Comparison of the aftershock activities of the 2016 M5.8 Gyeongju and 2017 M5.4 Pohang earthquakes, Korea

In this study the properties of the aftershock activities for the 12 September 2016 Gyeongju (M-L= 5.8) and 15 November 2017 Pohang (ML= 5.4) earthquakes are compared and analyzed. The areas of aftershock activity region estimated from the spatial distribution of aftershocks for the Gyeongju and Pohang earthquakes are much smaller than the estimated area from the empirical relationship between the mainshock size and the area of aftershock activity region. This implies that the ruptures caused by the aftershocks are considerably limited in a small area. The b-values on the Gutenberg-Richter relation for the aftershock sequences of the Gyeongju and Pohang earthquakes estimated with the maximum likelihood method are found as 1.12 +/- 0.03 and 1.05 +/- 0.02, respectively. The values are significantly larger than those of 0.85 obtained for the whole Korean Peninsula. The decay parameters of p-value in the modified Omori's formula for the aftershock sequences of the Gyeongju and Pohang earthquakes are estimated to be 0.965 and 0.954, respectively. These values are slightly smaller than the value of 1.0 in the original Omori's formula. It is concluded that the estimated parameters for aftershocks of two earthquakes show no unusual values. Finally, the probability versus time for ten months that at least one aftershock of which magnitude is equal to or greater than 4.0 will occur is predicted using the aftershock parameters estimated in this study.