Episodes 2023; 46(4): 521-535
Published online December 1, 2023
Copyright © International Union of Geological Sciences.
Hoil Lee1, Jin-Hyuck Choi1,2, Tae-Ho Lee1, Dong-Eun Kim1, Youngbeom Cheon1, Keun-Soo Lee3, Yire Choi1*
1 Active Tectonics Research Center, Geologic Hazards Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Korea
2 Geological Science, University of Science & Technology, Daejeon 34113, Korea
3 Water Infrastructure Safety Research Center, K-water Research Institute, Daejeon 34045, Korea
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Detecting faults and their paleoseismic parameters is important for understanding earthquake-fault behaviors and assessing earthquake hazards. We conducted a multidisciplinary investigation of the Yangsan Fault around Gyeongju, an old capital city in South Korea, one of the challenging regions for paleoseismic surveys due to the protection of cultural properties. Based on topographic analysis and field observations, we identified a 10-km-long fault trace in a north-south direction. We then investigated alluvial fans covering the fault to evaluate geologic evidence of paleoearthquakes based on airborne LiDAR-image, drilling core, and cosmogenic exposure age- and OSL age-dating. Our results showed that the alluvial fan at a minimum age of 63 ka was deformed by transpressional earthquakes during the Late Pleistocene. A total of ~2.5 m of vertical offsets were estimated by geomorphic restoration and stratigraphic correlations and were mostly accommodated on a western subsidiary fault. The distribution of exposure ages of the boulders on the deformed alluvial fans implied that horizontal offsets were accommodated by the main fault. These less-destructive approaches can play an essential role in cultural heritage protection zone and highly urbanized cities, where excavation is impossible.