The Eupcheon Fault, one of more than 20 identified Quaternary faults recently reported in the area around the Yangsan and Ulsan faults, SE Korean Peninsula, was discovered during the construction of a primary school in an area close to a nuclear power plant. We analysed a trench excavated across the Eupcheon Fault with the aim of understanding its movement history and kinematics. The Eupcheon Fault is a reverse fault (strike/dip: N20°E/40°SE) that records 6–7 m of displacement (3–4 m vertical separation). The fault consists of a series of branch faults, associated fault-related folds, and rotated pebbles. A previous study reported that the fault originally recorded normal slip but was reactivated during the Quaternary as a reverse fault. We identified four or five Quaternary faulting events upon the fault based on an interpretation of the trench logs including an analysis of colluvial wedges, and measurements of displacement–distance (dx) relationships along the fault. We used a quantitative analysis of dx relationships, commonly used for consolidated sedimentary rocks, to interpret the deformation history of the fault. From this we estimated the amount of slip in the range of 0.7–1.8 m for each of the five identified faulting events and the earthquake magnitude in the range of Mw 5.4–7.4. We propose two ideal dx profiles for syndepositional and post-depositional faulting, and a tectonic evolution model for the area around this fault. This approach may be applicable to analyses of earthquake hazards and studies of fault evolution elsewhere.

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