Deformation of the NE Tibetan Plateau revealed by velocity and azimuthal anisotropy structures

Because of the complex interaction among the Tibetan Plateau, the Alxa block and the Ordos block, the defor-mation mechanisms of the northeastern (NE) Tibetan Plateau have been controversial for decades. To shed light on these mechanisms, we obtain high-resolution 3-D images of the P-wave velocity and azimuthal anisotropy structures beneath the NE Tibetan Plateau and adjacent area. We invert high-quality manually picked first ar-rivals using an eikonal equation-based seismic traveltime tomography method. Tomographic results mainly present two features. First, low-velocity anomalies are evident in the lower crust of the Qilian block. In addition, weak azimuthal anisotropy with NW-SE-oriented fast wave directions (FVDs) is detected in the lower crust. These characteristics indicate compressional deformation of the Qilian block. Second, the lower crust and up-permost mantle below the eastern Qilian orogenic belt display high-velocity anomalies. These high-velocity anomalies are collected with the high-velocity body beneath the Alxa block. This feature may imply that the Alxa block is underthrusting beneath the eastern Qilian orogenic belt. Our tomographic results show that the 1920 Ms 8.5 Haiyuan earthquake occurred near the boundary of the high-velocity anomaly, where the FVDs change dramatically. In addition, we observe that the 1927 Ms 7.9 Gulang earthquake occurred in the normal upper crust and above the strong low-velocity anomalies. From our tomographic results, we propose that two different seismogenic structures were associated with these two great earthquakes.