Engineering seismological characteristics of the 12 September 2016 Gyeongju earthquakes

On 12 September 2016, M_L 5.1 (foreshock) and M_L 5.8 (mainshock) earthquakes occurred about 48 minutes apart in the historic city of Gyeongju. Among the numerous aftershocks within the Yangsan Fault System, the largest aftershock of M_L 4.5 occurred on 19 September 2016. In particular, the M_L 5.8 earthquake with the focal depth of 13 km is the largest seismic event observed in South Korea since instrumental earthquake monitoring began in 1978. Up to now (January 2018), more than 600 aftershocks (> M_L 1.5) have followed. The focal mechanism showed a right-lateral strike-slip fault plane with a strike of 26° (or 118°), a dip of 68° (or 85°), a rake of 175° (22°), and the moment magnitude (M_W) was deduced into M_W 5.5. The strikes of major events match the trend of the distribution of aftershock epicenters with depths ranging 11 to 16 km. According to the onsite survey, the fault-plane, which caused the Gyeongju earthquakes, did not extend to the surface. Finally, seismic site classification B and C, are primarily distributed near damaged buildings, where low-rise structures and non-structural members were damaged due to energy concentration of Gyeongju earthquakes on the high-frequency band.