Seismic Array Imaging of Teleseismic Body Waves from Finite-frequency Tomography to Full-waveform Inversions: With Applications to South-central Alaska Subduction Zone

• Main Author: Song, Xin
• Other Authors: Liu, Qinya, Physics
• Format: Thesis
• Language: unknown
• Published: 2019
• Subjects: 0605; Pacific; Yakutat; Alaska
• Online Access: http://hdl.handle.net/1807/97682

Seismic tomography has been one of the most essential tools in mapping the images of the Earth’s interior. This thesis aims to present an introduction to various seismic tomography techniques for utilizing teleseismic body-wave data, and their applications in south-central Alaska subduction zone. Due to the continuous subduction of Pacific plate beneath the North American plate, south-central Alaska has great tectonic and geodynamic complexities that needs to be further illuminated by seismic waves. First, we invert for 3-D variations of compressional and shear wave speed as well as Vp /Vs ratio for the south-central Alaska crust and uppermost mantle based on the traditional finite-frequency tomographic methods. Approximate 5400 teleseismic events are specifically selected with magnitude ≥ Mw5.5, epicentral distance between 30 ◦ and 95 ◦ , and good signal-to-noise ratio. A large number of teleseismic P and S wave multi-channel cross-correlation (MCCC) traveltime data are collected from available records in the past two decades. Finite-frequency traveltime sensitivity kernels of P and S wavespeed are constructed by the ray-based algorithm, and the P and S velocity perturbations are directly inverted by the Newton method. Our final images provide constraints on the geometry of the subducted oceanic lithosphere as high-velocity anomalies, including its extent, thickness, and subduction angle. In addition, other lithospheric heterogeneities, such as upper crustal structures like sedimentary basins, Wrangell subduction zone, and thickened Yakutat crust, are all presented in our inversion results. Then we strive to develop a high-resolution seismic imaging technique based on full waveform inversion (FWI) of teleseismic body waveforms. We use a state-of-the-art hybrid method interfacing analytic frequency-wavenumber (FK) calculation for 1D background models with a 3-D spectral-element (SEM) solver for a 3-D regional model to accurately simulate the direct and scattered waves in short-period teleseismic body waves. The velocity models can be inverted by reducing the waveform misfit between data and synthetics through an iterative Quasi-Newton method. In particular, we explore the feasibility of teleseismic body-wave FWI based on SEM-FK hybrid method for refining the 3D velocity structures in the crust and upper mantle, with applications to south-central Alaska subduction zone. Ph.D.