Seismic anisotropy, attenuation, and site amplification study of Southeast Asia and northern Los Angeles Basin
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[EMBARGOED UNTIL 08/01/2026] Understanding the structure and dynamics of the Earth's crust and upper mantle is fundamental to advancing our understanding of plate tectonics and seismic hazard assessment. This dissertation presents four integrated studies focused on high-resolution imaging of the upper mantle, lithosphere, and near-surface effects of ground amplification, using seismic anisotropy, attenuation, and site amplification analyses. Together, these four chapters span diverse tectonic environments, from the highly oblique Indo-Burma subduction system in Southeast Asia to the sediment-filled basins of northern Los Angeles, providing critical insights into mantle deformation, crustal rheology, and seismic ground motion amplification. First, shear wave splitting analysis of teleseismic phases (SKS, SKKS, and PKS) is used to characterize mantle flow and deformation beneath the Indo-Burma subduction zone and the eastern Indochina Peninsula. Results reveal trench-parallel N-S mantle flow beneath the forearc region, induced by prolonged northward motion of the Indian plate and preserved lithospheric fabric due to ongoing convergence, as well as E-W fabric in the east of the Sagaing Fault, likely associated with asthenospheric flow from the Hainan Plume. Second, lithospheric-scale attenuation structures are investigated across Southeast Asia by seismic quality factor (Q) tomography of regional seismic phases. Using both the two-station method and reverse two-station method applied to regional Lg and Sn phases, this study uncovers a heterogeneous attenuation structure that correlates strongly with crustal thickness, volcanic centers, and tectonic boundaries. Particularly notable is the low Q and frequency-dependent attenuation beneath the Tengchong volcanic arc and Indo-Burman Ranges, indicating the presence of partial melt and pervasive crustal scattering. Third, a novel site response mapping is conducted in Yangon, Myanmar, using a dense nodal seismic array, demonstrating that dense small-aperture arrays can resolve site amplification variability at city-block scales, crucial for improving urban seismic hazard models. Finally, reverse two-station analysis is used to map crustal attenuation and site response across the northern Los Angeles Basin, revealing complex amplification patterns tied to basin depth, geometry, and fault proximity. Collectively, this work addresses critical observational gaps in tectonically complex and urbanized regions, enhances our understanding of deep and shallow Earth structures, and provides a framework for improving physics-based ground motion simulations and seismic hazard models.
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Ph. D