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The M 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitoring began in 1978. It was preceded by an M 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor aftershocks was installed at about 1.5 km east of the announced epicenter. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ∼38 × 32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the aftershock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to monitor aftershocks of major earthquakes is advised. [ABSTRACT FROM AUTHOR]

Two earthquakes ( M 5.1 and 5.8) ruptured branches of the Yangsan Fault System in Gyeongju, S. Korea on September 12, 2016. After the M 5.8 earthquake, aftershock earthquakes continued to occur, including two notable earthquakes ( M 4.3 and 4.5) on September 12 and 19, 2016. This paper details the early reports of the Yangsan Fault System in the Gyeongsang Basin from various geological and geophysical/seismological perspectives. Based on a review and an initial seismological analysis of the results of the three earthquakes ( M 5.1, 5.8, and 4.5), we present and discuss the following topics: (1) the tectonic setting and the geophysical/seismic structure of the Yangsan Fault System, (2) historical seismicity and inferred seismic hazard from historical (literature) data, and (3) source mechanisms of the three earthquakes. In the end, we highlight some of the outstanding issues with regard to earthquakes and future research topics. [ABSTRACT FROM AUTHOR]