Seismic Imaging of Earth’s Core-Mantle Boundary using Diffracted Waves

Earth’s core-mantle boundary marks the transition from the rocky mantle to the liquid outer core and exhibits the most extreme velocity jump in the Earth’s radial seismic structure. In recent decades, seismological studies demonstrate that the seismic structure close to this boundary is far more complicated than a one-dimensional velocity model could describe. In particular, anomalous heterogeneities such as Large Low-Shear-Velocity Provinces (LLSVPs) and Ultra-Low Velocity Zones (ULVZs) have been reported in this region. We analyse seismic diffraction in different aspects including real data observations, numerical synthetics, asymptotic theory, and use it as a tool to resolve the fine-scale structure close to the core- mantle boundary. We present seismic observations of rare core-diffracted signals sensitive to the interior of the Hawaiian ULVZ at the highest frequency yet observed. This signal indicates a pronounced vertical structure on the scale of kilometres inside the ULVZ. We also explored a novel data set revealing a new ULVZ close to the area beneath Macdonald Island. These new observations reinforce the proposed correlation between the ULVZs and surface hotspots. Utilizing the latest computational advances in 3D synthetic waveform modelling, we also compare the waveforms of ULVZ models in different dimensions and demonstrate the consistency and nuance among the waveform in these modellings. We also discuss a new theoretical framework for modelling diffracted waves asymptotically, which extends surface wave ray theory to core-diffracted waves. To complement the discussion of the dynamics at the core-mantle boundary, we also analyse thermal boundary layer theory, where we validate the scaling of the core-mantle heat flux and the thermal boundary temperature under a strong temperature-dependent viscosity and compositional heterogeneity.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2021

Author dc:creator

• Li, Zhi

Advisors dc:contributor.advisor

• Cottaar, Sanne
• Al-Attar, David

• Global Seismology
• Deep Earth Interior
• Core-Mantle Boundary
• Seismic Diffracted Waves

Licence

• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

Author Identifier

0000-0001-5657-655X

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/334398