An 5.8 earthquake, which is large for a stable continental region, occurred in southeastern Korea on 12 September 2016. Ten days of data from a temporary seismic network deployed immediately after the mainshock are combined with data from permanent seismic stations to determine high‐precision locations of early aftershocks to reveal the geometry of the causative structure at depth. Well‐constrained relative earthquake hypocenters and focal mechanisms are used to define the subsurface fault plane with a strike of and dip of to the east‐southeast. This fault plane extends from 12 to 15 km depth and may have been responsible for most of the early earthquakes in the Gyeongju earthquake sequence. A pre‐existing weak zone in a strike‐slip duplex that formed from subsidiary Riedel shears beneath the Yangsan fault system may have been reactivated to nucleate the mainshock and aftershocks.