Paleoseismic study on the central part of the Ulsan Fault Zone, SE Korea

Abstract

The Ulsan Fault Zone is a NNW-SSE trending reverse fault with a length of about 50 km from Ulsan to Gyeongju. The study area is middle part of the Ulsan fault zone, Cretaceous and early Tertiary granitic bedrock which is covered by the Quaternary deposits. Large-scale alluvial fans from the eastern mountain area are linearly arranged in the N-S direction and a large number of Quaternary faults that displace the alluvial fan have been reported. However, the association between these previously reported faults and the kinematic characteristics of the Quaternary fault are insufficient. This study aims to solve these problems and to establish of fault segment in the central part of the Ulsan fault zone. The 4-6 rows of lineament in the N-S or NNE-SSW directions were extracted from DEM based on LiDAR and aerial photograph through topographic analysis. Based on the results of the lineament analysis, a surface geological survey was conducted along the locations where the fault-related landforms were clearly recognized, and several Quaternary faults were additionally discovered. Each fault develops with different displacements depending on the location in the form of granite thrusted over the Quaternary deposits. In particular, trench survey conducted at 350 m northwest of the Malbang Reservoir confirmed a fault displacement of approximately 50 cm. Based on the behavior characteristics of the bedrock fault and the Quaternary fault in the study area, the Malbang segment in the central part of the Ulsan fault was established. The Malbang segment with a total length of at least about 4.4 km is a reverse fault that develops in the N-S direction and dips to the east. The number of fault movement is at least 3 times, the cumulative displacement is about 7 to 12 m or more, and the period of the last fault movement is estimated to be between about 25,000 years and about 10,000 years ago. Additional studies are needed to derive fault parameters of the fault segment and to establish a detailed fault segment model.