogs, geomechanical simulations, and geological factors including fault zone lithology and structural segmentation. This integrative framework is essential for enhancing the physical realism of deformation modeling and improving the reliability of regional seismic hazard assessments.
We appreciate the reviewer’s suggestion and acknowledge that incorporating region-specific geological parameters into future modeling efforts will strengthen the predictive accuracy of strain-based seismic risk analyses.
Reviewer Comment 5:
The main objective is to gain a more accurate understanding of the crustal deformation and seismic potential in this area. The main conclusion in the text is too long and needs to be appropriately concise to highlight the main objective of the research.
Response 5:
Thank you for your helpful comment. We fully agree with your suggestion. To improve clarity and better emphasize the main objective of the study, we have revised the conclusion section by removing detailed technical content that overlapped with the Discussion, including the specific description of Triangle 16 and the angular deviations of adjacent triangles.
These detailed analyses are already thoroughly addressed in the Discussion section, and their removal from the Conclusion allows for a more concise summary that directly highlights the core contributions of the study—namely, the identification of localized deformation patterns and their implications for seismic hazard assessment based on GNSS-derived elastic strain fields. We believe this revision enhances the overall readability and focus of the manuscript.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors present an original, well-written, and comprehensive approach to crustal deformation analysis along the Yangsan Fault, effectively integrating high-precision GNSS data with strain tensor analysis. Their research demonstrates scientific consistency in addressing seismic hazard assessment in stable continental regions. The manuscript appears to have a clear structure and valuable insights into the evolution of spatiotemporal strain. The integration of geodetic monitoring with paleoseismic evidence represents a noteworthy contribution to the understanding of intraplate fault dynamics and earthquake forecasting methodologies. The following comments and suggestions should be examined.
COMMENT#1
While the study compares deformation fields between 2018-2020 and 2019-2020, the authors should consider incorporating more frequent temporal snapshots (quarterly or semi-annual) to better capture transient deformation signals and distinguish them from secular trends. If this case is not realistic for their approach, a brief discussion is required.
COMMENT#2
The manuscript would benefit from a more detailed treatment of measurement uncertainties and their propagation through the strain tensor calculations. Including formal error ellipses for the strain parameters and discussing how GNSS positioning uncertainties affect the reliability of maximum shear strain estimates would enhance the scientific credibility and allow for better comparison with future studies or alternative methodological approaches.
COMMENT#3
Figure 2, which illustrates basic strain-induced deformations with original and deformed shapes, appears to present undergraduate-level conceptual material that may not align with the sophisticated analytical content of this manuscript. For a research article of this quality targeting specialists in geodetic strain analysis, such elementary schematic representations could be omitted or replaced with more advanced visualizations.
COMMENT#4
The conclusions section contains substantial repetition of findings already presented in the discussion, particularly regarding Triangle 16's characteristics and shear stress orientations. The authors should streamline these sections by reserving the discussion for interpretation and analysis, while limiting conclusions to concise summary statements and clear implications for future research. The redundant presentation of specific numerical values (θγₘₐₓ = 87.59°, 2.325 μstrain/yr) weakens the manuscript's impact and suggests insufficient editorial refinement.
COMMENT#5
While the authors cite Wallace (1951) and subsequent studies on dynamic stress perturbations, the connection between these theoretical concepts and their specific GNSS-derived strain measurements remains superficial. The discussion would benefit from a more rigorous theoretical framework that explicitly links their observed shear strain anomalies to established fault mechanics principles, including a quantitative assessment of how their measured strain rates relate to critical stress thresholds for fault reactivation.
COMMENT#6
The concluding recommendations for "continuous monitoring" and "integration of elastic and viscoelastic models" are overly generic and lack actionable detail. The authors should provide specific methodological recommendations, such as optimal GNSS station spacing for their study area, recommended temporal sampling intervals for capturing transient signals, or particular viscoelastic modeling approaches most suitable for the Korean Peninsula's geological setting.
Author Response
Reviewer Comment 1:
While the study compares deformation fields between 2018-2020 and 2019-2020, the authors should consider incorporating more frequent temporal snapshots (quarterly or semi-annual) to better capture transient deformation signals and distinguish them from secular trends. If this case is not realistic for their approach, a brief discussion is required.
