Evolution modeling of the λ-type Yangsan-Ulsan fault system

Although Korean Peninsula has been considered as a relatively safe area against earthquake because it is relatively far from plate boundaries. Instrumental earthquake distribution in Korea shows that many moderate, light, minor earthquakes occur around the central and southeastern part of the Korean Peninsula. Especially, some major earthquakes including Gyeongju Earthquake (779, Mw=6.5) have been reported around Yangsan fault and Ulsan fault, major young faults in Korea. Recently, more than 40 Quaternary faults have been discovered near the Yangsan-Ulsan fault system (YUFS). The analysis of the lineament, geology and geomorphology around YUFS indicates that YUFS is analogous to the characteristics of λ-fault. Numerical modeling on the evolution of YUFS with stress changes has been performed, using the USGS open program Coulomb 3. This modeling is carried out based on a previously suggested tectonic evolution model for the YUFS. The result of this numerical modeling shows the evolution of the YUFS through λ-fault. During the propagation of the Ulsan fault to the Yangsan fault in the late Miocene, coulomb stress increses at the northwestern 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 northeastern region of the Yangsan fault and the eastern region of the Ulsan fault. These regions are consistent with the distribution of Quaternary faults and epicenters of recent earthquakes around the YUFS. This kind of study can contribute to estimation of earthquake hazards around active faults.