Stratigraphic and structural evidence of transpressional Quaternary fault reactivation along the SE Korean Peninsula

Comprehending the nature, extent, and distribution of seismogenic structures, along with their seismogenic potential to induce destructive earthquakes in low seismic strain regions, holds significant socio-economic significance. Analyzing historical and paleoearthquakes becomes essential to understanding the influencing factors controlling the origin of earthquakes in intraplate regions, such as the Korean Peninsula. Based on current knowledge, the seismic pattern within the intraplate region, particularly along the Korean Peninsula, exhibits greater heterogeneity. This observation is drawn from historical seismic records, instrumentally detected seismic patterns, and paleoseismological analyses. Comprehensive paleoseismological studies play a crucial role in providing new evidence for fault activity, and hence they are an important contribution to the understanding of the seismic and structural evolution of faults. Existing paleoseismic investigations have predominantly focused on southeastern Korea, primarily along the Yangsan or Ulsan faults, with less attention given to other faults in the southeastern part of the Korean Peninsula. In the ongoing Korean Active Fault Mapping Project, this study specifically identifies a fault trace approximately 5.5-7 km long along the Ulsan-Dongnae Fault system named as Seokgye fault, named after the locality. Detailed paleoseismic investigations were carried out along the Seokgye fault. The fault presents a clear example of fault reactivation under the present compressional tectonic setting with a paleoearthquake during the late Pleistocene (similar to 121 +/- 6 ka). Based on the length of the fault traced and observed displacement in the trench, Seokgye fault is capable of generating an earthquake of magnitude similar to the 2016 Gyeongju earthquake (Mw 5.8). This research enhances our understanding of the ongoing tectonic processes and Quaternary activities of fault systems in the southeastern Korean peninsula. Additionally, it contributes valuable insights for local-scale seismic hazard analysis in the area.