Characteristics of Microearthquakes in the Southeastern Area of the Korean Peninsula

We installed a temporary seismograph network which is consisted of 20 broadband seismic stations in the vicinity of Ulsan and Gyeongju to study the characteristics of microseismicity in the southeastern area of the Korean Peninsula. During the period from September 2011 to February 2013, we observed the seismicity occurred in the study area. As the result of automated process, a total of 1202 events were triggered initially. Then a set of 850 events occurred in the study area only was chosen for further analysis. This data set includes events from artificial source as well as those from natural sources, so the discrimination of events between artificial and natural sources was performed. Hourly variation of event distribution was considered to confirm the amount of the events in daytime because most of quarry blasts is performed in the time. Every event was classified into either a set of natural events or that of artificial events throughout checking all the waveforms by eye inspection. The discrimination results indicate about 70 percent artificial events of all triggered events, resulting in about 250 natural events. Most of the events classified into artificial sources were occurred in daytime, while natural events were evenly scattered in overall hours. The epicenter of the natural events is located predominantly in the southeastern area of Gyeongju and the northern area of Ulsan. we relocated these events using Velellipse location algorithm. The relocated events are distributed densely in the northeastern area of the so-called Ulsan fault. Focal mechanism analysis was performed for the relocated events, which explain a complex of various faulting senses with couple of strike-slip and dip-slip component. The pressure axes with the east-west direction in the focal mechanisms are compatible with the predominant stress field in the southeastern Korean Peninsula. Combination of the results of seismic analyses with the neotectonic features indicates that comparatively more microearthquakes occur at the hanging wall block of the so-called Ulsan fault with the regional stress field than other area in the southeastern Korean Peninsula.