A POSSIBLE SEGMENTATION OF QUATERNARY EUPCHEON FAULT AROUND NPP, SE KOREA, THROUGH MULTIDISCIPLINARY APPROACHES

A Quaternary Eupcheon fault (N30E, 50-60SE) crosses an apartment complex, which is located on the third marine terrace sediments (thickness: 10 m), around NPP, SE Korea. The fault (EFT) is 1.5 km long and shows 4 m vertical separation in average. To know the characteristics of the EFT, multidisciplinary approaches such as fracture mapping, terrace mapping, geophysical exploration, boring, trench and age dating have been done for lack of outcrop. From the extracted fault gouge in the marine sediments, it revealed that the EFT movement has occurred twice during Quaternary. The first movement was normal dip slip and ESR data indicated ca. 1,000 ka. The second one is reverse to the-top-up-to-the-west and cuts the third terrace (MIS 5e) and the upper sediments. Within a twelve-meter's horizontal distance at the northern tip of the EFT, three branch faults occurred in succession as convex shape. Each fault segment reduces gradationally the amount of dip separation from deep level to the shallow and forms a positive flower structure. Various movement vectors were induced along the curved fault plane. The shallowest segment terminates below top soil horizon and shows a local colluvial wedge, which indicates reverse sense after the normal. Other two segments die out within the third marine terrace sediments and are parallel to bedding at the top part of reverse fault. A mosaicked sketch of four trench sections indicates that the length of the northern segments of the EFT is ca. 20 m. In the northern middle part of the EFT and the southern part, each of them seems to have a segment of 200 m in average. It revealed that the EFT does not extend as a straight fault according to geophysical grid section lines and seven trenches among apartment buildings. Alignment of the EFT segments shows a right step pattern in the northern half of the EFT and a left step in the southern half. It seems that lots of shear faulting of NW trend cut the EFT and that shear faulting is younger than the EFT. Therefore, the EFT might not affect the safety of the NPP as a capable fault, but shear faults of NW trend might be a new candidate of capable fault to the NPP.