Mapping P-wave Azimuthal Anisotropy near the Core-Mantle Boundary using Novel Observations of Core-Diffracted waves PcSdiff

Lead Research Organisation: University College London
Department Name: Earth Sciences

Abstract

While the Earth's surface marks the boundary between the atmosphere and the solid earth, the core-mantle boundary (CMB) separates the solid mantle from the liquid core, modulating the loss of heat and driving the earth's dynamo and magnetic field.

Constraining seismic anisotropy in the CMB region is challenging. Previous results on S wave anisotropy are subjected to some limitation in geographical sampling, azimuthal coverage or/and proper correction of upper mantle seismic anisotropy, which is not trivial.

None of previous studies, neither in isolation or in combination, regional or global scale, has yielded any estimate of P-wave azimuthal anisotropy in the D'' layer. We do NOT know P wave anisotropy property of ultra-low velocity zones (ULVZs), large low-velocity provinces (LLVPs) or slab graveyards.

We will use novel observations of core-diffracted PcSdiff wave in the radial P wave receiver function stacks to constrain P wave azimuthal anisotropy in the core-mantle boundary, with a particular focus on the regions of possible slab graveyard, ULVZs and the base of LLVPs.

These renewed descriptions of P wave anisotropy models will be used to explore outstanding questions including the rheology or the deformation mechanism, the mantle flow patterns near the core-mantle boundary and the nature of LLVPs and ULVZs.

A series of outstanding questions can be much better addressed with our new seismic observations:

What's the rheology and deformation mechanism operating in the D'' layer?

How did the slab graveyard, LLVPs and ULVZs deform and their anisotropy properties?

Are LLVPs and ULVZs compositional distinctly from the rest of the D'' layer?

Is the deformation accommodated by bridgmanite, post-perovskite or secondary minerals such as ferropericlase and calcium-perovskite?

Is the dislocation creep, responsible for upper mantle seismic anisotropy, operating in the D'' layer?

These unprecedentedly rich observations will provide renewed constraints on fundamental processes relevant to the Earth's interior and evolution.

Planned Impact

This is a blue-skies, curiosity-driven research project that aims at using new seismic observations for a comprehensive characterization of P wave seismic anisotropy near the core-mantle boundary, which will provide key information on the dynamics, deformation mechanisms and flow patterns in one of the most mysterious regions in the interior of the Earth. Given the fundamental nature of this project, the main non-academic beneficiaries of this project will be school children, teachers and the wider public. The dynamics of the Earth's interior and its consequences - earthquakes, volcanoes, mountain building - are all topics that greatly interest these users, having a strong potential to help educate and motivate the public for STEM subjects. Ultimately, this project may also benefit the geophysics industry (e.g., hydrocarbon reservoir characterization), as the comprehensive imaging techniques developed in the project may be transferrable to small-scale, industrial applications.

School children, teachers and the wider public will benefit from the project through a higher visibility of geophysics in schools, notably through a revived UCL "Seismology in schools" project. Two-way partnerships between the project's team and school teachers and students will enable students to carry out end-to-end seismic analyses, from ground motion observations to the construction of profiles of mantle's composition. In addition, teachers will be able to refresh their teaching materials and knowledge in geophysics. More generally, the wider public will benefit from building on existing outreach efforts at UCL (e.g., "Build your planet" interactive website and the GeoBus) and from new demonstration materials on the dynamics of the Earth's interior that will be used in regular outreach events and at UCL's Geology festival.

The geophysics industry will benefit from this project by being informed about the project's outputs and new seismic methods developed through regular discussions during the project. This will be enabled by the PI and collaborator's extensive network of contacts and by their involvement in events with industry's participation (e.g., workshops on Big Data and industry's presentations at UCL). The outcomes of the project will be published in a popular science article in a journal accessible to the wide academic and non-academic community, such as the RAS journal 'Astronomy and Geophysics', the AGU Eos newspaper, New Scientist or NERC's Planet Earth magazine for wide-ranging dissemination of the project.