PALEOSEISMOLOGICAL APPROACH ALONG THE YANGSAN FAULT IN THE SOUTHEASTERN PART OF THE KOREAN PENINSULA

NNE-Yangsan fault (YFT) composed of four segments is the 1st grade-neotectonic fault in the Korean peninsula and extends to 200 km in land. Several fault events included intensive Cenozoic dextral-strike-slip movement have been reported.

Right-lateral offset of stream and ridge is observed at Yugye-ri~Bogyeong-sa areas along the northern reach of the YFT. The YFT dips to the east (45SE) with a reverse sense and cuts the lower-mid fluvial terraces. The latest fault event of Yuge-ri would have occurred during 2,400- 2,000 yrs BP (14C). Vertical slip rate of the fault yields 0.04-0.05 mm/yr.

The southern part of the YFT keeps a vertical dip of strike-slip movement. The YFT in Eonyang~Tongdo-sa areas reveals cumulative vertical offset of the mid-higher river terrace surfaces indicating the reactivated events for the late Quaternary. Vertical displacement after the higher terrace sediments (250-500 ka) is ca.10 meters. In those areas, on the basis of fieldwork, the YFT shows a slip rate of 0.02-0.03 mm/yr and the right lateral component of 0.05-0.1 mm/yr (Okada, 1995). Trench excavations were carried out along the southern reach of the YFT. It reveals that massive gravel layer of the higher terrace and cataclastic zone of bed rock are bounded by fault gouge of 5-30 cm thick and shows a consistent displacement. Long axes of slender gravels adjacent to the gouge show a sub-horizontal and sub-parallel to fault striation. It indicates that the gravel layer of the higher terrace was deformed by strike-slip faulting. However, systematic and detailed mapping of river terraces along and across the YFT is needed to constraint the latest fault event.