Paleotectonic controls on earthquake nucleation in stable intraplate regions

Earthquakes in intraplate regions pose significant seismic hazards since they can occur close to populated areas. In particular, large intraplate earthquakes tend to have long recurrence intervals, making it difficult to identify potential source regions prior to their occurrence. Understanding the mechanisms of earthquake nucleation is therefore crucial for seismic hazard assessment in low-seismicity regions. Historical earthquakes provide valuable constraints on such assessments. To address this issue, we investigate a magnitude ~6 earthquake that occurred in December 1952 in the southwestern suburban area of Pyongyang, North Korea, the largest instrumentally recorded event on the Korean Peninsula. We constrain its poorly known source parameters from analysis of long-period analog seismic records. The event is identified as a normal-faulting earthquake with a moment magnitude of Mw 6.3 and a focal depth of approximately 28 km. The source region is located along the eastern margin of the paleo-collision zone between the North China and South China blocks, where crustal-scale seismogenic structures are inferred to have developed. Beyond this large-event nucleation example, we further examine seismicity associated with other paleotectonic structures in the Korean Peninsula. We find persistent seismicity within reactivated paleo-rifting structures in East Sea (Sea of Japan). Mid- to lower-crustal earthquakes continue to occur offshore along the eastern coast of the Korean Peninsula, where laterally progressive variations in focal depth suggest ongoing neotectonic thrust evolution across inherited rift-related structures. In addition, paleovolcanic structures also host significant earthquakes.The 14 December 2021 Mw 4.9 Jeju offshore earthquake ruptured an aseismic paleovolcanic structure that responded sensitively to changes in the regional stress field and crustal properties. This mid-crustal moderate-sized event generated strong ground motions and local stress perturbations, triggering aftershocks on adjacent, preferentially oriented subparallel (NE–SW) faults. These observations indicate that paleotectonic structures act as preferred sites for earthquake nucleation in intraplate regions. We suggest that systematic monitoring of seismic activity associated with paleotectonic inheritance is essential for assessing the potential for future large earthquakes. This study presents seismic evidence for earthquake nucleation along paleotectonic structures surrounding the Korean Peninsula.