利用瑞利面波相速度和方位各向异性研究鄂尔多斯块体的岩石圈变形特征

利用在鄂尔多斯块体内部布设的45个宽频带流动台站和固定台站的资料，用双平面波方法反演了20~143 s共12个周期的基阶瑞利面波的平均相速度和方位各向异性，并反演了一维S波速度结构.反演结果显示50~100 s中长周期的瑞利面波相速度高于AK135速度模型的相速度，为高速异常，S波速度显示高速异常主要位于180 km深度范围内，表明鄂尔多斯块体保留有厚的高速岩石圈.20~111 s周期的方位各向异性强度小于1%，较小的各向异性表明鄂尔多斯块体岩石圈变形较弱.20~50 s周期的平均快波方向为近EW向，67~143 s周期的平均快波方向为NW-SE向，相对发生了整体改变，快波方向的转变约开始于80~100 km深度范围，这表明岩石圈上下部存在着由不同变形机制导致的各向异性.上部岩石圈中各向异性可能主要为残留的“化石”各向异性，而下部岩石圈各向异性可能是现今板块构造运动导致的变形而形成.鄂尔多斯块体岩石圈垂向上的变形差异可能主要与岩石圈温度随深度的变化以及青藏高原NE-NNE向挤压引起的上部岩石圈逆时针旋转有关.

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.