title: Structure of Earth's deep interior from seismic and correlation wavefields
Creators: name: Costa De Lima, Thuany
Publisher: name: The Australian National University
Year: 2024

description: My thesis focuses on the investigations of inner core (IC) and the mantle transition zone (MTZ), as two highly complex internal regions of the Earth with a wide-ranging impact on the rest of our planet's dynamics. Seismic investigations of the IC are challenging due to its remote location and the restrictive spatial coverage of data sensitive to its volume. This limitation impedes the complete characterization of its physical properties, particularly anisotropy - the directional variation of wave speed. Under IC conditions, different iron mineral phases can stabilize and form anisotropy, with hcp and bcc as the most plausible candidates predicted by mineral physics experiments. However, there is no consensus about which phase likely composes the IC. Both structures and subsequent patterns of anisotropy may imply different IC evolution, thus constraining the spatial distribution of anisotropy in the IC leads to a better understanding of the IC's composition and growth. Building on recent advances in theory and observational power of the Earth's coda-correlation wavefield, we develop a method to achieve unprecedented seismic coverage of the IC and probe its anisotropic structure. We study the correlation feature, I2*, formed by similarities in the waveforms of seismic phases with the same slowness sharing two P-wave legs traversing the IC. The dataset of I2* provides a comprehensive azimuthal sampling of the IC across its full-depth range. We show that the travel times of I2* sensitive to the bottom half of the IC radius are compatible with cylindrical anisotropy in Vp with a 3.3% strength and a zonal slow-axis pattern oriented 55deg from the Earth's rotation axis. Our findings provide evidence for a structure within the IC, confirming the existence of the innermost IC using the correlation wavefield framework with implications for Earth's core evolutionary processes, such as dynamic events affecting the texturing of crystals during the IC growth. Another aspect of the IC addressed in my research is its solidity. Shear properties of the IC are important constraints of its solidity and composition, density, possible amount of melting, and viscosity. In this study, we focus on the I-J, a correlation feature formed by similarities of P and S waves sampling the IC. We show the travel times of the I-J are independent of the Earth reference model, allowing estimation of the absolute shear-wave speed (Vs) in the IC. Our observations reveal a Vs reduction compared to the existing Earth velocity models, reducing previous estimations of IC rigidity. Our measurements indicate a 3.4% lower Vs than in PREM. This discrepancy highlights the need for a re-evaluation of IC composition, light elements and dynamics during its solidification, thus enriching our understanding of the growth and evolution of the core. Moving beyond the IC, we explore the MTZ, a region with substantial influence on global mantle circulation patterns, controlling heat and material flux between the lower and upper mantle. Within the MTZ is the 520km, with spatial variability and complexities that remain poorly understood. The observation of double signals in MTZ has led to debates regarding the nature of the 520, with some studies attributing this signal splitting to another seismic discontinuity, the 560km. Here, we explore the non-uniqueness of models that mimic a 520 signal and its splitting. We find that the splitting of 520 occurs in both cold and hot regions and that we can model it with a compositional jump at 520 and a phase transition near 560 (at high temperatures). We argue that compositional layering mimics the splitting of 520 km signals without invoking the presence of a secondary discontinuity at 560 km depth. Mineral physics establishes the influence of basalt content and temperature variations on the detectability of seismic signals within the MTZ, providing critical information about its local temperature, composition, and mantle convection patterns.