Eikonal equation-based seismic anisotropy tomography and its application to the imaging of the crust and uppermost mantle

• Main Author: Liu, Yongsheng
• Other Authors: Tong Ping
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Mathematics
• Online Access: https://hdl.handle.net/10356/155049
• Institution: Nanyang Technological University

Seismic anisotropy describes how the velocity of seismic wave varies with propagation direction in the Earth’s interior. Studying depth-dependent anisotropy variations is a way of estimating the crustal deformation and mantle flow patterns, which may help us better assess seismic hazards and understand ongoing tectonic dynamics. In this thesis, we develop a novel eikonal equation-based seismic anisotropy tomography method with multiple-grid model parametrization to construct 3D high-resolution anisotropic velocity model of the crust and upper mantle by inverting large amounts of P-wave first arrivals. The traveltime field and ray paths are accurately determined by numerically solving the Eikonal equation with the fast marching method. Multiple grids are utilized to parametrize the model space in the inversion. At each iteration, the output model is the average of the multiple models achieved from all the inversion grids. After that, I apply the method in some seismologically active and/or densely populated regions (northern California, southern Sumatra and Salton Trough) by inverting newly available datasets or public high-quality datasets. In the northern California, a 3D high-resolution P-wave anisotropic velocity model for the crust is developed. In the upper crust, the fault-parallel fast velocity directions (FVDs) generally prevail in the northern Coast Range due to the geological structure (e.g., fault-zone fabric and sedimentary bedding); while the Great Valley and the northern Sierra Nevada mainly show the NE-SW FVDs, possibly caused by the regional maximum horizontal compressive stress (SH_max). Contrarily, seismic anisotropy in the mid-lower crust may be attributed to the lattice-preferred orientations of mica schists. The San Francisco bay area (2-6 km) is characterize by both strong velocity contrasts and fault-oblique FVDs (~450 or above), suggesting that the faults there are mechanically weak. In the southern Sumatra, new P-wave anisotropic velocity models as well as the S-wave isotropic velocity model of the crust and uppermost mantle are derived from a newly available dataset. All the tomographic results in the uppermost mantle reveal a clear high-velocity belt associated with the subducting Indo-Australian slab. The upper crust exhibits complex variations in the fast P-wave velocity direction associated with the SH_max and/or geological structure, while the orientated plastic flow possibly triggered by the oblique subduction contributes to the trench-normal FVDs in the lower crust. The anisotropy in the mantle wedge is closely related to the 2D corner flow or 3D complex mantle flow. Trench-parallel FVDs within the subducting slab may suggest the presence of the frozen-in anisotropy. In the Salton Trough, new crustal models of the P-wave azimuthally anisotropic velocity, S-wave isotropic velocity and Vp/Vs ratio are developed. High Vp and Vp/Vs ratio (> 1.8) in the lower crust of the Salton Trough possibly reflect the underplated gabbroic rocks as a result of the extension-induced partial melting of the upwelling asthenospheric materials. The FVDs in the study area generally correlate with the direction of the SH_max except along the main fault traces where fault-parallel FVDs are found. However, the FVDs in the Salton Trough show complex features possibly due to the compression, faulting, block rotation or thermal anomaly. In all, our velocity models suggest that the faults in southern Salton Trough basin may have a high risk of potential mechanical failures due to the complexities of stress distribution and geological structures as well as the presence of the large volume of fluids. The applications in above target regions demonstrate that the novel P-wave seismic anisotropy tomography method is a useful and reliable tool to map anisotropic velocity structures in complex media.