Seismic Investigations into the Nature and Scale of Thermochemical Heterogeneity in Earth's Mantle

Abstract

Thermochemical convection distributes kilometer-scale heterogeneities throughout Earth's mantle. These kilometer-scale heterogeneities, or scatterers, reflect seismic waves which allow us to infer their composition, scale, and distribution. Thermochemical convection also influences the mantle transition zone (MTZ), distorting its topography and potentially leaving signatures of convection patterns in the upper mantle. Understanding mantle heterogeneities at the kilometer scale as well as MTZ topography is a critical component of understanding the nature of mantle convection, which drives all tectonic processes. This dissertation includes three chapters that contribute to our understanding of Earth's mantle.

In chapter 2 I interpret broadband US Array recordings from a 633 km deep South American earthquake as an S-to-P conversion originating from a scattering heterogeneity 1750 km beneath Western Brazil. I model the amplitude and polarity of the signal assuming the heterogeneity is a single scatterer. From the modeling, I show that this block has a characteristic scale length of 10 km and must have a shear velocity slower than the surrounding mantle by up to 12%. This elastic contrast is consistent with the post-stishovite transition of silica, which indicates that the subduction process can entrain and preserve 10 km scale crustal fragments in the deep mantle.

In chapter 3 I construct seismic models of lower mantle structures computed from the mantle convection models of textit{Brandenburg et al. (2008)} and use them to simulate global observations of PKP scattering. This was the first study to simulate PKP scattering using a joint geodynamic and spectral element waveform simulation method. The results show that simplified incompressible, cylindrical, two-component mantle convection simulations developed to explain geochemical isotope systematics also explain lower mantle scattering.

In chapter 4 I use data from the Arrays of China to recover MTZ thickness beneath the China Sea and surrounding regions by ray-theoretically migrating ScS reverberations. With spectral element waveform simulations I tested the ScS reverberation migration's ability to recover transition zone topography assuming inhomogeneity of sources and 3-D velocity structure. This study revealed that the ScS reverberation migration method is unable to recover degree-20 transition zone topography and that volumetric velocity heterogeneities strongly influence the recovered mantle transition zone thickness.