PRELIMINARY RESULT OF TRENCH STUDY IN CENTRAL-SOUTH YANGSAN FAULT IN THE KOREAN PENINSULA

Yangsan Fault extends for over 170 km and has been active since the late Cretaceous period. It has been linked to the formation of the East Sea (Sea of Japan) and considered a major tectonic fault in the Korean Peninsula. The fault has a holds a number of historical earthquake records over a period 2000 years, most recently the Mw 5.8 Earthquake which occurred in its vicinity. The fault has been studied by various disciplines, for example, structural geology to determine the characteristics of the fault, geophysical exploration to determine extension of the fault, mineralogy for fault gouge and so on. However, the last fault movement remains unknown. Trench studies on the Yangsan Fault have been performed in the central-south of the Yangsan Fault to obtain its last movement. The trench survey revealed that the fault had been reactivated at least twice during the Holocene period, at approximately 2ka and 4ka. Before the Holocene, another fault movement occurred in approximately 50ka, with a strike slip motion that created a meter-wide fault damage zone. In addition, from LIDAR and aerial photograph observations, we determined that a higher terrace estimated to be younger than 320ka had moved by 1.5 km with left lateral strike slip motion. We now surmise that the Yangsan Fault is being continuously reactivated for more than 60 million years, and could potentially generate serious geohazards in the near future. Furthermore, even if the fault is inside an intraplate, we infer that it has continuously been reactivated from the late Cretaceous to the present by subduction of the Pacific Plate and the Philippine Sea Plate.