Ambient noise tomography of upper crustal structures and Quaternary faults in the Seoul metropolitan area and its geological implications

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 shear-wave 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 VS 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.