Response 1:
Thank you for your insightful comment. We fully agree that incorporating more frequent temporal snapshots would provide a valuable means of detecting transient deformation signals and distinguishing them from long-term tectonic trends. However, as noted in the manuscript, the current study relies in part on campaign-mode GNSS observations, which are resource-intensive and limited by logistical constraints such as field accessibility, personnel availability, and funding cycles.
Therefore, while quarterly or semi-annual analysis would indeed enhance temporal resolution, it is currently not feasible within the scope of our campaign-based observation framework. To address this, we have now added a brief discussion in the revised manuscript, acknowledging this limitation and emphasizing the need for a permanent, high-density GNSS network—particularly in critical areas such as the Gyeongju–Pohang region—to enable more continuous and temporally detailed monitoring of crustal strain accumulation. We believe this clarification provides a realistic context for the temporal design of our study while highlighting future directions for improved observation strategies. (page 29, lines 1057–1085)
Reviewer Comment 2:
The manuscript would benefit from a more detailed treatment of measurement uncertainties and their propagation through the strain tensor calculations. Including formal error ellipses for the strain parameters and discussing how GNSS positioning uncertainties affect the reliability of maximum shear strain estimates would enhance the scientific credibility and allow for better comparison with future studies or alternative methodological approaches.
Response 2:
The background on the Yangsan Fault's tectonic history and seismogenic potential is Thank you for this valuable suggestion. We agree that addressing the propagation of measurement uncertainties through the strain tensor calculations is critical for enhancing the scientific rigor and comparability of our results. In response, we have incorporated a dedicated figure (Figure 10) illustrating the formal error ellipses associated with maximum shear strain (γₘₐₓ) estimates for the periods 2018–2020 and 2019–2020.
These ellipses were derived by propagating GNSS positional uncertainties through the strain tensor computations using standard least-squares covariance analysis. As described in the revised text, the ellipses provide a visual representation of both magnitude and directional uncertainty in shear strain estimates for each triangle. Notably, regions such as Triangles 10, 12, and 16 exhibit relatively large and directionally consistent error ellipses, indicating that while strain accumulation is substantial, the underlying positional uncertainties are also significant and must be taken into account.
This enhancement allows for a more robust assessment of result reliability and facilitates better comparison with future studies or alternative strain modeling approaches. We appreciate the reviewer’s guidance in strengthening this important aspect of our methodology. (page 26 lines 945-960 and Figure 10. )
Reviewer Comment 3:
Figure 2, which illustrates basic strain-induced deformations with original and deformed shapes, appears to present undergraduate-level conceptual material that may not align with the sophisticated analytical content of this manuscript. For a research article of this quality targeting specialists in geodetic strain analysis, such elementary schematic representations could be omitted or replaced with more advanced visualizations.
Response 3:
Thank you for your valuable comment. We agree with your suggestion. Figure 2 was determined to be less appropriate for the main text in its current form. Accordingly, we have removed it from the main body and relocated it to the Appendix as Supplementary Figure A1. Additionally, we included a complementary schematic (now Supplementary Figure A2) to provide further clarification. These figures were moved to ensure that the overall flow and analytical focus of the manuscript remain uninterrupted.
Reviewer Comment 4:
The conclusions section contains substantial repetition of findings already presented in the discussion, particularly regarding Triangle 16's characteristics and shear stress orientations. The authors should streamline these sections by reserving the discussion for interpretation and analysis, while limiting conclusions to concise summary statements and clear implications for future research. The redundant presentation of specific numerical values (θγₘₐₓ = 87.59°, 2.325 μstrain/yr) weakens the manuscript's impact and suggests insufficient editorial refinement.
Response 4:
Thank you for your thoughtful comment. We fully agree with the reviewer’s observation. To avoid redundancy and improve the overall editorial clarity of the manuscript, we have removed the sentence referring to Triangle 16’s shear strain value and orientation — “Triangle 16, situated at the structural junction between the Yangsan and Ulsan Faults, exhibits a high maximum shear strain (2.325 µstrain/yr)…” — from the Conclusions section.
