Time-advanced occurrence of moderate-size earthquakes in a stable intraplate region after a megathrust earthquake and their seismic properties

The distance-dependent coseismic and postseismic displacements produced by the 2011 M(W)9.0 Tohoku-Oki megathrust earthquake caused medium weakening and stress perturbation in the crust around the Korean Peninsula, increasing the seismicity with successive M(L)5-level earthquakes at the outskirts of high seismicity regions. The average M(L)5-level occurrence rate prior to the megathrust earthquake was 0.15 yr(−1) (0.05–0.35 yr(−1) at a 95% confidence level), and the rate has increased to 0.71 yr(−1) (0.23–1.67 yr(−1) at a 95% confidence level) since the megathrust earthquake. The 2016 M(L)5-level midcrustal earthquakes additionally changed the stress field in adjacent regions, inducing the 15 November 2017 M(L)5.4 earthquake. The successive 2016 and 2017 moderate-size earthquakes built complex stress fields in the southeastern Korean Peninsula, increasing the seismic hazard risks in the regions of long-term stress accumulation. The increased seismic risks may continue until the medium properties and stress field are recovered.