Seismicity of the 2016 ML 5.8 Gyeongju earthquake and aftershocks in South Korea

Since the modern seismograph network by the Korea Meteorological Administration (KMA) had been operated from 1978, the largest earthquake (ML 5.8) occurred in Gyeongju, South Korea on 12 September 2016. ML 5.1 and 5.8 earthquakes occurred successively and damaged mostly old building structures around Gyeongju. Aftershocks continued to occur, clustering around the epicenters of the two events. Number of observed aftershocks with ML ≥ 0.1 was 3,376 from 12 September 2016 to 31 March 2017 and these aftershocks were located by manual procedure. Since the origin time of the ML 5.8 earthquake, 339 and 672 aftershocks (ML ≥ 0.1) occurred on 12 and 13 September 2016, respectively. The occurrence rate of aftershocks decreased exponentially with time after 16 September. It is observed that the seismicity of the 2016 Gyeongju earthquake and aftershock sequences is dominant in the Yangsan Fault System. The distribution of aftershocks is NNE-SSW direction along the Yangsan Fault System, which agrees with the fault plane solutions of main shock and foreshock showing the strike-slip faulting. These aftershocks concentrate within the epicentral distance of 5 km from main shock and the depth distribution of aftershocks is mainly around 11–15 km with vertical trends. With regard to the seismicity of the 2016 Gyeongju earthquake’s aftershocks the full catalog with ML ≥ 1.2 based on Mc is suggested in this study. Finally the distribution of aftershock sequence correlated to the focal mechanisms can provide specific seismicity in the Gyeongsang Basin.