The Haiyuan fault is a large strike-slip fault in the northeastern (NE) margin of the Tibetan plateau. Using new data recorded by a temporary seismic array, this study obtains crustal seismic anisotropy measurements in different scales across the Haiyuan fault by analyzing splitting of shear-waves from local earthquakes. Parameters of shear-wave splitting (SWS) indicate clear zoning characteristics in seismic anisotropy in the upper crust. The study area is divided into two different anisotropic subzones by the Haiyuan fault. The dominant orientation of fast polarizations is NNE or NE north of the faults and WNW to EW south of the fault. This pattern of spatial variation indicates the controlling impact of Haiyuan fault to observed seismic anisotropy. With a zone of dozens of kilometers from the Haiyuan fault, the dominant orientations of fast polarizations are WNW, generally coincidental with the strike of the Haiyuan fault except for several stations. It indicates that the influence of the fault on stress could reach to about 10 km from the fault distance. Spatial pattern of anisotropy indicates multiple effects by stress, faults and local tectonics. It suggests different anisotropic mechanisms including stress-induced anisotropy and tectonic (or structural) anisotropy. The dominant orientation of fast polarizations can indicate in situ maximum horizontal principal compressive stress although there is always influence of tectonics.

The normalized time-delays are higher around the Haiyuan fault than those away from the fault, suggesting stronger crustal seismic anisotropy in a narrow belt around the strike-slip fault zone. The SWS parameters suggest that the Haiyuan fault is the actual crustal boundary of NE Tibetan plateau, about 200 km north of the reported boundary of the Tibetan plateau block. Combining with other studies in the crust and the lithosphere, this paper infers possible two-layer crustal anisotropy beneath the Yinchuan graben, north of the Haiyuan fault. The dominant orientation of fast polarizations are NNE or NE in the upper crust, probably originated from crack-induced anisotropy, but become WNW or NW in the middle-lower crust, possibly originated from deformation anisotropy.