LAUSR

An M-L 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 similar to N28 degrees E and dip of similar to 78 degrees 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.