Mantle Seismic Anisotropy beneath the Amur Plate According to the Data of ScS Waves from Deep-Focus Earthquakes

Abstract

Mantle seismic anisotropy beneath the Amur Plate is studied using the data of ScS waves reflected from the outer core of the Earth from local deep earthquakes in the area of five stations located in Primorye and Priamurye regions. The results of measuring the parameters of the split ScS waves near the stations show the dominance of the polarization azimuths of the fast wave along the eastern directions and are consistent with azimuthal anisotropy and the direction of motion of the Pacific Plate (300°) and Amur Plate (~120°) depending on the epicenter–station direction. In Primorye, in the vicinity of the TEY station, the polarization azimuths of the fast ScS wave dominate in the interval of NE–E directions orthogonally to the lines of the mantle flow along the complex 3D surface of the subsiding Pacific Plate. It is revealed that the delay time of ScS waves increases to 2 s and 3.4 s in the upper mantle and in the mantle transition zone, respectively, as the depth of events increases. The highest degree of anisotropy manifests itself in the upper mantle. In the case of the vertical propagation of waves under conditions of horizontal mantle flow, the difference in the arrival times of the ScS waves is the lowest. The anisotropy in the upper zone of the transition mantle can be related to the wadsleyite texture with the polarization of the fast ScS wave parallel or orthogonally to the motion of the stagnant plate. The anisotropy in the upper part of the lower mantle is associated with the texture of perovskite and periclase with the orientation of the symmetry axis and the polarization of the ScS waves in parallel to the plate subsidence and the subduction direction.