Partitioned Fault Movement and Aftershock Triggering: Evidence for Fault Interactions During the 2017 M_w 5.4 Pohang Earthquake, South Korea

The 2017 Pohang earthquake (M_w 5.4) is the largest earthquake associated with fluid injection activities. We report new characteristics of the earthquake and propose a dislocation-type model explaining previously reported observations. We identify fault geometry by relocating the hypocenters of 1,132 events that occurred during the first 3 months and then resolve their source regions as the northern, central, and southern patches, based on event groups with similar waveforms. The spatial features of these similar waveform groups, in addition to our obtained source mechanisms, indicate that oblique contraction is prevalent in the source region: Reverse faulting dominates the southern fault and the deeper part of the central fault; near-parallel strike-slip sense controls the northern fault and the shallower part of the central fault. Furthermore, we identify a migrating aftershock pattern that matches the fluid diffusion process along both sides of the northern and central faults. This observation suggests the interconnection of the two faults, allowing fluid transport, and implies mainshock coseismic movement along the fault intersection. The coseismic slip of the fault intersection can induce a fault-valve process, which explains the aftershock migration pattern along the two intersecting faults. The proposed fault interaction accounts for the previously reported uplift between the two intersecting faults and successfully reproduces the non-double-couple mechanism of the mainshock. Our results raise the question of fluid-faulting interactions in the aftershock seismicity of the Pohang earthquake, and the complex fault movement provides insight into the rupture process that allowed the Pohang earthquake runaway.