Evolution of slip susceptibility of Quaternary faults in southeastern Korea from Quaternary to present day

We study the geomechanical stability of faults in southeastern Korea that have been active during the Quaternary (mostly Pleistocene), to investigate whether these faults can still be considered active today. We compile fault plane data and slip orientations for 40 Quaternary faults in southeastern Korea distributed along the two major fault systems, Yangsan Fault (group 1) and Ulsan Fault (group 2), and in the younger block to the east of the Ulsan Fault (group 3). Our aim is to determine the representative tectonic stress states in the Quaternary and in the present day, and to compare the slip susceptibility of these faults under the stress fields of these two periods. We estimate Quaternary stress from inversion of Quaternary fault data, and present-day stress from inversion of contemporary earthquake mechanisms. Both stress fields exhibit a maximum principal stress axis that is sub-horizontal and trends ENE-WSW, but the intermediate and minimum principal stress axes are switched, suggesting a change in stress regime from reverse-faulting during the Quaternary to strike-slip-faulting in the present day. We demonstrate that this change in stress regime is due to a marked reduction in the magnitude of horizontal tectonic stress from the Quaternary to the present day. We assess the slip susceptibility of the faults under the Quaternary and present-day stress fields using two indices: slip tendency and fault instability. This analysis shows an increase in slip susceptibility for most of group 1 faults and some of group 3 faults. Some of these faults exhibit a slip susceptibility comparable to the average obtained for contemporary earthquake focal planes, suggesting that these faults can be considered active in the present day.