These detailed numerical findings are retained in the Discussion section, where they are more appropriately interpreted in the context of localized stress reorganization and fault interaction. The Conclusions section has been revised to provide a concise summary of the key findings and to emphasize their broader implications for seismic hazard assessment and future geodetic monitoring strategies. We believe this revision enhances the manuscript’s coherence and scientific impact.
Reviewer Comment 5:
While the authors cite Wallace (1951) and subsequent studies on dynamic stress perturbations, the connection between these theoretical concepts and their specific GNSS-derived strain measurements remains superficial. The discussion would benefit from a more rigorous theoretical framework that explicitly links their observed shear strain anomalies to established fault mechanics principles, including a quantitative assessment of how their measured strain rates relate to critical stress thresholds for fault reactivation.
Response 5:
Thank you for this valuable and insightful comment. We agree that the original discussion did not sufficiently elaborate on the theoretical link between GNSS-derived shear strain anomalies and established fault mechanics concepts. In response, we have substantially revised the Conclusions section to address this concern and to propose clear directions for future research that will strengthen the connection between geodetic observations and fault reactivation criteria.
The revised conclusion now includes a discussion on how experimental geomechanics and recent modeling studies (e.g., Noël et al., 2023; Ma & Elbanna, 2018) support the interpretation of localized strain concentrations in terms of evolving fault strength and rupture initiation. We also outline specific research priorities, such as quantifying region-specific critical shear strain thresholds, incorporating laboratory-based fault rheology parameters into GNSS-based models, and developing real-time monitoring frameworks for dynamic stress perturbations.
We believe this addition provides a more rigorous theoretical foundation and enhances the scientific depth of the manuscript, aligning the interpretation of strain field anomalies with contemporary advances in fault mechanics and seismic hazard analysis. (page 30 lines1145-1157)
Reviewer Comment 6:
The concluding recommendations for "continuous monitoring" and "integration of elastic and viscoelastic models" are overly generic and lack actionable detail. The authors should provide specific methodological recommendations, such as optimal GNSS station spacing for their study area, recommended temporal sampling intervals for capturing transient signals, or particular viscoelastic modeling approaches most suitable for the Korean Peninsula's geological setting.
Response 6:
Thank you for your valuable comment. We agree that the original concluding statements were too general and lacked sufficient methodological specificity. In response, we have revised the Conclusions section to include concrete and regionally tailored recommendations that address both the spatial and temporal requirements for effective crustal deformation monitoring in Korea.
Specifically, we now recommend a GNSS triangular network with approximately 5 km spacing in high-strain zones to ensure accurate strain tensor estimation. For tectonically active and infrastructure-sensitive regions such as Gyeongju–Pohang, we propose the installation of additional CORS stations at 10 km intervals along and across the Yangsan and Ulsan Faults. We also emphasize the need for long-term, high-frequency observation strategies to capture both steady-state deformation and transient events.
In terms of modeling, we suggest that future studies adopt viscoelastic forward modeling frameworks calibrated to the Korean crustal setting, such as multilayer Maxwell or Burgers rheologies, to more accurately simulate interseismic and postseismic deformation processes.(page 30 lines 1131-1144)
Additionally, (Discussion section) we have expanded the discussion to highlight how GNSS-derived strain rates can serve as critical input parameters for Probabilistic Seismic Hazard Models (PSHM), particularly in regions with sparse or incomplete earthquake records. Recent studies (e.g., Stevens and Avouac, 2021; Rollins et al., 2023; Magistrale et al., 2020) have demonstrated that incorporating geodetic strain can improve the physical basis and spatial resolution of PSHM by informing moment rate budgets and refining source zone geometries. As Materna and Maurer (2023) note, accounting for strain rate uncertainties further enhances the robustness of hazard forecasts. Therefore, we propose that future work on the Korean Peninsula incorporate GNSS strain data into PSHM frameworks to support more accurate regional seismic risk assessments.
