ORCID

http://orcid.org/0000-0002-7248-1849

Date of Award

Winter 12-15-2022

Author's School

Graduate School of Arts and Sciences

Author's Department

Earth & Planetary Sciences

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

This dissertation utilizes multiple techniques of earthquake tomography to investigate the seismic structure of the crust and mantle beneath Antarctica. The isotropic structure of the Antarctic crust and upper mantle is now constrained by several seismic studies, but until now few studies have investigated the anisotropic structure. Therefore, in this study, I focus on the anisotropic structure, which is crucial information to understanding current or historic deformation and flow patterns, as well as the organic process of Antarctica. I use the data from the seismic stations deployed on the Antarctica continent during the past 20 years, as well as other stations in the southern hemisphere. The first project focuses on the radial anisotropy by inverting the Rayleigh and Love wave phase and group velocities from ambient noise cross-correlation to develop a new radially anisotropic velocity model for West and Central Antarctica with improved shallow crustal resolution. Group and phase velocity maps for Rayleigh and Love waves are estimated and inverted for shear wave velocity structure using a Monte Carlo method. The shallow structure is better resolved by including the Love wave data, allowing me to construct the first continental-scale sediment thickness map for Antarctica. The radial anisotropic result indicates large deformation history in both the crust and mantle of West Antarctica, as well as some potential vertical compositional heterogeneities in the crust.To better understand the anisotropic structure, the second project focuses on azimuthal anisotropy. Rayleigh wave data from ambient noise correlation is first analyzed using anisotropic phase velocity tomography at periods from 8-55 s. These results are then inverted for two azimuthal anisotropic layers, one in the shallow crust and the other in the uppermost mantle. Azimuthal anisotropy is widespread in the shallow crust of West Antarctica and is caused by the lattice-preferred orientation of the crustal minerals rather than the shaped preferred orientation caused by cracks and faults. The azimuthal anisotropy result of the uppermost mantle is similar to the teleseismic shear wave splitting measurements in much of West Antarctica, showing that the lithosphere and asthenosphere have undergone similar deformation. However, other regions, particularly in East Antarctica, show differences between the azimuthal anisotropy in the uppermost mantle from this study and shear wave splitting observations that sample a much larger depth range, suggesting that the shallow lithospheric mantle has a different anisotropy orientation from the mantle below. The adjoint tomographic inversion method, using the spectral element solver SPECFEM3D, has been used to produce a high-resolution isotropic tomographic model for Antarctica and nearby ocean basins. In the third project, I take advantage of the fact that waveform differences between two nearby seismic stations recording the same distant earthquake must be localized near the stations rather than along the entire wave path to significantly improve this model. I use double-difference measurements of the earthquake data, along with double difference kernels produced using adjoint methods, to better resolve the structure beneath the Antarctic continent. The radial anisotropic structure, in particular, is significantly improved and shows strong positive radial anisotropy beneath the Southern Transantarctic Mountains and Ellsworth Mountains, most likely due to lattice preferred orientation of mantle minerals by horizontal deformation. Our result indicates a transition from positive radial anisotropy in the uppermost mantle to low amplitude and possibly negative anisotropy at 150~250km depth in the Antarctica mantle, which is also observed in other major continents.

Language

English (en)

Chair and Committee

Douglas A Wiens

Committee Members

Weisen Shen

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