Seismic effects of deep crustal high-density material in the southeastern Korean Peninsula

The 2016 MW 5.5 Gyeongju and 2017 MW 5.5 Pohang earthquakes occurred near the Yangsan Fault in the southeastern Korean Peninsula (SKP), a region marked by a prominent positive isostatic anomaly. Here we investigated how high-density material (HDM) influences the local stress field, particularly around the Pohang (PHF) and Gyeongju (GJF) fault planes and other major fault systems in the SKP. Using three-dimensional numerical models constrained by high-resolution gravity-derived density structures, we calculated Coulomb failure stress changes (ΔCFS) on the GJF, PHF, and other major faults. The models show that ΔCFS is controlled by HDM thickness, increasing in the eastern part (with greater thickness) and decreasing in the western part (with smaller thickness). Our findings emphasize that accurate seismic hazard assessments in intraplate regions must integrate both regional tectonic stress regimes (e.g., far-field tectonic stress) and local subsurface density structures (e.g., high-density magmatic underplating). Numerical models constrained by gravity data suggest that crustal underplating in the southeastern Korean Peninsula may alter the local stress field and enhance moderate-magnitude seismicity, including the 2016 Gyeongju and 2017 Pohang earthquakes.