Observation of P-wave velocity anisotropy of the shallow crust in the northern Xiaojiang fault zone: Principle and numerical tests

This paper proposes a new technique， residual slowness vector technique (RST)， for estimating anisotropy of the seismic velocity using the resi-duals of traveling times assuming that 2-D observed velocity field is taken as the superposition of anisotropic field and isotropic one. And then the technique is applied to P traveling time residuals of 3181 events recorded by 24 stations of the Qiaojia Array， a mobile array of seismic broadband stations installed across the northern Xiaojiang fault zone. The residual slowness vectors are calculated within a scale of 0.5°×0.5° in the horizontal direction and 0—5 km in the vertical direction around the observation stations. Finally the polarization directions of the fast P-velocity and slow P-velocity are obtained at all the stations by using the RST. The results show that the directions of the P-velocity at most of the stations are in good agreement. The fast-P directions point to ESE， which nearly agrees with the P axis direction of stress field in northern Xiaojiang fault zone， while the slow-P directions point to NNE， which nearly agrees with the T axis direction， implying that long period of stress action might be one of the important factors causing the P wave velocity anisotropy in this region.