Evidence for large Holocene earthquakes along the Yangsan fault in the SE Korean Peninsula revealed in three-dimensional paleoseismic trenches

The Yangsan fault is the most prominent NNE-SSW−striking active right-lateral strike-slip fault crossing the Korean Peninsula, with a continuous trace of ∼200 km. It can likely generate large earthquakes; however, the paleoseismic information on slip per event, slip rate, and timing of past ruptures along this fault remains sparse. To explore these parameters for the Yangsan fault, we excavated trenches across the central segment of the fault, which showed evidence for at least five surface-rupturing earthquakes preserved in Quaternary fluvial deposits. The timing of these earthquakes is discussed based on luminescence and radiocarbon ages. A close examination of three-dimensional trench exposures revealed that the most recent event(s) occurred during or slightly after the third century CE (one-event interpretation) or sixth to eighth century CE (two-event interpretation), and it was associated with 4.5 m to 5.3 m of lateral displacement of a paleochannel. The observed lateral displacement indicates that large earthquakes with a magnitude of around Mw 7 have taken place in the recent past, which is the first-ever direct evidence of large-magnitude earthquakes along the Yangsan fault. The penultimate event occurred after 17 ± 1 ka, whereas an earlier late Quaternary event occurred in the late Pleistocene, suggesting a recurrence interval in the range of 10,000 yr, and a consequent slip rate on the order of 0.5 mm/yr. The oldest observed ruptures are preserved below an erosional unconformity that probably dates back to the last interglacial period, based on infrared stimulated luminescence ages. An unknown number of ruptures may have occurred between the unconformity and subsequent sedimentation during the latest Pleistocene to Holocene period. Historical earthquake records indicate clustered behavior of moderate and large earthquakes along the Yangsan fault. Past faulting events and implied recurrence intervals constrain the long-term faulting behavior along the Yangsan fault and will contribute to a better seismic hazard assessment in the southeastern part of the Korean Peninsula.