On 2016 September 12, a moderate earthquake (ML 5.8) occurred in Gyeongju, South Korea, located hundreds of kilometres away from the nearest plate boundaries. The earthquake, the largest instrumentally recorded event in South Korea, occurred in a sequence of thousands of earthquakes, including a ML 5.1 event 50 min before the main quake and a ML 4.5 event a week later. As a case study, we analyse the source parameters of the 2016 Gyeongju earthquake sequence: precise relocations, fault structures, focal mechanisms, stress tensor analysis and Coulomb stress changes. To determine high-resolution hypocentres and focal mechanisms, we employ our temporary seismic network for aftershock monitoring as well as regional permanent seismic networks. The spatiooral distribution of events and inverted moment tensors indicate that the ML 5.1 event and the ML 5.8 event occurred on two parallel dextral faults striking NNE-SSW at a depth of 11-16 km, and the ML 4.5 event occurred on their conjugate fault with sinistral displacements. Seismicity on the fault for the ML 5.1 event abruptly decreased as soon as the ML 5.8 event occurred. This is not solely explained by the Coulomb stress change and requires more complex processes to explain it. The tectonic stress field obtained from inverted focal mechanisms suggests that the heterogeneity between the intermediate and minimum principal stresses exists along the NNE-SSW and vertical directions. The Coulomb stress changes imparted from the ML 5.1 event and the ML 5.8 event are matched with the off-fault seismicity, including that from the ML 4.5 event. Multifaceted observations, such as Coulomb stress interactions between parallel or conjugate faults and the heterogeneity of the tectonic stress field in the aftershock area, may reflect the reactivation processes of a complex fault system. This study offers a distinctive case study to understand the general characteristics of intraplate earthquakes in multifault systems.
- Continental neotectonics
- Dynamics and mechanics of faulting
- Dynamics: seismotectonics
- Earthquake interaction, forecasting, and prediction
- Earthquake source observations
ASJC Scopus subject areas
- Geochemistry and Petrology