The geometrical characteristics of causative faults related to clustered earthquakes in the southeastern Korean Peninsula

The southeastern part of the Korean Peninsula is known to have high seismic activity and many Quaternary faults. Nonetheless, there have been uncertainties in estimating seismic hazards due to insufficient information on potential seismic sources. We investigated the geometrical characteristics of causative faults related to clustered earthquakes in the southeastern Korean Peninsula by detecting microearthquakes and determining their source parameters. We used the seismic data recorded at the Gyeongju hi-density broadband seismic network, the temporary seismic networks operated to monitor the aftershocks of two moderate earthquakes (the 2016 M_L 5.8 Gyeongju and 2017 M_L 5.4 Pohang earthquakes), and the national seismic network of South Korea. An earthquake catalog for the southeastern Korean Peninsula was built using automatic earthquake detection methods based on measurements of energy ratio. We identified the five clustered earthquake regions via the microearthquake distribution: the 2016 Gyeongju earthquake region (GJ), the 2017 Pohang earthquake region (PH), the eastern part of the Ulsan Fault (UF), eastern offshore Gyeongju (EG), and the western part along the Miryang Fault (MF). We determined the relative location and focal mechanisms of the earthquakes occurring in those regions using the double-difference location method and the P-wave first motion polarity method, respectively. Finally, the geometry of the earthquake causative faults was inferred using the spatial distribution of the relative locations and the focal mechanisms. It was found that there are at least two NNE-SSW trending fault segments and multiple NE-SW trending fault segments in the GJ and PH, respectively. In the case of MF, UF, and EG, it is difficult to relate directly to the surface faults, but the strikes of the causative faults, which are confirmed by the spatial distribution of earthquakes, are similar to those of the surface faults.