We believe these additions provide both actionable guidance and a strong rationale for future methodological development in this field. (page lines 1096-1101)
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsI have received the manuscript: Crustal Deformation and Seismic Hazard Assessment along the Yangsan Fault, South Korea. The title can be improved to be more catchy. The abstract is messy and lacks proper scientific abstract sections. The authors must follow the guidelines for the abstract. Also, there is no problem statement or an innovative point in the manuscript. Keywords are too long. The introduction is too long, should have some subheadings. The study area can be discussed under introduction. Material and methods chapter should focus only on its relevant methodologies details. Fig 1 belongs to introduction chapter not materials chapter. Materials chapter is too long, many things belongs to results chapter. Move the results into results chapter. 3.1 Data collection. This belongs to the materials and methods section. Fig 7,8,9 inside writings are unreadable, increase font a little bit more. The discuss chapter is to small as compared to the results section. Also discussion chapter should have 2,3 chapters with clear subheadings, diagrams. There are very few references in the discussion chapter. Compare your findings with international literature and cite them like cite this reference (10.1016/j.marpetgeo.2023.106508) in line numb 987. This conclusion is massive, we dont need this kind of conclusion. shrink it to 150-200 words max.
Comments on the Quality of English LanguageEnglish can also be improved.
Author Response
Reviewer Comment 1:
I have received the manuscript: Crustal Deformation and Seismic Hazard Assessment along the Yangsan Fault, South Korea. The title can be improved to be more catchy
Response 1:
Thank you for pointing this out. We agree with this comment. Therefore, we have revised the title to improve its clarity and appeal. The new title better reflects the scientific focus of the study by emphasizing strain localization and stress evolution, which are central to the geodetic analysis conducted in this work.
The revised title is: “Strain Localization and Stress Evolution along the Yangsan Fault: A Geodetic Approach to Seismic Hazard Assessment”
Reviewer Comment 2:
The abstract is messy and lacks proper scientific abstract sections. The authors must follow the guidelines for the abstract. Also, there is no problem statement or an innovative point
in the manuscript. Keywords are too long.
Response 2:
We sincerely thank the reviewer for pointing out the issues related to the abstract structure and clarity. In response to your comments, we have substantially revised the abstract to ensure that it follows the recommended scientific format, including a clear background, methodology, results, and conclusion—as outlined in the journal guidelines.
In particular, we have now clearly articulated the problem statement (the lack of high-resolution geodetic monitoring for identifying localized strain concentration within stable continental interiors), and emphasized the innovative aspects of this study, which include:
•the application of high-density GNSS triangular strain network analysis along an active intraplate fault zone,
•the identification of significant directional anomalies in shear stress near the Yangsan–Ulsan fault junction, and
•the implications for localized stress reorganization and seismic hazard mapping in tectonically stable regions.
We have also shortened and refined the list of keywords from seven to six concise terms, removing redundancy and enhancing clarity:
Revised Keywords: Yangsan Fault; GNSS geodesy; strain tensor; shear strain; principal stress direction; seismic hazard
We hope these revisions adequately address the reviewer’s concerns.
Reviewer Comment 3:
The introduction is too long, should have some subheadings. The study area can be discussed under introduction. Material and methods chapter should focus only on its relevant methodologies details. Fig 1 belongs to introduction chapter not materials chapter.
Response 3:
We appreciate the reviewer’s constructive suggestion. In response, we have reorganized Section 1 into four thematically structured subsections to enhance readability and logical flow:
•1.1 Scientific Background and Motivation
•1.2 Regional Seismotectonic Context of the Korean Peninsula
•1.3 Research Gap and Objectives
•1.4 Study Areas
In particular, the study area description (previously located in Section 2.1) has been relocated and integrated into the revised Introduction under Section 1.4, as recommended.
Consequently, the subheadings in Section 2 have been renumbered and adjusted accordingly to reflect this structural change. Additionally, the duplicated subheading “3.2 Strain Tensor Analysis” has been resolved by revising the second occurrence to “3.3 Strain Tensor Analysis and Spatial Characterization of Deformation” for clarity and consistency.
These structural revisions improve the logical progression of the manuscript while maintaining the integrity of the reference sequence.
Reviewer Comment 4:
Materials chapter is too long, many things belongs to results chapter. Move the results into results chapter. 3.1 Data collection.
