Dynamic seismic response of a stable intraplate region to a megathrust earthquake

- Strong occurrence of dynamically-triggered earthquakes in a stable intraplate by a megathrust earthquake is observed.
- The earthquake triggering decays with time following the Omori law.
- The occurrence rate of triggered earthquakes is comparable to those of active tectonic regions.

The 2011 M9.0 Tohoku-Oki megathrust earthquake produced strong ground motions with a peak ground acceleration of 1.52 cm/s 2 in the Korean Peninsula, inducing large dynamic-stress changes in the crust. Sixty-one triggered earthquakes with magnitudes of 0.5-2.5 including 17 unusual, spatially-clustered events were identified from continuous seismic record sections of dense seismic networks for 18 h following the megathrust earthquake. The triggered earthquakes occurred in regions of high seismicity. The earthquake occurrence frequency increased after the megathrust earthquake, keeping the Gutenberg-Richter b value invariant. The seismic occurrence rates decreased with time following a modified Omori law. The focal mechanism solutions of triggered earthquakes are consistent with those observed before the megathrust earthquake. An unusual earthquake swarm displays apparent migration with a speed of 28.6 ± 4.4 m/h. The changes in pore fluid pressure induced by strong seismic waves may have caused the triggering of earthquakes. The duration and strength of dynamic triggering may be highly dependent on the magnitude and distance of megathrust earthquake.