New paleoseismological results in the central part of the Ulsan-Yangsan Fault system, SE Korea

Korean Peninsula is relatively stable in terms of large damaging earthquake compared with the neighboring countries such as Japan, Taiwan and China. The recent damaging M_L 5.8 Gyeongju earthquake (12^th Sept. 2016) and M_W 5.4 Pohang earthquake (15^th Nov. 2017) in the SE Korean raised the concern about the seismic hazard in Korean. Earthquakes prompted the reassessment of seismic hazard in the southeastern part of the Korean Peninsula bounded by the Yangsan-Ulsan Fault system. The historic records of past seismic events along the fault system are poorly documented and only a few site specific paleoseismological studies have been carried out to recognize causative active faults and the paleoearthquakes. The 2016 Gyeongju earthquake was the highest ever instrumentally recorded earthquake in the Korean Peninsula and caused moderate structural damages around the city.

Despite of these long historical records of earthquake damage, only a few paleoseismic studies have been reported with proper trenches and age dating around the city. Thus, the evidence of recurring paleoearthquakes from this study is significantly important for recognizing the earthquake hazard. In this present study with the help of 1954 aerial photo, recent LiDAR data and field observation, we have identified two N-S striking parallel running active faults displacing Quaternary alluvial fans and fluvial terraces for a length of ~6 km. OSL dating was employed to constrain the chronology of the events. Our detailed paleoseismological investigations have provided unambiguous evidences of at least three large earthquake events along this newly identified fault, which is named as Cheongun-dong fault following the locality name.

The oldest earthquake occurred between 100 to 80 ka and another surface rupturing earthquake occurred between 37 to 11 ka.The most recent earthquake occurred along the fault is between 4.2 to 4 ka. The present study firstly reports multiple large paleoearthquakes along the southern part of the Gyeongju city during the Late Pleistocene to Holocene. The repeated reactivation of the Cheongun-dong fault substantiates high seismic potential along this fault and calls for more extensive paleoseismic study along the southeastern part of the Korean Peninsula.