Multiple surface wave tomography methods and their applications to the Tibetan Plateau

Surface wave tomography is a widely used geophysical method to measuring seismic velocity and anisotropy in the crust and upper mantle. This paper briefly reviewed the history of surface wave tomography, and summarized the principles and advantages of multiple surface wave tomography methods (i.e., two-station method, two-plane-wave method, Eikonal and Helmholtz surface wave tomography, ambient noise tomography, and direct surface wave tomography). The theory and application of the two-station method are straightforward, but this method requires the earthquake and seismic stations to be located in the same great circle. This restriction results in a low lateral resolution phase velocity map for regions where stations are sparse and deployment times short. The two-plane-wave method can overcome the effect of scattering and multipathing of seismic wave propagation on phase velocity dispersions, but this method requires high-quality surface wave waveforms and is usually suitable for regional seismic networks. The Eikonal and Helmholtz surface wave tomography can directly compute the phase velocities and azimuthal anisotropy straightforwardly without any processes of forward modeling and inversion, but this method is limitedly suitable for high-density and orderly seismic arrays. In comparison to the Eikonal surface wave tomography method, the Helmholtz surface wave tomography approach analyzes both the phase and amplitude of the waveform and can produce better results. Ambient noise tomography can obtain high-resolution crustal structure without seismic events. However, this method is difficult to obtain long-period phase velocities, resulting in a lack of constraints on the mantle and lithosphere structure. Direct surface wave tomography can obtain the shear-wave velocity and azimuthal anisotropy from dispersive curves without the inversion process of phase velocity maps. We conducted detailed comparisons of the phase velocity maps at the short-intermediate periods (20～40 s) previously obtained from these surface wave tomography methods in the central-northern, northeastern and southern Tibetan Plateau. The comparisons demonstrated that the Rayleigh and Love wave phase velocity maps from different tomography methods show highly consistency in velocity variation patterns at same periods. The results show that the plateau's interior by characterized with low-velocities in phase velocity maps at intermediate and long periods, whereas relatively high velocities dominate adjacent regions, such as the Qaidam Basin and Sichuan Basin. These features indicate that the Tibetan Plateau's mid-lower crust and upper mantle tend to be weaker and easily deform during the northward movement of the Indian plateau and the barrier of the strong blocks (e.g., the Qaidam Basin and the Alashan Platform). The short periods phase velocity maps (i.e., 20～40 s) reveal that the mid-lower crustal flow of the Tibetan Plateau escapes southward surrounding the weakening zones (i.e., the Red River Fault and the Xianshui River Fault) due to the barrier of the strong Chuandian tectonic block. The Qilian Orogen also shows low velocities in the phase velocity maps at the short-intermediate periods, which is likely due to thermal anomaly with localized mantle upwelling. It is noteworthy that the Rayleigh-wave and Love-wave phase velocities at different period ranges (e.g., 4～200 s) can be obtained via incorporating earthquake surface wave tomography and ambient noise tomography. Those phase velocities can be used as input to inverting three-dimensional shear wave velocities and radial anisotropy models simultaneously. This paper envisions that integrating higher-mode earthquake surface wave tomography, adjoint tomography, and joint inversion could obtain higher resolution and more reliable crustal and upper mantle structures.