Velocity Anisotropy in the Crust and Upper Mantle of North China

Abstract

AbstractThis paper presents the result of a surface wave study on azimuthal anisotropy in the crust and upper mantle of North China, and makes a preliminary comparison with the result of S‐wave splitting in the region. The anisotropy pattern of Rayleigh waves of different periods exhibits obvious lateral variation, which is closely related with the tectonic divisions and vertical layering of the crust and upper mantle of North China. In the stable blocks of Ordos and Alxa significant anisotropy is rather uniform in the lithospheric mantle down to 160 km; while in the eastern part of the North China Craton, where lithosphere thinning occurred in Meso‐Cenozoic time, no azimuthal anisotropy is detected in the depth range of 80~150 km, which may indicate that no significant horizontal tectonic movement occurred during the process of lithospheric thinning. The anisotropy in the study region is characterized by obvious layering, as evidenced by the inversion result from surface waves. On the other hand the remarkable scatter of apparent splitting parameters may also be attributed to multi‐layering and/or slanting symmetry axis of the anisotropy. Assuming a multi‐layer anisotropy model, the differences between surface wave and S‐wave splitting in most cases can be qualitatively explained. In the future study the detecting depth and resolving power of surface wave should be increased and more splitting measurements are needed in order to establish a quantitative or semi‐quantitative 3D anisotropy model.