Observation of migrating aftershocks in the 2016 Gyeongju earthquake sequence

It is known that aftershocks occur due to stress changes caused by the mainshock. However, the detailed mechanism of aftershock occurrence are not fully understood yet. The 2016 M L 5.8 Gyeongju earthquake is the largest event occurred since an official seismic monitoring started in South Korea. The Gyeongju earthquake sequence contains two major earthquakes, M L 5.1 (Event A) and M L 5.8 (Event B), and two sub-parallel independent fault planes (Fault A and Fault B) host each of them. A dense aftershock monitoring network installed right after Event B makes a detailed study on aftershocks possible. Among numerous aftershocks occurred on Fault A and Fault B, we selected two groups (Group A and Group B) of aftershocks with very similar waveforms and analyzed their characteristics. Here Group A and Group B are located at lower edges of Fault A and Fault B, respectively. The hypocenters of events for the two groups are clustered within a circle with a radius of ~50 m and ~100 m, respectively. If we assume a stress drop of 10 MPa, which is reported as the stress drop of both events, rupture areas of events overlap each other. However, slip distribution models for relatively large events in each group determined by an empirical Green's function method show that major slip areas do not overlap. In addition a stress drop estimated by the slip inversion is much larger than 10 MPa. If we assume a larger stress drop (200 MPa), rupture areas of events do not overlap anymore and events are likely to migrate with increasing time. The directions of migration for Group A and Group B are SE and NW, respectively. These migration directions are consistent with the directions of rupture propagation for Event A and Event B. The migration velocity of Group A and Group B are estimated as 2 m/day and 4 m/day, respectively. Although further analysis will be required to interpret the physical meaning of the observations, our study provides important observational data to understand the detailed mechanism of aftershock generation.