Major earthquakes in continental regions may cause significant damage. Preexisting fault system across megacity receives high attention for possible seismic damages. Earthquake occurrence mechanism is important to assess the geohazard potentials. Continental‐scale Quaternary fault system is developed across the Seoul metropolitan area where the population is the largest in the Korean Peninsula. Historical seismic‐damage records suggest potential seismic hazards in the Seoul metropolitan area. We investigate the fault motions and spatial distribution of earthquakes in the Seoul metropolitan area using a matched‐filter technique that is based on stacked waveform crosscorrelation functions among densely‐deployed seismic stations. The analysis detects 1103 earthquakes that include 360 events with magnitudes (ML) of −0.6 to 2.0 around the Chugaryeong fault and 34 events with magnitudes of −0.5 to 2.7 around Wangsukcheon, Pocheon, and Yeseonggang faults. The seismicity suggests a set of near‐vertical subparallel (or orthogonal) faults that develop from the major faults. A major fault system behaves as a backbone structure that makes branch faults develop, producing seismicity including major earthquakes. The backbone structure may control the fault development that conforms to the ambient stress field. The backbone faults may play a role to increase geohazard potentials. Plain Language Summary The Seoul metropolitan area is the most populated region in the Korean Peninsula. Historical seismic damage records and major Quaternary faults suggest high seismic hazard potentials in the region. The identification of earthquake‐spawning faults is crucial for seismic hazard mitigation. We illuminate the earthquake‐spawning faults from micro to small earthquakes in the Seoul metropolitan. We perform a matched‐filter analysis to find earthquakes in adjacent locations of reported events. Recently‐deployed temporary dense seismic stations allow us to detect microearthquakes in high ambient‐noise environments of the megacity. We detect 1103 earthquakes that include 360 events with magnitudes (ML) of −0.6 to 2.0 around the Chugaryeong fault and 34 events with magnitudes (ML) of −0.5 to 2.7 around Wangsukcheon, Pocheon, and Yeseonggang faults. The earthquake distribution suggests the presence of active near‐vertical subparallel or conjugate faults along the major Quaternary faults that may behave as backbones of earthquake‐spawning faults. Key Points Micro to small earthquakes occur around Quaternary faults in the Seoul metropolitan area Subvertical strike‐slip faults develop along the Quaternary faults, responding to the ambient stress field Clustered earthquakes occur along conjugate or subparallel faults