Active Fault Topography and Fault Outcrops in the Central Part of the Ulsan Fault System, Southeast Korea

Abstract

The Ulsan fault system situated in the southeastern part of Korean Peninsula marks the western range front of the Mt. Tohan (745.1 m) to the Sandae Peak (629.1 m) with the remarkable Ulsan fault scarp. This system extends with a N-S trend for a distance of more than 40 km. The NNE-SSW trending 200 km-long Yangsan fault and this Ulsan fault are recognized as the most important active faults on the Korean Peninsula.From interpretations of aerial photographs and detailed topographic maps, and field surveys along the central part of the Ulsan fault system, the major results are summarized as follows : 1. The Ulsan fault system extends in the direction of NNW-SSW to N-S and the trace is slightly sinuous (Fig. 1). The middle to higher river terrace surfaces on this system are displaced up thrown to the east side (Fig. 2). The streams and the ridges across this fault trace are not accompanied by systematic lateral displacement. The Ulsan fault system is a typical example of reverse faulting, considered from the behavior of fault outcrops (Figs. 3, 4, 5).2. The vertical displacements are about 5 m on the middle terrace, and about 15 m on the higher terrace from measurements of topographical cross-section across the fault scarplets. The cumulative vertical displacement is recognized during and after the formation of the terraces. The sense of vertical displacement of fluvial terraces coincides with the magnificent fault scarp. This mode of mountain-building movement has continued repeatedly at least since the middle of Quaternary.3. The terrace surfaces originated from the dissected fan surfaces in the late Quaternary. From geomorphic correlation, facies analysis, development of soil covering the terraces, and results of dating of humic material, etc., the ages of terrace surfaces are roughly estimated and the average slip rates of vertical displacement are measured to be about 0.1-0.08 mm/y.4. Fault outcrops are clearly observed along the Ulsan fault in this area. The shattered granite rocks thrust over the gravel deposits composing the middle terrace at the north bank of Sagok Pond, Malbang-ri, Wedong-eop, Kyeongju-gun, Kyeongsang-bukdo (Fig. 3-A, B). The fault strikes in the direction of N-S and dips eastward at an angle of 20-30°E. The shattered granite is in contact with the slightly weathered and deformed gravel at the northeast of Kaegok-ri settlement (Fig. 5).5. Liquefaction phenomena are observed within the upper horizon of the middle terrace deposits at the northwest of the Sagok Pond (Fig. 6).6. As the NNW-SSE (or N-S) trending Ulsan fault system is predominantly a reverse fault and NNE-SSW trending Yangsan fault system has been dislocated with a predominant right-lateral slip in the late Quaternary, it is estimated that this area is situated under the maximum horizontal stress field in the ENE-WSW (or E-W) direction.