Properties of clustered earthquakes in stable intraplate region: a case study for the Korean Peninsula

Intraplate seismicity has been poorly understood relative to interplate seismicity, which may be partly due to low seismicity and long earthquake recurrence intervals and ambiguity in the responsible structures. Locally clustered earthquakes provide essential information on the nature of intraplate seismicity such as earthquake-spawning mechanisms, earthquake source properties, seismogenic depths and structures, stress accumulation and release patterns, rupture processes, and potential seismic hazards. The Korean Peninsula, located in a stable intraplate region around the eastern margin of the Eurasian Plate, offers a unique opportunity for studying intraplate seismicity due to the dense seismic network and diverse geological and paleo-tectonic structures. We investigate clustered earthquakes from 2018 to 2024 at 39 sites in the Korean Peninsula using a matched filter analysis based on 1057 stations. The earthquake locations are refined using a double-difference method (hypoDD) based on interevent phase traveltime differences. The earthquake magnitudes are determined using an amplitude-scaling method based on the waveform amplitude ratios between earthquakes. We determine the focal mechanism solutions using a phase polarity analysis (FOCMEC). The analysis detects 4 to 1644 earthquakes by site, identifying a total of 6957 clustered earthquakes with magnitudes of M_L–0.9 to 3.7. The earthquake distributions and focal mechanism solutions reveal NNE-SSW to NE-SW and WNW-ESE directional subvertical strike-slip faults at most sites, which agree with the ambient stress field. Coast-parallel reverse faults are identified around the eastern margin of the Korean Peninsula, which may be associated with the reactivation of paleo-rifting structures. Normal faults and relatively low-angle faults are observed around the western margin of the peninsula, which may be related to the paleo-collision structures. The earthquake clusters are generally populated at upper crustal depths of ~5–15 km, consistent with typical seismogenic depths in the region. We also observe earthquakes at mid-crustal depths of ~18–25 km in low-heat-flow regions, which may indicate relatively deep brittle-ductile transition zones. Either episodic or one-off earthquake swarms are observed depending on the site, suggesting site-dependent stress accumulation and release patterns. The one-off swarms may be associated with fluid diffusion along faults. The earthquakes at most sites present prominent temporal migration that is potentially due to the stress transfer and fluid diffusion, which may expand the fault ruptures with time. Some sites exhibit up-dip migration of seismicity, implying potential seismic hazards. These observations suggest that intraplate seismicity is controlled by combined effects of various geological and geophysical factors, including the ambient stress field, geological and tectonic structures, earthquake interactions, and medium properties.