Paleoseismic records around epicenter of the 2017 Pohang earthquake, SE Korea

The 2017 Pohang earthquake (M_L 5.4) ranked as the second-largest instrumental earthquake in Korea and the most destructive seismic event. Before this event, the absence of documented instrumental seismic activity and no mapped Quaternary faults near the epicenter raised questions about the paleoseismic history of the region. This study aims to gather and trace evidences of the paleoseismic ruptures along the surface projection of seismogenic fault, reported by previous study, and interpret their implications. To achieve it, we conducted comprehensive paleoseismological investigations, including geomorphic analysis, fieldwork, drilling survey, trench excavation and numerical age dating. Through geomorphic analysis and drilling survey, we identified two lineaments: NNE–SSW-striking Fault-1 and NE–SW to NNE–SSW-striking Fault-2. In the excavation site of fault-1, stratigraphic features and numerical ages indicate that the PE event occurred between 11±1 ka and 2.6±0.1 ka, and then the MRE event activated after 0.17±0.01 ka.  On the other hand, the combined results of two outcrops of Fault-2 show that the MRE and PE of Fault-2 could be constrained to have occurred between 148±7 ka and ca. 40 ka and around 200 ka, respectively. Our findings present that even before the 2017 Pohang earthquake, seismic events causing surface ruptures of moderate to large magnitude have occurred at least three times in this area during the late Quaternary.

This work was supported by a grant (2022-MOIS62-001) of National Disaster Risk Analysis and Management Technology in Earthquake funded by Ministry of Interior and Safety (MOIS, South Korea).