PALEOSEISMOLOGICAL STUDY IN GEYONGJU CITY, KOREA

Gyeongju City, situated in the southeastern part of the Korean Peninsula (KP), and which was the capital city during the Silla Dynasty from 57 BC to 935 AD, and according to the large number of historical earthquake records, is situated in the most dangerous, seismically active, district of the KP. Many active faults have been located in the southeastern part of the KP. Most of the active fault structures characteristically swarm around the Yangsan and Ulsan faults. However, the age of the last faulting episode for the majority of fractures is still unclear because of the absence of effective materials for age dating.

We recently reexamined an active fault site at Jinhyun, a component of the Ulsan Fault swarm,to clarify the time of thelast event and the average slip rate. The host rock is Tertiary granite overlain by a surficial sediment cover of more than 20 m in thickness, which is divided into two sequences. The lower sequence is old Quaternary sediment (OQS) and the other is young Quaternary sediment (YQS), a distinction based upon their respective composition and geometry. The OQS consisted of subrounded to rounded pebble or cobbles with a well-developed stratification that indicates this deposit to be fluvial. The OQS is overlain by the YQS debris flow deposit consisting of mega-sized granite boulders. Theoptically stimulated luminescence (OSL) age of the OQS is 55 ka and the carbon 14 (C-14) age of the YQS is 43 ka. The reverse fault strikes N5-10°E / 70° SE and cuts through the OQS. It is considered unlikely that it penetrates the YQS. The hanging wall of the fault consists of basement Tertiary granite and the overlying debris flow of YQS, whereas the footwall is within the 20 m of superimposed OQS and YQS. Estimation of the throw of the fault was based on the observed displacement between the hanging and footwalls and is at least 10m. The average vertical slip rate of the fault is 0.02 mm/yr, which is one of the highest rates of displacement for active faults in the KP. We concluded that the approximate 10 m of offset indicates an accumulation of fault displacement following the deposition of the fluvial sediments at 55 ka, and is associated with a main fault along the toe of the mountain escarpment. The main faulting led to the deposition of the thick OQSfollowing erosion on the hanging wall after the faulting. Further intermittent movement on the faultcontinued, and induced the YQS.

We concluded that the Jinhyun Fault located in Gyeongju City might not be directly related to the historical earthquakes during the Silla to Chosun Dynasty (57 BC1910 AD), although we observed a terrace surface adjacent to a small bench as apossible fault trace. However, the Jinhyun Fault has a notably large slip rate.