Isotropic and Anisotropic Seismic Tomography Using Active Source and Earthquake Records

This thesis involves two regional seismic tomography studies: One for the isotropic P-wave velocity in the upper crust beneath Taiwan using first-arrival times from active sources; the other for the shear-wave anisotropy structure under Southern California using SKS splitting intensity measurements. In the isotropic tomography for Taiwan, we use waveform records from the 10 explosions in 2008 conducted by the TAiwan Integrated GEodynamics Research (TAIGER) project. A large dataset of high-quality ground-truth first-arrival times are hand-picked from the active-source records at ~1400 sites throughout Taiwan, which greatly enhance our capability to determine the crustal velocity beneath Taiwan with unprecedented accuracy and resolution., especially along the two north-south and two east-west island-wide linear transects with densely-deployed receivers. At first, these first-arrival times are used to evaluate the existing tomography models for Taiwan. Results show that tomography models obtained from traditional travel time inversions provide consistent and qualitatively correct locations of larger-scale velocity perturbations. However, small-scale features are inconsistent among different models, and their velocity perturbations are mostly underestimated. Then we use our ground-truth first-arrival times to refine the P-wave velocity model. With a trial-and-error procedure, we acquire the best 2D models along a number of shot-to-station profiles by fitting the first-arrival times. Finally a partition modelling approach is employed to invert for a 3D model in northern Taiwan based on a collection of the crisscrossing 2D models that densely transect across the region. The resulting structural variations in our 3D model correlate remarkably well with the surface geological features that are distinctly shaped by the orogenic and tectonic history in Taiwan. In the anisotropic tomography for Southern California, our purpose is to resolve the spatial variation of anisotropy in the upper mantle which plays an important role in our understanding of the Earth’s internal dynamics. Shear-wave splitting has always been a key observable in the investigation of upper-mantle anisotropy. However, the interpretation of shear-wave splitting in terms of anisotropy has been largely based on the ray-theoretical modelling of a single vertically incident plane SKS or SKKS wave. In our study, we use sensitivity kernels of shear-wave splitting to anisotropic parameters calculated by the normal-mode theory, which automatically accounts for the full-wave effects including the interference of SKS with other phases of similar arrival times, the near-field effect, and multiple reflections in the crust. These full-wave effects can lead to significant variations of SKS splitting with epicentral distance and are neglected in ray theory. We image the upper-mantle anisotropy in Southern California using nearly 6000 SKS splitting data and their 3D full-wave sensitivity kernels in a multiscale inversion enabled by a wavelet-based model parameterization. We also appraise our inversion by estimating the spatial resolution lengths using a statistical resolution matrix approach, which shows the finest resolution length of ~25 km in regions with better path coverage. The anisotropic model we obtain displays the structural fabrics in relation to surface geologic features such as the Salton Trough, the Transverse Ranges and the San Andreas Fault. The depth variation of anisotropy does not suggest a strong decoupling between the lithosphere and asthenosphere. At long-wavelengths, the orientations of the fast axis of anisotropy are consistent with the absolute plate motion in the interiors of the Pacific and North American plates.