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Mapping the Core-Mantle Boundary Using Sdiff Postcursors


Type

Thesis

Change log

Authors

Abstract

The core-mantle boundary – the interface between the rocky mantle and the fluid iron outer core – is host to a diverse collection of phenomena across all length scales. At the largest length scales, continent-sized structures characterised by low velocity anomalies lie opposite to each other on the core-mantle boundary. These structures, known as large low shear velocity provinces (LLSVPs), are predominantly situated beneath the Pacific Ocean and the African continent. Smaller, more extreme phenomena (10s km thick, up to 100s km wide, dVs -10–50%, dVp -5–25%) have been identified on top of the core-mantle boundary, called ultra-low velocity zones (ULVZs). The largest of these anomalies have been associated with whole-mantle plumes at the base of major hotspots (Hawaii, Iceland, Samoa, and Galapagos), and have been suggested to act as a plume root and geochemical reservoir.

In this thesis, we use the S core-diffracted phase (Sdiff) to detect ULVZs. Sdiff wavefronts propagating across the core-mantle boundary refract through low velocity anomalies and create additional wavefronts, called ‘postcursors’, whose move-out pattern is determined by the size, velocity, and shape of the ULVZ. These structures can then be detected by searching for the move-out of postcursory energy across a large seismic array sequenced by azimuth and inferences made about the ULVZ. We perform an extensive global search for Sdiff postcursors from earthquakes between 1990–2022 with magnitudes ≥5.7 at any depth in the IRIS earthquake catalogue. This results in the identification of 100 events sampling the Hawaiian ULVZ, and 100s more sampling the Iceland, Galapagos, and several newly discovered ULVZs. By pushing the dataset to higher frequencies than previously used, we find evidence for a thinner ULVZ beneath the mid-Pacific (less than 10 km) – which we refer to as the ‘ultra-thin ULVZ’.

Modelling of these structures using Sdiff postcursors has, so far, required computing 3D full waveform synthetics to the periods corresponding to the length scales of ULVZs (approximately 10 s). Doing this is computationally expensive, resulting in simplified models and limiting the exploration of the parameter space. Accurate models of ULVZ morphology and physical parameters is essential to understanding their origins, and role in mantle dynamics and core-mantle interaction. We adapt a wavefront tracking software to model waves propagating across a spherical shell in 2D to decrease the forward model computational time from 100s CPU hours to a few seconds. We implement this forward model – the 2D wavefront tracker (2DWT) – into a Bayesian inversion to better explore the parameter space of these structures and, for the first time, to estimate the associated errors.

For the Hawaiian ULVZ, we selected five high quality events with good azimuthal coverage to apply the inversion. We found a 2D elliptical shape with a shear velocity reduction of 22 ± 4%. The ULVZ is aligned along the edge of the LLSVP, suggesting it might be piled up there. Using our 2D shape in full waveform modelling, we constrained the height to be 25 ± 5 km based on comparison of the frequency content to the waveforms. We also apply the inversion to the ultra-thin ULVZ. Data coverage for this structure is limited to one azimuthal direction, so we are unable to resolve a precise location or morphology. However, we are able to build a probabilistic picture of its properties: it is likely quasi-cylindrical, with a radius of 280 ± 50 km, shear velocity reduction of 20 ± 4%, and height of 8–12 km.

We go on to perform Bayesian inversions with the 2DWT on other ULVZs. We characterise two known ULVZs (Iceland and Galapagos) and two newly identified ones (near Vanuatu and near Marquesas), and show evidence for several other new structures as well. Despite detections of ULVZs in the literature being prevalent across the core-mantle boundary, all of the ULVZs detected with Sdiff are unambiguously clustered around the edges of the LLSVPs. This suggests a multi-scale dynamical relation between the two phenomena. Furthermore, all of the observations and inferred properties of ULVZs are consistent with an iron-enriched (Mg,Fe)O compositional origin.

Description

Date

2023-09-26

Advisors

Cottaar, Sanne

Keywords

Core-Mantle Boundary, Deep Earth, Earth Sciences, Lower Mantle, Seismology, ULVZs

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
European Research Council (804071)
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 804071 -ZoomDeep).