Influence of the 2011 M9.0 Tohoku-Oki Megathrust on Intraplate Seismicity in Regional Distance

Megathrusts produce large permanent lithosphericdisplacements as well as strong transient ground shakingup to regional distances. The lateral permanentdisplacements construct stress shadows in a wide backarcregion. The Korean Peninsula is placed in the far-easternEurasian plate that belongs to a stable intraplate regionwith a low earthquake occurrence rate and diffusedseismicity, and is located in the backarc at ∼1300km in the west from the epicenter of the 11 March 2011M9.0 Tohoku-Oki earthquake. The seismicity around theKorean Peninsula was increased significantly after the2011 M9.0 Tohoku-Oki earthquake, which is not consistentwith the expected seismic-quiescence. Strong seismicwaves cause large dynamic stress changes, incurring fluidmigration and increasing pore fluid pressure in themedia. The lithospheric displacements directing to theepicenter on the convergent plate boundary developtransient radial tension field over the backarclithospheres, which is subparallel with the preseismicambient compressional field. The pore pressure growth andradial tension field decrease the Coulomb failure stress,increasing episodic increases of seismicity in both faultzones and intact media. The ambient stress field isrecovered gradually as the induced stress fielddiminishes with time by tectonic loading. The seismicitychanges with the temporal evolution of stress field. Aseries of moderate-size earthquakes and earthquake swarmsoccur as a consequence of medium response to the temporalevolution of stress field. The long-term evolution ofseismicity is expected to continue until the preseismicambient stress field is fully recovered.