Response 4:
We appreciate the reviewer’s insightful comment. In response, we have revised the manuscript to improve the clarity and structural balance between the Materials and Results sections. Specifically, we have:
•Concisely revised Sections 2.1, 2.3, 2.4, and 2.6 to focus solely on methodological content, removing any interpretative or result-oriented discussions.
•Moved the ‘3.1 Data Collection’ subsection into Section 2.6, which now appropriately consolidates all relevant observational and acquisition procedures.
•Additionally, we have revised the caption of Figure 6 to clarify that the red lines represent active fault zones, namely the Yangsan Fault and Ulsan Fault, rather than simply indicating high-strain-rate baselines.
These changes have been implemented to ensure that all methodological elements are confined to the Materials section, while analytical findings and interpretations are presented exclusively in the Results section, as per standard scientific reporting guidelines.
Reviewer Comment 5:
Fig 7,8,9 inside writings are unreadable, increase font a little bit more.
Response 5:
Thank you for pointing this out. We agree with this comment. Therefore, we have increased the font size of internal labels in Figures 7, 8, and 9 to improve readability, particularly for GNSS station names, triangle IDs, and directional annotations.
Updated caption for Figure 8: "Maps of principal strain magnitude and direction derived from GNSS-based strain tensor analysis for two observation periods: (1) 2018–2020 (left) and (2) 2019–2020 (center). The rightmost panel illustrates the triangulated geodetic network used for strain computation. Black bars indicate the orientations and magnitudes of maximum and minimum principal strain axes, with bar lengths scaled to strain intensity (μstrain/yr). The red lines denote the Yangsan Fault (western trace) and the Ulsan Fault (eastern trace). Strain accumulation is notably concentrated between the two fault zones, particularly in the central segment of the Yangsan Fault."
Reviewer Comment 6:
The discuss chapter is to small as compared to the results section. Also discussion chapter should have 2,3 chapters with clear subheadings, diagrams.
Response 6:
Thank you for your valuable comment. We agree with your observation regarding the disproportion between the Results and Discussion sections. Therefore, we have substantially expanded and reorganized the Discussion section into three structured subsections with clear subheadings: 4.1 Elastic Strain Fields and Seismic Hazard Implications, 4.2 Temporal Variability and Network Design Considerations, and 4.3 Integrated Approaches and Hazard Modeling Perspectives. This restructuring improves thematic clarity and analytical depth.
Additionally, to reinforce the conceptual understanding and practical relevance of our findings, we have incorporated a new schematic diagram—Figure 11—that illustrates the overall framework linking GNSS-based strain analysis with seismic hazard assessment and policy applications. The figure supports the discussion by visualizing the flow of data, analysis, and decision-making, and further addresses the need for diagrammatic explanation.
[Figure 11 Caption:] Conceptual framework for integrating GNSS-based strain analysis into seismic hazard assessment and policy planning. The process begins with Data Acquisition and Preprocessing, utilizing GNSS network observations alongside supplementary geological and seismological datasets. This is followed by Strain Analysis, where digital strain tensor solutions are derived to extract principal and maximum shear strains and infer regional stress orientations. In the Hazard Assessment phase, anomalies in strain patterns are identified and mapped to delineate zones of elevated seismic risk. Finally, outputs feed into Policy and Planning Applications, enabling the formulation of risk mitigation strategies and the development of probabilistic seismic hazard models and forecasts (PSHM). This framework highlights a multidisciplinary approach that enhances the reliability and applicability of strain-based assessments in tectonically active yet low-seismicity intraplate regions.
Reviewer Comment 7:
There are very few references in the discussion chapter.
Compare your findings with international literature and cite them like cite this reference (10.1016/j.marpetgeo.2023.106508) in line numb 987.
Response 7:
Thank you for your insightful suggestion regarding the inclusion of more international references in the Discussion chapter. We agree that incorporating relevant global literature enhances the scientific rigor and contextual relevance of our findings. Therefore, we have cited the recommended reference (DOI: [10.1016/j.marpetgeo.2023.106508]) in line 987(now 937) of the revised manuscript to strengthen the discussion on lithological boundary effects on strain localization.
