Earthquake-spawning faults in the Seoul metropolitan area and their seismic implications

Quaternary faults run across the Seoul metropolitan area that is the highest population region in the Korean Peninsula. Active fault identification and seismic hazard potential assessment are crucial for public safety. Densely-deployed permanent and temporal seismic stations enabled us to detect micro to small earthquakes, allowing us to identify earthquake-spawning faults in Seoul metropolitan area. The source parameters of 455 earthquakes in 2004-2020 are refined. The Gutenberg-Richter b value is 0.94. Dominant focal depths are 4-15 km. The focal mechanism solutions of 64 earthquakes are determined using seismic-wave polarities and amplitude ratios. Strike-slip earthquakes are dominant in the region. Earthquakes are clustered around the Chugaryeong fault system. The dominant strikes of fault planes range from N20◦E-N45◦E in the northern and southern Seoul metropolitan areas, suggesting branch fault development locally. The earthquakes in middle-northern Seoul present N-S directional strike-slip motions at depths ∼7.5 km along the Chugaryeong fault. The spatial earthquake distribution and fault-plane orientations suggest near-vertical continuous faults subparallel with Chugaryeong fault, being seismically active.