Upper‐Mantle Anisotropy and Dynamics Beneath Northeast Asia: Insight From SKS and Local S Splitting Analysis

Abstract

AbstractTectonics in Northeast Asia are characterized as the deep subduction of the Pacific Plate and the induced volcanism in Northeast China. Seismic anisotropy due to deformations is essential for understanding the mantle dynamics. However, the routine SKS splitting captures the accumulated seismic anisotropy from core‐mantle boundary to the surface, so the interpretation of the SKS splitting measurements is nonunique. Here, benefiting from abundant deep‐focus earthquakes in the subducting Pacific Plate, we measured shear wave splitting using both the SKS and local S waves recorded at Station MDJ to discriminate the anisotropy at different depths under Northeast China. We obtained 61 SKS and 22 local S wave splitting parameters. The SKS splitting measurements show apparent azimuthal variations that can be modeled by two‐layer anisotropy. The fast polarization directions (FPDs) of anisotropy in the upper and lower layers are NE‐SW and NW‐SE, which may be explained by the lithospheric deformation accompanying the Mesozoic fault activity and the asthenospheric deformation due to motion of the Eurasian Plate, respectively. The FPDs of the local S wave splitting measurements are parallel to the direction of the Pacific Plate subduction; the variation of the delay times is related to the take‐off angles. The observations can be explained by the inclined symmetry axis of the aligned metastable olivine in the subducting Pacific Plate.