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Top1. Introduction
At 7:44 p.m. and 8:32 p.m. on Sep. 12, 2016, there were two devastating earthquakes of magnitude 5.1 and 5.8 on the Richter scale occurred in succession (Korea Meteorological Administration, 2017; Ministry of Land, Infrastructure and Transport, 2017; Korea Institute of Geoscience and Mineral resources, 2017). This earthquake was recorded as the highest magnitude since the first seismological observation in 1978 in Korea (Kim & Lee, 2015, pp. 181-192; Cho, 2015, pp. 485-495; Park et al., 2013, pp. 2481-2486); this earthquake made a shock that was discernable nationwide and continuous aftershocks developed national anxiety. The Yangsan fault where Gyeongju is located caused many discussions about earthquakes because it is an active fault (Yang & Lee, 2014, pp. 17-27; Kim & Lee, C.K., 2015, pp. 181-192; Nam & Song, 2015, pp. 305-316). As a fault in the Korean Peninsula, the Yangsan fault starts from Yeongdeok-gun, Gyeongsangbuk-do, goes to Yangsan-si, Gyeongsangnam-do and ends at the mouth of the Nakdonggang River in Busan Metropolitan City. It is an active fault with the highest probability of seismic outbreak in Korea and it is a strike–slip fault about 170 km in length and is named the Yangsan fault or Eonyang fault on geological maps. As fragmental zones are accompanied inside bedrock and the dislocation distance is huge, it is known as a geologically important fault zone in Korea (Park et al., 2015a, pp. 247-254; Senapati et al., 2011, pp. 29-48; Park et al., 2015b, pp. 215-224; Park & Kim, 2016, pp. 9-14). Earthquake monitoring requires analyzing the dislocation due to earthquake and detecting crustal movements in areas with high probabilities of seismic outbreaks. GNSS is a positioning technology using satellite signals developed for military purposes, but is now widely used in navigation, geophysical research, and surveying (Li et al., 2015, pp. 607-635; Song, 2009, pp. 535-544; No et al., 2012, pp. 529-533; Power et al., 2006, pp. 373-376; Lee et al., 2005, pp. 242-255; Park & Um, 2016, pp. 567-572). The GNSS positioning technology has become a core technology not only in various mobile navigation fields, but also in location information based information fusion applications such as ITS (Intelligent Transportation System), LBS (location based system) and telematics. The accurate determination of the time using GNSS provides precise time information to the field of mobile communication and the related fields of electronic information such as electronic finance, electronic commerce, and ubiquitous, newly emerging according to the development of communication network and mobile environment (Hong, 2015, pp. 123-129; Lee et al., 2015, pp. 537-545; Wang et al., 2013, pp. 656-667; Yang et al., 2000, pp. 783-788). This research used GNSS data of national CORS to investigate seismic dislocations caused by the Gyeongju earthquake, to analyze the location change of CORS, and to suggest a measure of efficient seismic monitoring for earthquake disaster prevention monitoring. Figure 1 shows the study flow. In data acquisition, collects NGII’s CORS data for the month in which the earthquake occurred. And Data Processing, the PPP method is used to calculate dislocation aspects caused by the earthquake and the relative positioning method is used to investigate the effects of national seismic events. In the analysis, the displacement and CORS changes due to the earthquake are examined.