Keywords:Pohang earthquake, Finite fault inversion, complex fault geometry
The 15 November 2017 Mw 5.5 Pohang earthquake produced strong ground motions in a city of Pohang, South Korea with damaged buildings and injured people. This earthquake was recorded as the largest earthquake in seismicity near enhanced geothermal systems. With its focal depth of 4 km, the earthquake generated a measurable static deformation, which gives an opportunity to quantify its source mechanism. In this study, we estimated the rupture process of the Pohang earthquake from regional seismograms by using multiple-time-window linear inversion and segmented fault geometry inferred from its aftershock distribution. The initiated rupture propagated southwest downward by 2 km and generated maximum peak slip up to ~30 cm. The rupture then propagated to the opposite direction, and slip patches located NE direction are sequentially reactivated over the potential barriers between two adjacent fault segments. Two largest aftershocks of MW 4.3 and MW 4.6 are located on the edge of the slip distribution, which represents the existence of local asperities. Rupture duration of 4 s and fault dimension of 40 km2 suggest relatively low stress drops, which may be caused by the retardation of rupture propagation by patches with strong asperities with a high stress state. The complicated rupture process of the Pohang earthquake illustrates how the rupture evolves in a complex fault system with possible fluid interaction and reconciles the importance of seismic barriers that can control following aftershocks as well as the slip history.