The Eastern Himalayas Structure Has Anisotropic Characteristics Of The Crust

The Himalayan orogenic belt is the frontier of the collision between the Indian and Eurasian Plates,and its orogeny has been extensively considered by many geoscientists.The Eastern Himalayan syntaxis is locate on the southeast margin of the Tibet Plateau,which is the axis of the large-scale shortening of the crust and the change of the tectonic movement direction in the eastern Himalayan orogenic belt.On the whole,the trend of the structural faults changed from NE to NW,the geophysical results also show the crustal motion velocity field measured by GPS and the fast polarization direction derived from the SKS wave splitting measurements both rotate 90° clockwise around the Eastern Himalayan syntaxis.There are dramatic topography,complex deep structures and frequent strong earthquakes in the Eastern Himalayan syntaxis.Therefore,the Eastern Himalayan syntaxis is a natural laboratory for geodynamics research.Seismic anisotropy is the most direct seismological means to reveal the deep deformation.One of the most clear-cut manifestations of anisotropy in seismic data is shear wave splitting.The two basic splitting parameters,the fast polarization direction and the delay time between the fast and slow shear waves,are measures of the orientation and magnitude of mantle deformation,respectively.Thus,shear wave splitting is widely used in the deformation characteristics and dynamic interpretation of the crust and mantle.For the upper crustal anisotropy,the anisotropic deformation characteristics is determined mainly by the near vertical incident Sg wave of local earthquake.In general,the upper crustal anisotropy is caused by the directional arrangement of micro-fractures.Shear wave splitting reveals the micro-fracture structure controlled by the stress field.Therefore,it is used to evaluate the characteristics of regional stress field and active faults.In this paper,we will collect the waveform data of local earthquake from the Lower Yarlung Zangbo River seismic array and measure the shear wave splitting parameters,then,based on the results of shear wave splitting,we will analyze the characteristics of the upper crustal anisotropy in the Himalayan syntaxis and its dynamic related to plate collision.Based on the local seismic recordings of 16 seismic stations from the Lower Yarlung Zangbo River seismic array in 2016,we present 369 valid shear wave splitting parameter pairs by analyzing shear wave particle motion in the shear wave window and analyze the upper crust anisotropy characteristics in the eastern Himalayan syntaxis.We analyze the spatio-temporal characteristics in the shear wave splitting parameter.The spatial distribution of fast polarization directions shows a clear change from west to east that are oriented near EW to NE,to near NS or NNE,and then to NW.The fast polarization directions of most stations close to or locate in the active fault zone are consistent with the strikes of the faults,which are mainly reflected in station ZOS in the Mozhugongka fault,four stations(WOL,NYG,ZIB and DOJ)in the Yarlung Zangbo River fault,two stations(BEB and DEX)in the Motuo fault,and two stations(BAX and DAM)in the middle section of Polong-Pangxin fault.At the stations mentioned above,the fast polarization directions are consistent with the strikes of the faults,which demonstrates upper crustal seismic anisotropy is mainly controlled by the structure.The fast polarization directions at two stations(LAD and YIG)far away the Yarlung Zangbo River fault and station TOM located at the intersection of the east section of the Yarlung Zangbo River fault and the Jiali fault are inconsistent with the fault strike.However,the fast polarization directions at the above three stations(LAD,YIG and TOM)are consistent with the NNE maximum horizontal compressive stress directions in the eastern Himalayan syntaxis,which reflects upper crustal anisotropy is also controlled by the local stress field.With regard to the temporal variation,the shear wave splitting parameters at each station do not show the regular change characteristics with time,which is related to the weak intensity of seismicity in the study area in 2016.The large difference of shear wave splitting parameters and the large dispersion of each station in the study area reflect the strong deformation and complicated structure in eastern Himalayan syntaxis.