Kinematic characteristics and movement timing of the Wonwonsa fault in the central Ulsan fault

Abstract

Deriving paleoseismological fault parameters of active faults is essential for earthquake disaster provision. The purpose of this study is to document the fault characteristics of Wonwonsa fault, which was recently reported in the 2nd fault outcrop on the eastern side of the Ulsan fault, and to provide fault parameters, such as the timing of activity. The fault slipped along the boundary between biotite granite and andesitic dike, and cuts Quaternary fluvial deposits, which deposited above basement rock. Vertical separation of about 42 cm is recognized based on the unconformity surface between the bedrock and the Quaternary fluvial deposit, but net-slip using fault geometry is calculated to be 46 cm. The burial ages of the fluvial deposits are ca. 6 ka (hanging wall part) and ca. 3 ka (footwall part) based on the optically stimulated luminescence (OSL) dating. The boulder's exposure age, which is located on the terrace surface using 10Be, was ca. 9,000 years. Although there is a difference between two dating methods, it is interpreted that this fault slipped at least once during the Holocene. Because we cannot rule out the possibility of the inheritance although it is minimal considering the steep mountain stream, the 10Be exposure had better be taken as a maximum. So, the OSL age may be near to the real age. This fault seems to have been activated after 3,600 years ago, implying that an earthquake with the surface rupture occurred during the Holocene. This result indicates the youngest active faulting age among the Quaternary faults reported along the Ulsan fault.