The cited study provides a detailed geochemical and tectonostratigraphic analysis of the Paleocene Hangu Formation in northwestern Pakistan, emphasizing how lithological and tectonic contrasts at convergent plate boundaries influence crustal deformation processes. This aligns well with our interpretation that the lithological contrast between the Mesozoic crystalline rocks and the Cretaceous–Neogene sedimentary units in Triangle 16 introduces mechanical heterogeneity and promotes localized strain accumulation. The citation thus deepens the geodynamic context of our argument.
Accordingly, the reference has been added as Reference [75], and all subsequent references have been renumbered to maintain citation consistency throughout the manuscript.
Reviewer Comment 8:
This conclusion is massive, we dont need this kind of conclusion. shrink it to 150-200 words max."
Response 8:
We sincerely thank the reviewer for the valuable suggestion to condense the conclusion section. In response, we have substantially shortened the conclusion from 692 words to 296
words, while preserving the core findings and their implications. The revised version provides a concise summary of the study’s objectives, major results from the GNSS-based strain tensor analysis, and their significance for seismic hazard evaluation in southeastern Korea. This modification improves the clarity of the manuscript and ensures a better balance between sections, enhancing the overall readability and focus of the paper.
Author Response File: Author Response.pdf
Round 2
Reviewer 4 Report
Comments and Suggestions for AuthorsThere are two parts of my response. Lets start from my positive review:
The author have very quickly conducted the revisions. I can see a lot of improvements and efforts from author which i greatly appreciate.
But there are some small and some large problems still exists. Small problems like, the second line of abstract should be a problems tatement. and 3rd should be how author is going to solve this problem. In last lines of abstract, instead saying :important insights i: "valuable frame-26
work for identifying structurally sensitive segments prone to future seismic activity." , tell readers what exactly are they? what are their global importances??
Now lets move to major problem. the author made this error because of hurry. I strongly recommend take your time and solve the following problems:
Materials and methods section should be section 2. It consists of lot of results. so move all of it to result section, and rewrite this sections of 150 words to 300 words in which you just tell us the names of result analysis, techniques software used etc etc. no results should be included in this section (see example and cite at proper place 10.1016/j.marpetgeo.2024.106693). Fig 5 and related writings should moved to materials and methods section. then start 3. result section (where probably move results from Fundamentals of Elastic Theory in Lithospheric Mechanics to whatever comes in results). Then move to 4. Discussion section. add 1-2 more figures like fig 11 in other subsections of discussion as well. Finally conclusion is still too large, you need to trim it more, and only discuss your innovation points. You should not cite results and references in it.
Take your time, dont hurry and put some effort in this as well. Hope to see all these changes properly.
Comments on the Quality of English LanguageCan be improved
Author Response
Reviewer Comment 1:
But there are some small and some large problems still exists. Small problems like, the second line of abstract should be a problems tatement. and 3rd should be how author is going to solve this problem. In last lines of abstract, instead saying :important insights i: "valuable frame-26 work for identifying structurally sensitive segments prone to future seismic activity." , tell readers what exactly are they? what are their global importances??:
Response 1:
Thank you for your constructive feedback. We agree with your comment regarding the structure and specificity of the abstract. Therefore, we have thoroughly revised the abstract to ensure a clearer logical progression. Specifically, we now begin with a precise problem statement (the lack of high-resolution geodetic assessment along the Yangsan Fault), followed by a brief description of the approach used to address the problem (GNSS-based elastic strain tensor analysis).
In addition, we have removed vague phrases such as “important insights” and instead provided specific findings and implications, including identification of localized high-strain zones and the significance of stress orientation deviation at fault intersections. We also clarified the global importance of our approach in the context of intraplate seismic hazard assessment worldwide.
This revision can be found on Abstract of the revised manuscript.
