the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Aug 2024
Quaternary surface ruptures of the inherited mature Yangsan fault: implications for intraplate earthquakes in Southeastern Korea
Abstract. Earthquake prediction in intraplate regions, such as South Korea, is challenging due to the complexity of fault zones. This study employed diverse methods and data sources to detect Quaternary surface rupturing along the Yangsan Fault to improve seismic hazard assessment. Paleoseismic data were analyzed to reveal insights into seismic activity, displacement, and structural patterns. Observations from five trench sites indicate at least three faulting events during the Quaternary, with the most recent surface rupturing occurring approximately 3,000 years ago. These events resulted in a cumulative displacement of 3.1–94.0 m and maximum estimated magnitude of 6.7–7.2. The average slip rate of 0.14 mm/yr suggests a quasi-periodic model with possible recurrence intervals exceeding 10,000 years. The structural patterns imply the reactivation of a pre-existing fault core with top-to-the west geometry, causing a dextral strike-slip with a minor reverse component. This study underscores the continuous faulting along the inherited mature fault, the Yangsan Fault, since at least the Early Pleistocene, contributing valuable insights for seismic hazard assessment in the region and offering a broader understanding of intraplate earthquake dynamics for earthquake prediction.
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RC1: 'Comment on egusphere-2024-1696', Anonymous Referee #1, 11 Aug 2024
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This manuscript addresses the paleoseismic and structural features of the Byeokgye section of the Yangsan Fault, SE Korea mainly obtained from the trench surveys. The Korean peninsula is a representative slow-deforming region, but recently, high-resolution seismological and paleoseismological researches have been actively conducted. In particular, the Yangsan Fault holds significant research value for its long-term geological evolution and as a seismogenic fault during the current stress field (Quaternary period).
I read whole manuscript thoroughly. The quality of the data presented throughout the manuscript is quite excellent. The academic logic and structure of the paper are also reasonably sound. Despite these strengths, the manuscript needs significant polishing in terms of English expression throughout, and there are many parts that are unnecessarily repetitive and overly lengthy. Consistent and systematic descriptions are also needed across the each result section. I would propose major and minor comments for each individual section of the manuscript. The detailed comments annotated in the PDF file should be also considered. Since I am not a native English speaker, I did not leave extensive comments on the English expressions. I would suggest the authors to ask for help to a native English-speaker or to someone with a better level of English.
The major comments for each section are as follows.
Major comments on “3.4. Displacement and earthquake magnitude estimation”
I know that there are recent studies on the slip rates of this area published in the GSA Bulletin and Geomorphology. Please present their methodologies and also describe the validity of your method for revealing displacement of a fault.
Using this methodology to determine horizontal displacement (or total displacement) has several weaknesses. Describe these weaknesses and clearly describe why, despite them, the methodology is still meaningful.
Major comments on “4.1. Characteristics of Quaternary faulting in the trenches”
1) The descriptions of faults for each trench need to be rewritten in a more concise and systematic manner. The descriptions are unfriendly.
2) The past tense is being used where it would be more appropriate to use the present tense. Please check carefully.
3) Clearly distinguish and describe the fault core, fault damage zone, and fault rocks of the bedrock faults (a few cm to m scales), and the fault splays of the Quaternary faults (<2 cm in thickness). For example, I suggest using the term "fault" for bedrock faults and "splay" or "rupture" or "Quaternary slip zone" for Quaternary faults. This part of the manuscript is quite confusing throughout.
4) At some trench sites, the authors divide the mature core into core 1 and core 2. Is this particularly meaningful? Focus on clearly distinguishing between the old fault core and Quaternary fault splay in your descriptions. And then, describe the relationship between the Quaternary splay and Quaternary layers.
5) Describe in the following order for age results and interpretations: presentation of OSL and C14 results, interpretation of event horizons using these results, presentation of ESR results, and interpretation of older events using these results.
6) Present the age dating results in a table, and in the main text, concisely and clearly describe only those results necessary for actual interpretation.
7) If the sample locations and dating results are included in each figure, it will make it easier for readers to understand.
8) Please make that the colors clearly differentiate between the bedrock parts (including old fault rocks) and the Quaternary layers in each figure.
9) When describe a range of ages, list the older age first.
Major comments on “4.3. Paleo-stress reconstruction”
In the text, Quaternary faults and bedrock faults are described separately, but the figures only present the results for the Quaternary faults. It would be better to state in the text that the focus is on the striations indicating Quaternary faults, without considering the bedrock faults.
Major comments on “4.4. Displacement and earthquake magnitude estimation”
Please consider the comments in Section 3.4
Major comments on “5.1.2. Quaternary slip rate and recurrence interval”
In this section, describe the slip rate compactly for each trench section that can present long-term data, and then for the entire section as well. As long as I know, the slip rate is generally not discussed using MRE. It is reasonable to discuss the slip rate only when there are two or more events with displacements. For example, if an earthquake with the surface displacement occurred 10 years ago, it is not possible to calculate the slip rate using the displacement associated with the earthquake and the 10-year time period. The slip rate is generally considered a long-term concept.
Major comments on “5.2. Structural patterns of Quaternary reactivation of the Yangsan Fault”
In this discussion section, please focus on how the western boundary of the preexisting mature fault core has been repeatedly reactivated during the Quaternary. I propose that first, describe the characteristics of the bedrock faults, and the features of the Quaternary faults at the western boundary, and then explain why ruptures are repeated propagated along the western boundary of the mature fault core.
Major comments on “5.3. MRE and activity for each segment of the Yangsan Fault”
This discussion addresses the MRE of the several active segments of the Yangsan Fault, specifically focusing on the Byeokgye-Bangok-Yugye segment. I suggest that the authors present your research findings first (Byeokgye-Bangok-Yugye), then follow with the results from the other sections. When describing, please enhance readability by making clear comparisons.
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