Microearthquake activity associated with the 2016 M L 5.0 offshore Ulsan earthquake sequence and its tectonic implications

The 2016 offshore Ulsan earthquake occurred in the southwestern margin of the Ulleung Basin, which had been formed by the extensional tectonics and has now undergone the back-arc closure by the compressional stress. It is believed that the geological structures would reversely reactivate. Such present-day active tectonics typically produce increased seismicity in the region where fold and thrust faults are developed. Thus, understanding the mechanisms of earthquake occurrence is necessary to evaluate the seismic hazard potential of the region. On July 5, 2016, a crustal earthquake with a local magnitude (M L ) of 5.0 occurred at a depth of 15.8 km with a characteristic strike-slip faulting motion. Four significant aftershocks in the offshore Ulsan area also displayed a mechanism similar to that of the mainshock. A matched-filter technique was applied to detect microearthquakes using a relatively large event as a template to identify seismic activity during July 2016. In total, 140 events were detected for the sequence. Microearthquakes were the most frequently detected when the aftershock on July 8, 2016, was used as a template. The location and focal mechanism of the aftershock with persistent co-located seismic activity indicate that the causative fault corresponding to the sequence has an E–W strike, and its rupture failed to trigger a patch of geological faults with high compressive strength at the Dolgorae Thrust Belt. The proximity, spatial distribution, and focal mechanisms suggest that the 2016 earthquake sequence might have resulted from the reactivation of faults developed during the formation of the Gorae-V structure after the Miocene. Faults with strikes in the E–W to NW–SE directions developed under the same tectonic environment with the structure would be dominantly responsible for the current seismic activity in the southwestern Ulleung Basin.