Subsurface fault imaging beneath the Seoul metropolitan area using joint inversion of S-wave travel times and Rayleigh-wave dispersion

Abstract

Constraining seismogenic fault structures is critical in the densely populated area such as the Seoul metropolitan area of the Korean Peninsula, where historical records document damaging earthquakes. Recent increase in clustered seismicity has highlighted the need for improved characterization of subsurface fault systems, motivating the deployment of dense seismic arrays that enable high-resolution imaging of the crust and upper mantle. In this study, we perform a joint inversion of S-wave travel times and Rayleigh-wave phase and group velocities recorded by the dense seismic arrays to resolve low-velocity anomalies associated with major faults down to 40 km depth. Our results reveal a shallow low-velocity anomaly along the Pocheon Fault and a deeper anomaly along the Wangsukcheon Fault extending into the upper mantle, including previously unrecognized fault extensions. An earthquake swarm in northern Seoul appears to be associated with the southwestward extension of the Pocheon Fault. We also image, for the first time, the Gyeonggang Fault dipping at ~ 60° to the northwest. These findings provide new constraints on subsurface fault geometry and highlight the importance of high-resolution seismic imaging for assessing seismic hazards in intraplate urban setting.