Transpressional earthquakes along the Southern Yangsan Fault, SE Korea and its paleoseismic significance during the late Quaternary

Abstract:

The NNE–SSW-striking Yangsan Fault in southeastern Korea has been regarded as one of the most prominent seismogenic structures in the Korean Peninsula. Its seismic behavior is, however, still uncertain due to long recurrence intervals of strong earthquakes and insufficient historical and geologic records. We have demonstrated that at least one strand of the Southern Yangsan Fault in SE Korea has experienced multiple surface-rupturing earthquakes during the late Quaternary by applying detailed paleoseismic analysis and interpretation at the Inbo trench site. The earthquake slip occurred on a subsidiary fault located about 200 meters east of the main fault core of the Yangsan Fault. Kinematic indicators and some stratigraphic features on the exposed section indicate that the fault has a significant amount of lateral displacement along with the observed vertical displacement resulting from multiple (≥2–4) surface-faulting events. Based on OSL ages of the units cut by faults, we suggest that the most recent event occurred after 29 ka and the timing of an earlier event is constrained to have occurred between 70 ka and 29 ka, although we are limited in determining the recurrence time of earthquake-faulting due to a variety of possibilities and the potential for unrecognized additional events on the fault strand in this study. The minimum vertical slip-rate of 0.11 mm/yr is estimated from the minimum vertical offset (7.5 m) of the oldest sedimentary unit and its OSL age (70 ka). Considering the transpressional kinematics of the fault, the minimum horizontal slip rate is probably two times that of vertical slip rate. We also conclude that the east-side-up dextral transpressional kinematics with upward decreasing dip pattern represents the recent earthquake-faulting kinematics of the Yangsan Fault, mainly controlled by pre-existing old fault zone geometry having an east dip direction in conjunction with the present-day maximum horizontal stress field.