ACTIVE FAULTS IN GAEGOK-RI, GYEONGJU, KOREA

Several active faults are found in the central-eastern part of the Ulsan Fault Zone near Gaegok-ri, Gyeongju, Korea. The faults in Gaegok-ri are developed around the contact between Early Tertiary granite and Quaternary gravel deposit which overlies unconformably the granite. They are called from southwest to northeast as Gaegok 1 fault, Gaegok 2 fault, Gaegok 3 fault, Gaegok 4 fault, Gaegok 5 fault, and Gaegok 6 fault. The Gaegok 1 fault strikes N08°W, dips 78° to the east, and two striations are developed; nearly horizontal one with sinistral shear sense and nearly dip-slip one with eastern side up. In the Gaegok 2 fault, strike of the fault plane and fault zone is N02°W, dipping 82° to the west. In the Gaegok 3 fault, strike of the fault plane is N30°E, dipping 65°to the southeast. The fault plane has two different striations, nearly horizontal one related with dextral strike-slip faulting and the other one showing slip along dip direction related with reverse faulting. In the Gaegok 4 fault, the strike of the fault is N30°W-N45°W with dips of 38°-48°to the northeast. In the main Quaternary fault plane, the orientation of striation is 17°, 142°. This fault has a 30-50 cm wide cataclastic shear zone mixed with Quaternary sediments and old fault breccia of granite, indicating a sinistral faulting with some reverse component. There is another striation (08°, 070°) with sinistral strike-slip sense, developed on the subsidiary plane which cuts the main Quaternary fault plane. In the Gaegok 5 fault, the strike of the fault is N25°E-N40°E, dipping 55°-60° to the southeast. The orientation of striation is 55°, 106°, showing reverse-slip sense. The Gaegok 6 fault strikes N02°-22°E, dips 45°-80° to the west. This fault has a 30-50 cm wide cataclastic shear zone with gouge zone, mixed with Quaternary sediments and breccia of granite by old faulting. In the main Quaternary fault plane, the orientation of striation is 17°, 356°, indicating a dextral faulting with some normal component. There is another striation (78°, 278° and 43°, 270°) with reverse-slip sense, developed on the subsidiary plane which cuts the main Quaternary fault plane. The Gaegok 6 fault has a similar orientation, westward dipping geometric pattern, and reverse sensed kinematic pattern with Gaegok 1 fault in the north. We consider that the Gaegok 6 fault is a southern extension of Gaegok 1 fault. In brief, six active faults in Gaegok-ri area juxtapose contact between the granite and the Quaternary gravel deposit. The eastern blocks of faults are mainly uplifted. The striations and movement senses of faults indicate multiple compressional stages in this region. The active faults are developed with an interval of several hundred meters from southwest to northeast in Gaegok-ri, not as a unique active fault, but as several sub-parallel faults forming an active fault zone. A trench survey is done near Gaegok 1 fault. In the trench two Quaternary faults are found. The old one is developed along the contact between Early Tertiary granite and Quaternary lower gravel and sand deposits, and do not continue to the upper sand layer. The younger one is developed between upper channel deposits and humified soil with liquefaction, along which channel deposits are reversely dragged. We describe and discuss also the possibility of surface rupture and liquefaction by earthquake occurred along Ulsan Fault Zone just before the Late Silla Dynasty (779 A.D.).