Lithospheric deformation of the Ordos block from Rayleigh wave phase velocities and azimuthal anisotropy

We use fundamental mode Rayleigh waves recorded at 45 temporary and permanent broadband seismic stations in the Ordos block, and invert for average phase velocities and azimuthal anisotropy of the 12 periods from 20 to 143 s by the two-plane-wave inversion technique. And one-dimensional shear wave velocity model is inverted from phase velocities. The results show that the phase velocities at the periods from 50 to 100 s are faster than AK135 and exhibit high velocity anomalies. The shear wave velocities show that the high velocity anomaly zone mainly lies at depths shallower than 180 km. The amplitudes of azimuthal anisotropy at periods from 20 to 111 s are below 1%. The small anisotropy indicates weak deformation of the lithosphere. The average fast direction at periods from 20 to 50 s is nearly EW, and the direction at periods from 67 to 143 s changes completely to NW-SE. The change of the fast direction happens in the 80~100 km depth range in the lithosphere. The difference of fast direction indicates that the azimuthal anisotropy in the upper and lower lithosphere may be caused by different deformation mechanisms. The azimuthal anisotropy in upper lithosphere may be frozen anisotropy, and that in lower lithosphere may be recently formed by the deformation resulted from present-day tectonics. The different deformation at different depths in the lithosphere is mainly related with the variations of temperature with depth and counter-clockwise rotation of upper lithosphere caused by NE-NNE direction compressional stresses of the Tibetan Plateau.