Seismic Anisotropy of the Mantle beneath Eastern Asia Based on ScS and S Waves from Deep-Focus Earthquakes

The seismic anisotropy of the mantle is studied based on the data of S and ScS waves from earthquakes occurred in the mantle transition zone over the period of 2007-2013 and recorded by seismic stations in the continental margin of Asia, on Sakhalin Island, and in the southern part of the Kamchatka Peninsula. The measurements of the azimuths of polarization of the fast S and ScS waves in the continental margin of Asia show that they are predominantly oriented in the E-SE directions. Based on the distribution of the shear wave splitting parameters, the symmetry of the medium can be described in terms of a transversely isotropic model with a horizontal symmetry axis and may correspond to horizontal flow in the upper mantle beneath the Amur Plate. The fast azimuths of polarization of ScS wave, which were determined to be of N-NE directions in the northern area of Sakhalin Island and in the continental part of Asia, may correspond to an inclined flow under the conditions of oblique subduction and complex geometry of the downgoing Pacific Plate. In the south of the Kamchatka Peninsula, the S-and ScS-wave azimuths of polarization from the M 8.4 Sea of Okhotsk earthquake are determined to be oriented along the direction of the Pacific Plate motion. The fast-S-wave azimuths of polarization from the aftershocks of the Sea of Okhotsk earthquake and from other large events of 2008-2009 are determined to be nearly parallel to the motion trend of the Pacific Plate, but orthogonal to it for the events of 2008-2009. On the basis of the distribution of azimuths of polarization of the fast S waves, the symmetry of the medium can be described in terms of a transversely isotropic model with the symmetry axis inclined orthogonally to the plane of downgoing plate and oriented westward orthogonally to the trench strike.