Multi-archive record of late Quaternary paleoseismicity along the surface projection of the 2017 Pohang earthquake seismogenic fault, SE Korea

The 2017 Pohang earthquake (M_L 5.4) ranks as the second-largest instrumental earthquake in the Korean Peninsula and the country’s most destructive seismic event. The earthquake history of the Pohang area prior to the 2017 event is unknown due to the absence of instrumental seismic activity and the lack of mapped Quaternary faults near the 2017 epicenter. The aim of the present study is to identify evidence for previous earthquake ruptures along the surface projection of the seismogenic fault and interpret their paleoseismic implications. The study involved comprehensive paleoseismological investigation, including geomorphic analysis, field-work, drillhole surveys, trench excavation, and numerical age dating. Geomorphic analysis and drillhole surveys revealed two lineaments presumed to have originated from Quaternary faulting: NNE-SSW-striking Fault-1 and NE-SW to NNE-SSW-striking Fault-2. At the excavation site of Fault-1, which is regarded as the seismogenic fault of the 2017 Pohang earthquake, stratigraphic features and numerical ages show that the penultimate event occurred between 11 ± 1 and 2.6 ± 0.1 ka and that the most recent event took place after 0.17 ± 0.01 ka. Combined results from two outcrops of Fault-2 give occurrence ages for the penultimate and most recent events of ca. 200 ka and between 148 ± 7 ka and the analytical limit of ¹⁴C dating (> 43,500 BP), respectively. Our findings reveal that at least three seismic events causing surface ruptures have occurred in the Pohang area during the late Quaternary before the 2017 Pohang earthquake.