Joint Inversion of S-Wave Relative Travel Times and Rayleigh-Wave Phase and Group Velocity Dispersion Curves for the Estimation of Subsurface Seismogenic Faults in the Seoul Metropolitan Area

The rise in seismic activity since the Mw 5.8 Gyeongju earthquake in 2016 has prompted a detailed study of subsurface fault systems in South Korea. Given the potential for a moderate earthquake to cause significant damage and loss of lives in the densely populated Seoul metropolitan area, investigating subsurface fault systems in this region is an urgent task. We jointly invert S-wave travel times and Rayleigh-wave phase and group velocities to identify subsurface seismogenic faults in the Seoul metropolitan area, taking advantage of the complementary resolutions of the three different datasets. We obtained 3,837 of S-wave relative arrival times from 74 broadband stations and 229 teleseismic earthquakes. Additionally, we obtained Rayleigh-wave group-velocity and phase-velocity dispersion curves with periods of 1 to 10 s from 962 and 1,822 station pairs, respectively, using ambient noise cross-correlations. We also incorporated phase velocity maps with periods from 10 to 30 s, previously measured using Helmholtz tomography. Our S-velocity model reveals clustered seismicity in northern Seoul aligned with a low-velocity anomaly, suggesting the presence of an underlying subsurface seismogenic fault. Sharp velocity contrasts are observed linearly along the Pocheon and the Wangsukcheon faults, which extend underneath Seoul to depths of 20~30 km in a southwestern direction. An isostatic gravity study suggests that the Daebo granite intruded during the Jurassic period and is widely distributed underground. This geological setting and the strong low-velocity anomaly observed along the Pocheon Fault indicate the potential presence of subsurface faults caused by rock heterogeneity.