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2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011) EARTHQUAKE ARCHAEOLOGY INQUA PALEOSEISMOLOGY AND ACTIVE TECTONICS STRUCTURAL CHARACTERISTICS AND EVOLUTION OF THE YANGSAN-ULSAN FAULT SYSTEM, SE KOREA S.-R. Han (1), M. Lee (1), J. Park (2), Y.-S., Kim (1*) (1) Dept. of Environmental Geosciences, Pukyong National University, Busan 608-737, Korea. *Email: ysk7909@pknu.ac.kr (2) GeoGeny Consultants Group Inc. 807-2 Bangbae-Dong Seocho-Gu, Seoul 137-831, Korea. Abstract (Structural characteristics and evolution of the YangSAN-Ulan fault system, SE Korea): The Yangsan-Ulsan Fault System (YUFS) is a major fault system in Korea. More than 40 Quaternary faults have been discovered around the YUFS. To understand the evolution and structural characteristics of the YUFS, we performed numerical modelling for the evolution of the YUFS and fault zone analysis on the eastern part of the Ulsan fault. The result of the modelling shows that the YUFS evolved into -fault, a low angle merging fault system. In a fault zone analysis, the hanging wall block of the fault shows relatively higher damage with higher fracture density and deformation structures compared with the footwall. This indicates that the hanging wall is more susceptible to deformation than the footwall during faulting events; this result is consistent with damage patterns in other Quaternary faults. Epicenters of recent earthquakes around the study area are also concentrated on the hanging wall of the Ulsan fault. This kind of study can help us to evaluate hazards resulting from future potential earthquakes in and around active fault systems. Key words: Yangsna-Ulsan fault system, -fault , Quaternary faults INTRODUCTION The Korean Peninsula is generally considered as a relatively safe region considering earthquake hazards, because it is located away from plate boundaries. However, many moderate and minor earthquakes occur around the Yangsan-Ulsan fault system (YUFS), in the southeastern part of the Korean Peninsula (Lee and Yang, 2006). Recently, more than 40 Quaternary faults were discovered near the YUFS (Jin et al., 2011). There are two nuclear power plant sites near the YUFS (Fig.1). To prevent nuclear catastrophe due to a potential large scale earthquake, it is necessary to understand the deformation characteristics and future evolution of the YUFS. For this purpose, a numerical modelling related to the evolution of the YUFS and a detailed field investigation on a reactivated fault were carried out in this study. STUDY AREA The study area is located in the Cretaceous Gyeongsang Basin, SE Korea (Fig. 1). The basement of the study area is composed of Cretaceous sedimentary rocks with later igneous and volcanic rocks. The Yangsan fault is a NNE-SSW trending right-lateral dominant strike-slip fault, and the NNW- SSE trending Ulsan fault is interpreted as a strike-slip fault overprinted by later reverse-slip. Recently, more than 40 Quaternary faults have been discovered near the Yangsan and Ulsan faults. Historical records show many earthquakes in this region, with an earthquake producing over 100 casualties recorded Fig. 1: Regional geologic map of Gyeongsang basin, the SE part of the Korean peninsula and Quaternary faults (circles) around the Yangsan and Ulsan faults. Two Nuclear Power plants are located near the YUFS (modified from Lee, 2000) in A.D. 779 (estimated M= 6.7), near the intersection of the two faults (Lee and Yang, 2006). 74
2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011) EARTHQUAKE ARCHAEOLOGY INQUA PALEOSEISMOLOGY AND ACTIVE TECTONICS Fig. 2: Comparison between coulomb stress changes, distribution of Quaternary faults and epicenters of earthquakes around the Yangsan-Ulsan fault system (Han et al., 2009). (a) Coulomb stress changes are concentrated in the eastern and southeastern region of the Yangsan fault and eastern region of the Ulsan fault in Quaternary. (b) Locations of Quaternary faults around the Yangsan-Ulsan fault system. (c) Earthquake distributions around the southeastern part of the Korean peninsula (1994-1998). Green circles indicate the epicenters of the recent earthquakes. Earthquakes are concentrated in the eastern part (hanging wall) of the Ulsan fault. EVOLUTION MODELLING OF THE YANGSAN- ULSAN FAULT SYSTEM Analysis of lineaments and geomorphology of the YUFS indicate that the YUFS is analogous to the characteristics of -fault. The concept of -fault was suggested by Du and Aydin (1995). This is the geometric relationship between two intersecting strike-slip faults. When a strike-slip fault propagates into a pre-existing strike-slip fault, the orientation of the propagation depends on the angle between the maximum principal stress and propagating fault. To understand the evolution of the fault system, a numerical modelling on the YUFS with regards to stress changes with time was performed, using the USGS open program Coulomb 3 (http://earthquake.usgs.gov). This modelling was carried out based on a previously suggested tectonic evolution model for the YUFS (Park, 2004). The result of this numerical modelling indicates that the YUFS is a -fault. During the propagation of the Ulsan fault into the Yangsan fault in the late Miocene, coulomb stress increases at the north-western tip of the Ulsan fault. After the Ulsan fault connected to the Yangsan fault in Pliocene, a new segment was initiated at the northern part of the Ulsan fault due to stress changes. Since the penetration of the Ulsan fault into the Yangsan fault in Quaternary, coulomb stress increased in the north-eastern region of the Yangsan fault and the eastern region of the Ulsan fault. These regions are well consistent with the distribution of Quaternary faults and locations of epicenters of recent earthquakes around the YUFS (Fig. 2). FAULT GEOMETRY AND SLIP ANALYSIS OF A FAULT ZONE IN THE WEOLSEONG AREA In the SE part of Korea, two nuclear power plant (NPP) sites are located near the YUFS. In the new Weolseong NPP site located in the southeastern coast of Korea, an east dipping, N-S trending fault was discovered during construction. The main fault zone shows various different coloured fault gouge bands and shear fabrics indicating opposite slips (Fig. 3). These features indicate that there were discrete stages of normal and reverse faulting events along this fault zone. The analyzed kinematic indicators, such as lineations, cleavages, and slickenlines in the fault zone suggest that the fault was initiated as a normal fault and was reactivated as a reverse fault under NW-SE compression. The hanging wall block of the fault shows relatively higher damage indicating higher deformation in the hanging wall than the footwall. CONCLUSION It is common that earthquakes occur by reactivation of pre-existing faults (e.g. Chen, 2002; Ota et al., 2004; Ota et al., 2005). It is well known that most of the damage from earthquake is concentrated on the hanging wall block during the faulting (Ota et al., 2005). A N-S trending, east dipping fault zone was discovered in the construction site of the new Weolseong NPP.Detailed analysis was performed on this fault zone and the result indicates multiple faulting events. According to the analysis, this fault zone initiated as a normal slip fault and it was 75
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