Upper crustal anisotropy in the eastern Himalayan syntaxis

Based on the local seismic recordings of 16 seismic stations from the lower Yarlung Zangbo River array in 2016, we present 369 valid shear wave splitting parameter pairs by analyzing shear wave particle motion in the shear wave window so as to analyze the upper crust anisotropy characteristics in the eastern Himalayan syntaxis. We analyze the spatio-temporal characteristics of 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 are close to or locate in the active fault zone are consistent with the strikes of the faults, which are mainly reflected in the 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 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 from the Yarlung Zangbo River fault and the 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 maximum horizontal compressive stress directions NNE in the eastern Himalayan syntaxis, which reflects that 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.