Seismic Anisotropy In The Upper Mantle Beneath The Northeastern Tibetan Plateau From Shear Wave Splitting Analysis

The northeastern Tibetan Plateau,a complex area which is bounded by the Tarim basin to the northwest,the Ordos plateau and Alashan block to the northeast and the Qiangtang block to the south,is one of the major tectonic units in mainland China.Systematic study of the deep structure,deformation mechanism and geodynamics of the region can help to better understanding the lateral expansion mechanism of the Tibet Plateau.Seismic anisotropy plays a great role in geodynamics such as mantle convection,the formation and destruction of plates,and orogeny.Shear wave splitting is an effective tool to investigate anisotropy.When seismic waves pass through an anisotropic medium,shear waves will split into two types of waves which propagate approximately orthogonal to each other with different velocities.The orientation of the fast shear wave provides constraints on the elastic structure of the medium and the delay time between the fast and slow shear waves reflects the intrinsic anisotropy and thickness of the anisotropic medium.The two splitting parameters,the fast polarization direction and delay time,offer insights into deformation processes.Using teleseismic events to identify SKS phases can reveal the upper mantle anisotropy beneath the receiver station.In this study,we collected teleseismic SKS phases recorded by 238 seismographic stations including temporary and permanent stations during 1992 and 2017 in the northeastern Tibetan Plateau.By simultaneously using Rotation-Correlation method,Minimum Energy method and Eigenvalue method,253 teleseismic events with clear SKS phases are selected,and a total of 798 pairs of good splitting parameters are obtained.Based on the comparison between our results and the GPS velocity field,we analyzed the relationship between crust and mantle anisotropy in the northeastern Tibetan Plateau.The results show that in the west and north sides of the Qilian block,the fast wave direction is in NW-SE direction,consistent with the direction of the main fault in this area,however,the direction of the GPS velocity field is in NE—SW direction.The differece between the fast wave direction and GPS velocity field may between crust and mantle suggests that the deformation mechanism in the crust and mantle is different and decopuled in this area.In the Xining area on the eastern side of the Qilian block,the upper-layer fast-wave polarization direction is 66°,consistent with the direction of the GPS velocity field;the lower-layer fast-wave polarization direction is 114°,which may reflect the flow direction of the asthenosphere.In the Qaidam block,the fast-wave polarization direction is in the NEE-SWW direction;in the Songpan-Ganzi block and the Qiangtang block,the fast-wave polarization direction rotates clockwise from NEESWW to E-W to NWW-SEE.The fast-wave directions of these blocks are generally consistent with the direction of the GPS velocity field which means that the crust and the upper mantle possibly have the same deformation pattern.