The 12 September 2016 M L 5.8 midcrustal earthquake in the Korean Peninsula and its seismic implications

The seismicity in the Korean Peninsula has increased since the 2011 M w 9.0 Tohoku‐Oki megathrust earthquake. Two strike‐slip earthquakes with magnitudes of M L 5.1 and 5.8 occurred in the southeastern Korean Peninsula on 12 September 2016. The two events occurred within 48 min. The M L 5.8 earthquake was the largest event in the Korean Peninsula since 1978 when national seismic monitoring began. Both events produced strong high‐frequency ground motions. More than 500 aftershocks with local magnitudes greater than or equal to 1.5 followed the events for 2 months. An unreported subsurface strike‐slip fault with a dip of 65° to the east and a strike of N27°E was responsible for the earthquakes. The fault ruptured at depths of 11–16 km, resulting in a rupture plane of ∼26 km 2 . The aftershock distribution displayed horizontal streaks at a depth of ∼14 km, which was consistent with the focal mechanism solutions from long‐period waveform inversion. The number of aftershocks decreased exponentially with time. The two M L 5.1 and 5.8 earthquakes produced regional Coulomb stress changes of −4.9 to 2.5 bar. The spatial distribution of the aftershocks correlated with the Coulomb stress changes. The peak dynamic stress induced by strong ground motions reached 14.2 bar. The groundwater levels changed coseismically in some regions of decreased static stresses. The earthquakes on previously unidentified faults raised attention for the potential seismic hazards by earthquakes with long recurrence intervals. Moderate‐sized earthquakes occurred on a subsurface hidden fault Midcrustal earthquakes occurred as a consequence of stress perturbation by a megathrust earthquake The static and dynamic stress changes caused by the main shock induced aftershocks