Temporal change of seismic velocities and induced seismicanisotropy in the backarc continental lithosphere after amegathrust earthquake

The large coseismic and years-long-lasting postseismicslips of the 2011 Mw 9.0 Tohoku-Oki earthquake caused adistance-dependent lithospheric deformation, whichresulted in the development of a transient uniaxialtensional stress field over the regional backarccontinental lithosphere. We report on the transientdepth-dependent seismic velocity change in thelithosphere of the eastern Eurasian plate and subsequentgradual recovery of medium properties for several yearsafter the megathrust from an analysis of seismic ambientnoises. The seismic velocity changes and recovery ratesdisplayed azimuthally modulating harmonic variationsdepending on the 4 times the horizontal azimuths from thegreat-circle direction to the epicenter of themegathrust. The coseismic shear velocity reduction alongthe great-circle direction to the epicenter variedbetween 1 and 5 % depending on the depth in thelithosphere of the Korean Peninsula. The depth-dependentmedium perturbation can be ascribed to thedepth-dependent rheological and lithological propertiesin the lithosphere. The recovery of seismic velocities issuggestive of the reconstruction of the ambient stressregime by tectonic loading as a consequence of decreasingpostseismic slips.