Full-waveform seismic tomography using earthquake and ambient noise data: with application to the Alaskan lithosphere

Abstract

Utilizing seismic waves propagating in the subsurface, seismic tomography is one of the most important methods to image the Earth's interior. Benefiting from the use of numerical simulations as forward operators, full-waveform-based seismic tomography is able to account for the 3-D complex geometries and heterogeneities of the Earth, and image the subsurface with high resolutions and high fidelity. Despite its success in tomographic studies over the past two decades, efforts are still needed to improve the efficiency and accuracy of the full-waveform seismic tomography workflow, and adapt full-waveform seismic tomography to imaging tasks at various scales and of different datasets. This thesis aims to address three key challenges in full-waveform seismic tomography, namely efficient continental-scale seismic wave simulation, joint inversion of ambient noise and earthquake data, and accurate and flexible implementation of wavefield injection for scattering imaging of teleseismic waves. A method is first developed to enable flexible and accurate seismic wave simulations with perfectly matched layers (PML) at continental scales (10-60 degrees), and a series of numerical examples are provided to validate its effectiveness. A workflow is then established to jointly invert ambient noise and local earthquake data, which, in combination with the method developed for continental-scale wave simulations, is applied to imaging the Alaskan lithosphere using recently available seismic datasets in Alaska. The obtained images provide new insights into several key questions regarding the tectonic evolution of Alaska. Finally, a new formulation for the wavefield injection problem is derived as a special case of interface discontinuity problem which not only allows for an accurate and flexible implementation of wavefield injection, but also ties together various published implementations of wavefield injection. Numerical examples demonstrate that the proposed implementation of wavefield injection is useful for modeling interactions between incoming waves and local structures at various scales, and inverting scattered waves based on full-waveform seismic tomography.