Detection of microearthquakes and identification of their causative structures in the eastern offshore region of South Korea

Abstract

A matched filter has been applied to continuous seismic records from the eastern offshore region of South Korea to identify small earthquakes undetected by routine processing. The resulting density of the regional earthquake catalog has increased by a factor of five relative to the detections reported by the Korea Meteorological Administration. High-precision locations of newly identified microearthquakes and previously located earthquakes cluster into streaks that delineate structures responsible for the seismicity associated with the Hupo Fault offshore of the southern Korean Peninsula. Microearthquakes in the northern and southern segments of the fault zone exhibit different behaviors: earthquakes in the northern segment occur at a regular temporal interval, while those in the southern segment occur in bursts. The identified fault system dips steeply to the west at shallow (< 10 km) depths, with the dip angle becoming shallower at greater depths. A comparison of the micro earthquake locations along the previously identified faults in the study area suggests reactivation of the Hupo Fault that formed as a listric normal fault via back-arc rifting associated with separation of the SW Japan Arc during the late Oligocene to early Miocene. The microseismicity data indicate that the fault has the largest fault plane identified to date using seismological techniques in onshore and offshore areas of the Korean Peninsula. Based on empirical relationships between earthquake magnitude and the potential rupture area, the fault system could generate a magnitude 6.5 earthquake if the entire fault plane outlined by the microseismicity ruptured in a single event.