Identification of Quasi-Love waves and its implication of east-westward azimuthal anisotropy gradient in the upper mantle of the central Tibet
青藏高原中部Quasi-Love波的识别及其转换点揭示的东西向方位各向异性变化

Seismic anisotropy can be studied by four basic seismic wave behaviors, e.g, direction-dependent travel time anomalies, shear-wave birefringence, surface-wave scattering, and direction-dependent conversion of compressional waves to shear waves. In this paper, Quasi-Love (QL) waves, generated by surface wave scattering, are utilized to determine the variation of uppermost mantle anisotropy of Tibet plateau. Firstly, QL waves are identified from the seismograms of the selected paths recorded by the CAD station. The observed QL waves are confirmed by distinguishing QL waves from higher mode Rayleigh waves or other body waves, which are absent in synthetic seismograms. Furthermore, the location of azimuthal anisotropy gradient is estimated from the group velocities of Rayleigh wave, Love wave and QL wave. Finally, the characteristic of azimuthal anisotropy of upper mantle in Tibet plateau is summarized. The reason of the existence of azimuthal anisotropy gradient can be explained by east-west variation of tectonics. Also the other reason is the force from the upper mantle flow which has led to voluminous rock deformation.