The 2016 Gyeongju earthquake sequence revisited: aftershock interactions within a complex fault system

Abstract

, a moderate earthquake (M L 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 M L 5.1 event 50 min before the main quake and a M L 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 spatio-temporal distribution of events and inverted moment tensors indicate that the M L 5.1 event and the M L 5.8 event occurred on two parallel dextral faults striking NNE-SSW at a depth of 11-16 km, and the M L 4.5 event occurred on their conjugate fault with sinistral displacements. Seismicity on the fault for the M L 5.1 event abruptly decreased as soon as the M L 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 M L 5.1 event and the M L 5.8 event are matched with the off-fault seismicity, including that from the M L 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.

Spatial and temporal evolutions of aftershocks between 2017 and 2023 at the Pohang EGS site and its vicinity

“…However, the minimum principal stress was subvertical compared with the regional stress field of the Korean Peninsula (Soh et al, 2018). For the southeastern part of the Korean Peninsula and the 2016 Gyeongju earthquake sequence (Chang et al, 2010;Woo et al, 2019b), although there was a slight difference in the trend of the principal stresses, the plunges of the intermediate and minimum stresses were determined to be sub-vertical, similar to the results of this study.…”

“…The estimated b-values were significantly lower than those for earthquakes on the Korean Peninsula (Noh, 2019;Park et al, 2021) and the 2016 Gyeongju earthquake sequence (Woo et al, 2019b), exhibiting a similar trend even with a short-term aftershock catalog (Woo et al, 2020). The b-values have been considered to be related to various factors, such as the faulting type (Schorlemmer et al, 2005), material properties (Mogi, 1962;Ogata et al, 1991), and crustal stress (Scholz, 2015;Rigo et al, 2018).…”

Seismic Scatter Associated with Earthquakes that Ended a Quiescent Period in Southeastern Korea

“…Most of the faults in Figure 3 have not been clarified for their activity. Recent studies (Lee et al 2018;Woo et al 2019), based on micro-seismicity related to the 2016 M 5.8 earthquake, differentiated active faults, including the YF and UF, into the Gaum fault (GF),…”

Measurement of seismometer misorientation based on P-wave polarization: application to permanent seismic network in South Korea

“…In South Korea, there are three permanent seismic networks: 1) the KG network operated by the Korea Institute of Geoscience and Mineral Resources (KIGAM), 2) the KS network operated by the Korea Meteorological Administration (KMA), and 3) the KN network operated by the Korea Hydro and Nuclear Power (KHNP) . These permanent stations provide valuable seismic waveform data for various studies aiming to elucidate earthquake properties and seismic structure of the Korean Peninsula (Kang and Baag, 2004;Kim et al, 2010;Kim et al, 2017;Kim et al, 2018;Woo et al, 2019;. The number of seismic stations has increased continuously since the modern KMA seismograph network operation started in 1978.…”