Assessment of tectonic activity using geomorphic indices along major faults in SE Korea

Geomorphic indices are a useful tool for rapid tectonic activity assessment over large areas. We assessed the tectonic activities along the Yangsan Fault where many Quaternary faults are observed, the Ulsan Fault where Quaternary faults and micro-earthquakes frequently occur, and the Miryang Fault where only micro-earthquakes are reported, using the geomorphic indices. The indices used in this study are Hypsometric Integral (HI), Hypsometric Curve (HC), and Basin Shape ratio (BS), which indicate the maturity of the drainage basins, Asymmetry Factor (AF), which implies the degree of asymmetry due to tilting of the basins, and Stream Length-gradient (SL), which represents the change in slope of the stream. These results were combined to evaluate Relative Tectonic Activity (RTA) of each basin. Drainage basins along the Yangsan and Ulsan faults dominantly show 'High' and 'Very High' RTAs and are also characterized with the asymmetry of RTA distribution of much higher tectonic activity in the eastern block of the faults. However, the drainage basins along the Miryang Fault dominantly show 'High' and 'Moderate' RTAs with their symmetric distribution. SL values generally depend on the lithology of bedrocks, but some SL anomalies in the Yangsan and Ulsan faults are nearly identical to the location of the Quaternary surface ruptures. These features indicate that the analysis results of geomorphic indices supported by geological interpretation can be useful for finding the Quaternary faults.