Characteristics of the Quaternary faulting detected along the Yangsan Fault in Yugye- and Jungsan-ri, northern Pohang City

In this study, we conducted a geomorphologic analysis, fieldwork, geophysical survey, and trench survey to trace the Quaternary faulting along the northern Yangsan fault. We also complement the study with the Electron Spin Resonance and luminescence datings of the fault rocks and Quaternary sediments, respectively, to constrain the timing of the Quaternary faulting. As a result of geomorphological analysis, two lineaments with lengths 6.8 km and 0.35 km are recognized by the linear distribution pattern of fault saddles and deflected streams. The amount of deflection of deflected streams is 2.05-2.82 km, 1.72 and 0.74 km, and 0.43 km in Seojeongcheon, Cheonghacheon, and Gwangcheon stream, respectively, which decreases gradually toward the north. Faults cutting the unconsolidated sediments were found at an outcrop and trench site, and both the sites have the following similar features: slickenlines on the fault surface in contact with the Quaternary sediments, indicating a dextral strike-slip including a reverse component, and a mixed zone of Quaternary sediment and fault gouge between the fault core and the Quaternary sediment. Their cross-cutting relationship indicates at least two faulting events during the Quaternary. Luminescence ages obtained from the sediment layers indicate that the latest faulting event occurred between 154 +/- 13 ka and 0.9 +/- 0.1 ka. The slip rate, calculated by dividing the net displacement by the age gap, is 0.007 to 0.008 mm/yr. Also, the conversion of the vertical displacement of the Yugye site reported in the previous study into the net displacement gives rise to an estimated slip rate of about 0.08 mm/yr. Combining the results, the slip rate of the northern Yangsan fault in the study area is estimated to be in the range of 0.007 to 0.08 mm/yr. The moment magnitude (Mw) obtained using the net displacement in the empirical equation is about 6.7, and the magnitude of about 6.0 is derived using the lineament length as the surface rupture length.