The updated abstract reads as follows (marked in red in the revised manuscript):
This study addresses the lack of detailed geodetic assessments of crustal strain accumulation along the central Yangsan Fault in southeastern Korea… [Full updated abstract as in previous message]
Reviewer Comment 2:
Materials and methods section should be section 2. It consists of lot of results. so move all of it to result section, and rewrite this sections of 150 words to 300 words in which you just tell us the names of result analysis, techniques software used etc etc. no results should be included in this section (see example and cite at proper place 10.1016/j.marpetgeo.2024.106693).:
Response 2:
Thank you for your valuable comment. We fully agree with this suggestion and have restructured Section 2 (Materials and Methods) accordingly. Specifically, the velocity comparisons of five continuous GNSS stations and their related illustrations (Figures 2 and 3), previously in Section 2.3, have been moved to a newly created Section 3.1 (GNSS Velocity Pattern) under the Results. Additionally, interpretive statements previously included in the latter part of Section 2.1 were relocated to the Introduction and Discussion sections to enhance narrative clarity. Furthermore, the analytical workflow diagram formerly presented in Section 2.5 has been relocated to Section 2.6, concluding the methods section in a more logical manner.
We also thoroughly reviewed the article you suggested (doi: [10.1016/j.marpetgeo.2024.106693]). This reference provides a detailed analysis of facies changes, depositional evolution, and the development of the Ceno-Tethys Ocean during the Paleocene–Eocene transition, emphasizing the importance of high-resolution geological assessments in structurally transitional domains. We found its approach and thematic relevance to align well with our study's objective of identifying and characterizing localized strain accumulation along fault interaction zones. Accordingly, we cited this work in the latter part of the third paragraph of the Introduction as reference [9], to reinforce the background motivation and scientific significance of our strain-based approach.
Reviewer Comment 3:
Fig 5 and related writings should moved to materials and methods section. then start 3. result section (where probably move results from Fundamentals of Elastic Theory in Lithospheric Mechanics to whatever comes in results).
Response 3:
Thank you for your thoughtful suggestion. In response, we have made the following revisions to improve the manuscript structure and consistency.
Figure 5 and its associated explanatory text have been moved to the end of Section 2 (Materials and Methods), where they more appropriately represent the methodological workflow rather than results. Additionally, the latter part of Section 2.1, which previously included literature review and theoretical background, has been separated:
- The literature review content has been relocated to the Introduction to better frame the research context.
- The analytical results and interpretation based on elastic theory have been moved to a newly created Section 4.1, where they are now discussed within the broader context of results and interpretation.
Additionally, we carefully reviewed and updated the reference order and all in-text citations to reflect these structural changes, ensuring consistency and accuracy throughout the manuscript.
Reviewer Comment 4:
Then move to 4. Discussion section. add 1-2 more figures like fig 11 in other subsections of discussion as well.
Materials chapter is too long, many things belongs to results chapter. Move the results into results chapter. 3.1 Data collection.
Response 4:
Thank you for the helpful suggestion. To improve clarity and facilitate reader understanding, we have added a new figure (Figure 11) corresponding to the content discussed in Section 4.1, following a format similar to the original Figure 11. This new figure provides a clear visual representation of the spatial and interpretative aspects introduced in that subsection.
Accordingly, the original Figure 11 has been renumbered as Figure 12 to maintain consistency in the sequence of figures throughout the manuscript. These adjustments enhance the visual support for key discussion points and help communicate the results more effectively to readers.
The updates can be found in the revised manuscript under Section 4.1 and Figures 11–12.
Reviewer Comment 5:
Finally conclusion is still too large, you need to trim it more, and only discuss your innovation points. You should not cite results and references in it.
Response 5:
Thank you for your valuable feedback. In response, we have carefully revised the conclusion to ensure conciseness and to focus exclusively on the core innovations of the study. All references to specific results and external literature have been removed, in accordance with the journal’s standard for conclusion sections.
The revised conclusion now briefly summarizes the key methodological contributions, such as the integration of elastic strain tensor modeling with dense GNSS observations, and emphasizes the framework’s potential for identifying structurally sensitive fault segments. Policy or infrastructure-related implications have been omitted to maintain scientific focus. The updated conclusion is now more concise, clearly delineating the novelty and broader applicability of the proposed approach without restating detailed findings or including citations.
Final Remark:
We sincerely appreciate the reviewer’s insightful and constructive comments, which greatly contributed to improving the clarity, structure, and scientific quality of our manuscript. All suggestions were carefully considered and incorporated into the revised version. We hope that the updated manuscript now meets the expectations and standards of the journal.
Thank you again for your time, effort, and valuable guidance throughout the review process.
Author Response File: Author Response.pdf
