During the last six years, South Korea has experienced two of its largest earthquakes since instrumental recording began. A specific set of aftershock criteria is necessary for earthquakes that occur near the mainshock. Despite the discovery of the fault associated with the mainshock, some aftershocks have a low relevance to the fault where the mainshock occurred. Providing detailed information about aftershocks to people living near fault areas is important, and the difficulty in specifying their end time has prompted the need for improved criteria. We propose improved criteria for determining the aftershocks of the ML5.8 Gyeongju and ML5.4 Pohang earthquakes.The criteria are divided into temporal and spatial components. The earthquake Mw-fault information relationship of Wells & Coppersmith (1994) is used for the spatial criterion to calculate the extent of ruptures that can occur based on magnitude. The rupture range is conservatively applied to the Mw5.6 Gyeongju and Mw5.4 Pohang earthquakes. The spatial aftershock distribution is defined within 4 km of the earthquake fault line associated with the mainshock, and it is based on the empirical relationship of the fault down-dip rupture width and the earthquake moment magnitude of past earthquakes. For the temporal criterion, the endpoint of aftershocks is defined as the time when the number of earthquakes per unit of time after the mainshock is equal to or less than the average seismicity in the five years before the mainshock. Using these criteria, the study concludes that the aftershock sequences of the Gyeongju and Pohang earthquakes are still ongoing. 
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)