The subsurface geological structures can be inferred from seismic velocity structures. The comprehension of seismic velocity structures is important for seismic hazard mitigation. Active source studies have limitations due to costs and imaging depths. The low seismicity in the Korean Peninsula causes difficulty to perform tomographic inversions based on passive sources. Ambient noise tomography is useful for low seismicity regions. Ambient noise tomography is applied to image shallow crustal

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... ocity structure at local scale (Picozzi et al., 2009; Pilz et al., 2013) . The method illuminates the geological structures (Green et al., 2017; Wang et al., 2019) , and gas and oil reservoirs (Chmiel Abstract We investigate the upper-crustal seismic velocity structure in the Seoul metropolitan area, where about 20 million people live. The Chugaryeong fault zone (CFZ) is placed in this area, but the seismic hazard potential remains unclear. We conducted ambient noise tomography to illuminate the high-resolution upper-crustal structure in the Seoul metropolitan area. We analyzed continuous vertical seismic records for ∼5 months from a dense seismic array with 77 broadband stations. Group velocity dispersion curves and tomographic maps were extracted between 0.5 and 10 s periods. We inverted 3-D group velocity tomography models up to a depth of ∼10 km from the group velocity maps. The shearwave velocity model is consistent with the geological features. High-velocity anomalies at shallow depths are correlated with the surface topography and geology. The CFZ is located at a low velocity below the 5 km depth and presented as the simplified model. The large V S contrast regions are located beneath NS-trending faults. The cross-sections coincide with the near-vertical strike-slip faults in this area. In the southern region of the Seoul metropolitan area, low-velocity anomalies correlate with high heat flow regions. Our results effectively suggest high resolution upper-crustal structures and subsurface hidden faults in the urban area. Plain Language Summary The Chugaryeong fault zone (CFZ) runs across the Seoul metropolitan area, where more than 20 million people live. The Chugaryeong fault system was repeatedly reactivated in the Quaternary. This study investigates the upper crustal structures in the Seoul metropolitan area using ambient seismic tomography based on densely deployed seismic stations. The seismic velocity structures are correlated with surface topography and geology. Low velocities are present in the southern Seoul metropolitan area of high heat flows. Vertically consistent low shear-wave velocity anomalies are observed at depths of 5-10 km in the CFZ. Lateral variations in seismic velocities reveal the fault structures, which is consistent with seismic activities. The lateral gradients of shear-wave velocities illuminate the near-vertical fault geometry. The upper-crustal shear-wave velocity model may provide information to assess seismic hazard potentials in the Seoul metropolitan area.