Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

58 PATA Days 2024 Unit 11 is composed of colluvial wedge materials sourced from the metamorphic fragments. For these reasons, the base of Unit 11 corresponds to the event horizon for a single earthquake. Although a fault strand cross-cuts the entire 2.8 m thickness of Unit 20, it is difficult to accept a true displacement when considering the intraplate tectonics of Korea. Moreover, a sub- vertical fault geometry is difficult to develop when reverse- dominant faulting occurs under near-surface confining pressure. Unit 10 exhibits poorly sorted boulders, which descend intensively on the boundaries of basement rocks. We infer that Unit 10 is composed of colluvial material derived from an unstable mountain slope. The kinematics of basement faults and fault core are indicative of a sinistral sense of shear (Fig. 4b), which was recorded during the Cretaceous periods. However, our structural mapping of the basement and microstructural analysis support that the latest faulting reactivates during Quaternary (Fig. 4c). Several OSL samples were collected from Unit 20, Unit 11, Unit 10 to estimate the timing of the latest faulting along the YF. Two samples collected from the base of Unit 20 suggests the age is around > 44 ± 3 ka and > 53 ± 8 ka, respectively. However, the middle of Unit 20 is 3.8 ± 0.3 ka. This much younger age may be due to the several reasons such as intrusion of younger grains into older deposits or the or contamination by roof-spall (i.e. decomposition of unbleached weathered rock into well- bleached sediments) (Roberts et al., 1998; Jacobs et al., 2011a). Therefore we have discarded this age from our paleoseismic event interpretations. Considering the oldest age in Unit 20, which is the only faulted unit exposed in the trench section, it can be inferred that the faulting event might have occurred after 53 ± 8 ka. The faulted unit, Unit 20 is covered by Unit-11 which will be helpful to bracket the event timing, assuming Unit-11 was deposited soon after the earthquake. But due to poor quartz and IRSL signals, the age of Unit-11 could not have measured. Therefore we have considered the age of the Unit-10 as upper limit of the timing of the faulting (11.0 ± 0.4 ka). D I S C U S S I O N & C O N C L U S I O N S Instrumental and historical earthquake records suggest the possibility of moderate earthquakes in the central part of the Korean Peninsula. To address the uncertainty of historical earthquake records and the short duration of instrumental