Characteristics of Seismic Activity and Seismogenic Faults in the Southeastern Korean Peninsula

Abstract

The 2016 Gyeongju and 2017 Pohang earthquake sequences, which occurred in the southeastern Korean Peninsula near assumed epicenters of several substantial historical earthquakes and Quaternary fault segments, underscores the importance of seismic hazard assessment in the region. However, uncertainties about potential seismic sources make evaluating the potential for a moderate-to-large earthquake challenging. Microearthquake monitoring through a dense seismic network can provide crucial insights into the regional seismic characteristics. An extensive temporary seismic array and temporary aftershock monitoring network were established to investigate microearthquake activity in the southeastern Korean Peninsula and source characteristics of the 2016 Gyeongju and 2017 Pohang earthquake sequence. A comprehensive earthquake catalog consisting of 18,422 events was detected using two automatic detection methods based on the measurement of the energy ratio from September 2016 to June 2024. The detected events were categorized into five seismic regions excluding quarry blasting sites, that is, the 2016 Gyeongju earthquake region, 2017 Pohang earthquake region, eastern part of the Ulsan Fault, eastern offshore Gyeongju, and western part of the Miryang Fault. A local magnitude scale was developed for the southeastern Korean Peninsula using detected events. This reflects the distance attenuation and site conditions of the seismic stations in the southeastern Korean Peninsula for earthquakes. The earthquake catalog provided high-resolution hypocenter parameters at a low completeness magnitude (MC) despite the seismic environment of the network being exposed to high cultural noise. The Gutenberg-Richter b-value was estimated as 1.00 ± 0.02. This implies that the seismicity reflects a representative intraplate seismic environment. The seismogenic faults identified in each cluster primarily exhibit NNE-SSW or NE-SW strike. In terms of dip angles, both low-angle faults ranging from 30° to 50° and high-angle faults ranging from 60° to 80° were confirmed. It is also consistent with the distribution of fault plane solutions. Stress inversion results using fault plane solutions determined the trend and plunge of the maximum principal stress (σ1) to be 265.4° and 3.9°, respectively, with a stress ratio of 0.89, indicating a transpressional stress regime. This is consistent with the regional stress environment of the Korean Peninsula. Slip tendency of the determined seismogenic faults exhibit high slip tendency values (>0.5). Depending on the relationship between the magnitude and frequency of earthquakes, a relatively large number of small earthquakes can provide detailed information on the geometric properties of the seismogenic faults and the state of the acting stress. High-precision microearthquake, including aftershock activity following large earthquake, observation and analysis through dense seismic array could provide an unprecedented opportunity with seismic datasets to understand the seismogenesis of the southeastern Korean Peninsula, including the zone of aftershocks of the 2016 Gyeongju and 2017 Pohang earthquake.