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

I N T R O D U C T I O N The Yangsan Fault (YF), a major NNE‒SSW oriented long-lived intraplate strike-slip fault developed in the south- eastern part of the Korean Peninsula, has undergone multiple deformations since the Cretaceous (Cheon et al., 2017; 2020). The fault predominantly exhibited dextral strike-slip motion due to the ENE‒WSW oriented compressive tectonic stress since the Quaternary. According to recent studies, paleo-surface rupturing earthquakes occurred along the entire section of the YF during the Quaternary (e.g., Choi et al., 2021; Ko et al., 2022). In Korea, there are no historical seismic records or instrumental records of large earthquakes accompanied by surface ruptures, and the geomorphic records of paleoearthquakes are extremely limited due to modifications in morphology caused by climate, human activities, and slow deformation. Consequently, studies on horizontal slip rates that consider the geomorphology have not been sufficiently conducted to understand the YF. This study aims to trace the paleo surface rupture trace along the YF, specifically the studied section, and estimate their offset. Also, paleoseismologic trencheswereexcavatedtodeterminepaleoearthquake timings and events. It involves conducting precise geomorphic analyses, field investigations, geophysical explorations, drilling surveys, and trench surveys, all as part of a multidisciplinary approach. M E T H O D For the geomorphic analysis in the study area, vintage aerial photographs taken in 1977 were utilized, along with a digital surface model (DSM) derived from these photographs and high- resolution airborne LiDAR data (0.5 m/pixel). Based on elevation and slope, characteristics of the alluvial fan surface were classified. Geomorphological features were used to estimate traces of paleoearthquake surface ruptures. To identify these traces, five lines of electrical resistivity tomography (ERT) surveys and two lines of seismic refraction surveys were conducted perpendicular to the traces. The ERT surveys were performed using the dipole-dipole array method, with electrode spacings of 2 m and 4 m, and results were combined for inversion. Seismic refraction surveys utilized a 2 m geophone spacing and were designed to explore to depths of 25 m. Based on the geomorphic and geophysical surveys, three drilling surveys were executed to verify the traced paleo surface rupture. Each borehole was drilled to a depth of approximately 25-30 m and until bedrock was encountered.