Cumulative offset analysis of the Central-Southern Yangsan Fault based on topography of Quaternary fluvial terrace .

In general, earthquakes larger than magnitude 6 involve surface ruptures. Therefore, geomorphic and stratigraphic offsets recorded in the surface provide clues to interpret history of moderate to major paleo-earthquakes. In this study, we carried out topographical analysis of the Quaternary fluvial terraces along the Yangsan Fault, one of the major geological structures in Korea. Our investigation focused on 4 sites: Yongjang-ri, Gyodong-ri, Sangcheon-ri and Chosan-ri, where river system flows across the central-southern part of the Yangsan Fault. We used aerial photographs (1968) and airborne LiDAR-based DEM data (2017) for precise topographical analysis. For each area, terrace tread and riser were used as geomorphic offset indicators. The horizontal offsets were statistically measured using the root-mean-square-error method for the sections where the straightness of the geomorphic offset indicator was secured. The vertical offsets were measured by comparing topographic progiles of fluvial terraces on both sides of the fault. As a result, various horizontal offsets over at least about 10 m (Yongjang-ri site: 19.09 ± 2.58 m, 21.92 ± 2.69 m, Gyodong-ri site: 195.43 ± 23.54 m, Sangcheon-ri site: 37.53 ± 6.56 m, Chosan-ri site: 9.68 ± 2.73 m, 34.44 ± 4.40 m) were identified. All these offsets indicate a right-lateral sense of slip. On the other hand, the vertical offsets were less than about 10% of the horizontal offsets at all sites. This implies that dextral deformation were dominated during the surface ruptures associated with paleo-earthquakes along the central-southern Yangsan Fault. Our results show that it is possible to acquire information on paleo-earthquakes, such as fault trace distribution, slip sense, offset, through accurate topographical analysis. These results can be used to estimate long-term slip-rate along with depositional age of fluvial sediments.