Crustal anisotropy in the Ural Mountains foredeep from teleseismic receiver functions

Radial and transverse teleseismic receiver functions (RFs) at GSN station ARU, in central Eurasia, display variation in back-azimuth Ψ consistent with a 1-D anisotropic crustal structure. In a broad Ψ range, the transverse RFs possess a strong phase at ∼ 5-sec delay relative to direct P, with a polarity reversal at Ψ ∼ 50°. The radial RFs peak at the transverse-RF polarity reversal for this converted phase. The first motion of the transverse RFs varies with Ψ also, reversing polarity at Ψ ∼ 345°. The azimuthal variation can be modeled by a 5-layer velocity profile with substantial (15%) seismic anisotropy in both the lowermost crust and a low-velocity surface layer. Assuming hexagonal symmetry, the lowermost crust has a tilted "slow" symmetry axis i.e. an oblate phase velocity surface. The strike of the axis is oblique to the north-south Urals trend, but deviates < 20° from the mantle fast-axis inferred from SKS splitting. The magnitude and tilt of the model's anisotropy suggests that fine layering and/or aligned cracks augment mineral-orientation anisotropy near the top and bottom of the crust.