Crustal and uppermost mantle S-velocity structure of the Seoul metropolitan area on the Korean Peninsula from Helmholtz tomography

The Seoul metropolitan area, the most densely populated part of the Korean Peninsula, features complex subsurface structures and seismogenic faults, though their characteristics remain ambiguous due to low seismicity and limitations in fault investigation. High-resolution velocity models can provide constraints for identifying subsurface faults by detecting elongated low-velocity anomalies along fault zones. Recently, a dense seismic network was deployed in this area, facilitating the use of Helmholtz tomography, an array-based method that accounts for finite-frequency effects. Utilizing Helmholtz tomography, we obtained a high-resolution S-wave velocity model down to a depth of 50 km with waveform data recorded at 74 broadband seismic stations. We found that a linear low-velocity anomaly along the Pocheon fault extends to the uppermost mantle, with an increasing width with depth. In contrast, the Dongducheon fault, which traverses Seoul from north to south, is not well imaged, indicating its current weak activity. Another linear low-velocity anomaly extends southwest through Seoul from northern Seoul, potentially representing the extension of the Pocheon fault based on similar strike and dip directions. Additionally, a large lateral low-velocity anomaly is identified in the lower crust beneath the northern part of the Seoul metropolitan area, interpreted as a ductile décollement, connected with the Pocheon, Wangsukcheon, and possibly Gyeonggang faults. This study successfully identified the extensions and orientations of subsurface faults beneath the Seoul metropolitan area down to the uppermost mantle, which is critical for seismic hazard predictions and earthquake simulations in this highly populated area.