Seismic structure of the lower mantle

Abstract

The lower mantle plays an important role in the thermal and chemical evolution of the earth. Although recent advanced seismological imaging displays the heterogeneous nature of the lower mantle, most results are constrained to large scale and longer wavelength structures. This thesis involved waveform modeling studies of the detailed structures of the lower mantle, especially the African Superdome and D layer. A simple uniform 3% shear velocity reduction model can explain the observed seismological anomalies for the African Superdome (also refer as Africa Large Low Shear Velocity Province or Africa Superplume), but it lacks small scale complexity inside. In parallel with the seismic model, a composition-dependent compressibility model with a high bulk modulus is developed to explain the African Superdome. To validate this dynamic model, we map the modeled chemistry and temperature into P and S velocity models. Synthetic seismogram sections generated for this 2D model are then compared directly with the corresponding seismic observations. These results explain the anti-correlation between the bulk velocity and shear velocity, as well as the sharpness of the edge. A lower mantle S-wave triplication with a Scd branch occurring between S and ScS has been recognized for many years and has been interpreted in a variety of ways. The triplication is particularly strong when sampling regions beneath the circum-Pacific lower mantle fast velocity belt seen in global tomographic models, where it has been modeled with a 2–3% jump in S-velocity. The D discontinuity may arise from a phase change for Perovskite to Post-Perovskite. We model the phase boundary height by mapping S-wave tomography into temperature. A few adjustable parameters involving reference phase boundary height and velocity jump are determined from comparing synthetic seismogram predictions with densely sampled observations. Adding 3D propagational effects caused by these structures through Perovskite to Post-Perovskite velocity jump predicted from mineral physics appears to generate compatible results with Scd waveform observations. In the last chapter, we develop a new tool based on a decomposition referred to as a multi-path detector which can be used to distinguish between horizontal structure (in-plane multi-pathing) vs. vertical (out-of-plane multi-pathing) directly from processing array waveforms. A lateral gradient coefficient based on this detector provides a direct constraint on the sharpness of the boundaries and material properties. We demonstrate the usefulness of this approach by processing samples of both P and S data from the Kaapvaal array in Southern Africa. The results further validate the case for distinct chemistry inside the African Superdome. We also present evidence of a narrow plume-like feature coming off the top of the large African low-velocity structure in the lower mantle. The plume's diameter is less than 150 km and consistent with an iso-chemical, low-viscosity plume conduit.