Date of Award

Spring 1-1-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Michael H. Ritzwoller

Second Advisor

Shijie Zhong

Third Advisor

Michael A. Calkins

Fourth Advisor

Craig H. Jones

Fifth Advisor

Vera Schulte-Pelkum

Abstract

Seismic anisotropy yields important constraints on the character of past and present deformation of the Earth’s interior. It is therefore of great interest to seismologists. In this thesis, I develop and apply a new method to estimate the tilted elastic tensor for a hexagonally symmetric medium based on seismic surface wave (i.e. Rayleigh and Love waves) data. I apply the method to infer crustal anisotropy in the western US and eastern Tibet. The goal is to obtain more accurate and reliable information about the anisotropic properties of the Earth’s crust to help improve the understanding of crustal composition and past deformation.

In terms of method development, my inversion technique simultaneously reconciles observations of surface wave azimuthal and radial anisotropy to provide novel information about the inherent anisotropy and the orientation of the foliated anisotropic material that composes Earth's crust. My inferences occur within the framework of a Bayesian Monte Carlo inversion, which yields posterior distributions for the components of the elastic tensor and its orientation and naturally propagates data uncertainties into model uncertainties.

In terms of application of the methodology, I process seismic data from several arrays in the US and China (USArray, PASSCAL, CEArray, ChinaArray) recorded between the years 2000 and 2012 to obtain high resolution measurements of Rayleigh and Love wave phase speeds and the azimuthal variation of Rayleigh wave phase speeds. Data in both regions can be fit well simultaneously by a tilted hexagonally symmetric medium. The resulting models of the tilted elastic tensor are geologically correlated. An example result is that in the interior of eastern Tibet, where the crust is thicker than elsewhere in the world, I infer a shallowly dipping middle-to-lower crust that I believe is caused by ductile deformation underlying a steeply dipping upper crust that I believe reflects brittle deformation. In contrast, near the periphery of the Tibetan Plateau the foliation is moderately-to-steeply dipping throughout the entire crust, which may reflect the redirection and shearing of crustal flows imposed by less deformable media surrounding Tibet. The spatial and vertical variations of the estimated elastic tensor and its orientation may provide new insights into the composition and deformation history of Tibetan Plateau in the future.

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