The 12 September 2016 Gyeongju earthquakes: 1. Observation and remaining questions

Two earthquakes (M L 5.1 and 5.8) ruptured branches of the Yangsan Fault System in Gyeongju, S. Korea on September 12, 2016. After the M L 5.8 earthquake, aftershock earthquakes continued to occur, including two notable earthquakes (M L 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 L 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.