ⒺCrustal Velocity Structure of the Northeastern Tibetan Plateau from Ambient Noise Surface-Wave Tomography and Its Tectonic Implications

Abstract Broadband seismic data from the regional seismic network operated by the China Earthquake Administration and 32 temporary seismic stations are used to image the crustal velocity structure in the northeast Tibetan plateau. Empirical Rayleigh- and Love-wave Green’s functions are obtained from interstation cross correlation of con-tinuous seismic records. Group velocity dispersion curves for Rayleigh andLovewaves between 10 and 50 s are obtained using the multiple-filter analysis method with phase-matched processing. The group velocity variations of Rayleigh and Lovewaves overall correlate well with the major geologic structures and tectonic units in the study region. Shear-wave velocity structures were then inverted from Rayleigh- and Love-wave dispersion maps. The results show that the Songpan–Ganzi terrane is associated with a low velocity at depth greater than 20 km. The northern Qilian orogen, with higher elevation and thicker crust compared to the southern Qilian orogen, is also dominated by low velocity at depth greater than∼25 km.However, there is no clear evidence of the low-velocity mid-to-lower crust beneath the southern Qilian orogen as the crustal flow model predicts. The low-velocity zone (LVZ) beneath the northern Qilian orogen may suggest that the crustal thickening and surface uplift of the northern Qilian orogen are related to the LVZ, and the LVZmay be considered as an intracrustal response to bear the ongoing deformation in the northern Qilian orogen. Online Material: Figures of crustal topography, number of group velocity mea-surements, checkerboard tests for NETS stations, and 1D velocity models