Direct Inversion for Three-Dimensional Shear Wave Speed Azimuthal Anisotropy Based on Surface Wave Ray Tracing: Methodology and Application to Yunnan, Southwest China

Azimuthal anisotropy retrieved from surface waves is important for constraining depth-varying deformation patterns in the crust and upper mantle. We present a direct inversion technique for the three-dimensional shear wave speed azimuthal anisotropy based on mixed-path surface wave traveltime data. This new method includes two steps: (1) inversion for the 3-D isotropic Vsv model directly from Rayleigh wave traveltimes and (2) joint inversion for both 3-D Vsv azimuthal anisotropy and additional 3-D isotropic Vsv perturbation. The joint inversion can significantly mitigate the trade-off between strong heterogeneity and anisotropy. With frequency-dependent ray tracing based on 2-D isotropic phase speed maps, the new method takes into account the ray-bending effect on surface wave propagation. We apply the new method to a regional array in Yunnan, southwestern China. Using Rayleigh wave phase velocity dispersion data in the period band of 5-40 s extracted from ambient noise interferometry, we obtain a 3-D model of shear wave speed and azimuthal anisotropy in the crust and uppermost mantle in Yunnan. This model reveals that two midcrust low-velocity zones are possible weak channels, and the azimuthal anisotropy at a depth of 5 to 30 km is mainly controlled by nearby strike-slip faults, some of which also approximately coincide with the lateral boundaries of the crustal low-velocity zones. Approximately south of 26 degrees N, the upper crustal azimuthal anisotropy from our model is significantly different from the upper mantle anisotropy inferred by shear wave splitting, indicating different deformation styles between the crust and upper mantle in southern Yunnan.