The 3D attenuation and anisotropy structure extracted by the Wavegradiometry method resolves the uplift mechanism of the southeastern Tibetan Plateau

The Wave Gradiometry Method (WGM) measures spatial gradients of the wavefield within a subarray to extract phase velocity, wave propagation direction, amplitude perturbation, and radiation pattern within a subarray (Langston, 2007; Cao et al., 2020; Liang et al., 2023). The phase velocity can then be analyzed with respect to azimuths to determine azimuthal anisotropy. The Azimuth-Dependent Dispersion Curve Inversion (ADDCI, Liang et al., 2020) method is used in conjunction with the WGM to extract both 3D velocity and 3D azimuthal anisotropy. Amplitude perturbation accounts for geometrical spreading relative to propagation distance, intrinsic attenuation, and wave scattering within the medium. By eliminating the effects of scattering and geometrical spreading, the dispersion curve of attenuation is obtained, allowing for the determination of the medium's 3D attenuation.

The method is applied to the seismic waveforms collected by the ChinArray conducted in the southeastern Tibetan plateau. The arrays have an average station spacing of 35km. Our results show large variations in fast propagation directions (FPD) and magnitude of anisotropy (MOAs) with depths and blocks. The FPDs are positively correlated with plate moving directions measured by GPS. Low-velocity zones (LVZs) in the middle to lower crust are widely distributed in the Songpan Ganze Terrane and the north Chuan-Dian block. However, the LVZs are not well represented across the Lijiang-Xiaojinghe fault towards the southeastern Tibetan plateau. Low 1/Q values are found in the Sichuan basin and Emeishan Large Igneous Province at all depths. For the Tibetan plateau, low 1/Q values are found at depths shallower than 50km, while high 1/Q values are present at 50km and deeper depths. The low attenuation, combined with the FPDs being dominantly perpendicular to the movement directions of the materials, contradicts the lower crust flow model. However, the pure shearing crust shortening model, which involves the thrusting and folding of the upper crust and the lateral extrusion of blocks, may be the primary mechanism responsible for the growth of the southeastern Tibetan Plateau.

References：

Liang, C., Cao, F., Liu, Z., & Chang, Y. (2023). A review of the wave gradiometry method for seismic imaging. Earthquake Science, 36(3), 254-281. https://doi.org/10.1016/j.eqs.2023.04.002

Cao, F., C.Liang*, Yihai Yang, Lu Zhou, Zhiqiang Liu, Zhen Liu (2022). 3D velocity and anisotropy of the southeastern Tibetan plateau extracted by joint inversion of wave gradiometry, ambient noise, and receiver function, Tectonophysics, https://doi.org/10.1016/j.tecto.2022.229690

Liang, C., Liu, Z., Hua, Q., Wang, L., Jiang, N., & Wu, J. (2020). The 3D seismic azimuthal anisotropies and velocities in the eastern Tibetan Plateau extracted by an azimuth‐dependent dispersion curve inversion method. Tectonics, 39, e2019TC005747. https://doi.org/10.1029/2019TC005747

Langston C A. Wave gradiometry in two dimensions (2007). Bulletin of the Seismological Society of America, 97(2): 401-416, https://doi.org/10.1785/0120060138