IAP2-22-373

Understanding ULVZs: Anisotropy around the mantles most mysterious structures

This project will produce new observations of seismic anisotropy within and around ultra-low velocity zones (ULVZs) – small, extreme and poorly understood structures on the Earth’s core-mantle boundary – to help understand what they are and what role they play in mantle convection.

The core-mantle boundary (CMB) is a controlling thermal boundary layer at the top of outer core convection and the base of mantle convection. Thus, knowledge of the structures and processes at this interface are essential for understanding the driving forces that control our planet. Within the complex landscape of the CMB, no features are more extreme, or more poorly understood, than ultra-low velocity zones (ULVZs), Figure 1. ULVZs are small features, only 10s km high and 100s km across, however, while most material within the Earth causes changes in earthquake (seismic) wave speed by only several percent, ULVZs show reductions of 10-50%, an extreme observation we cannot currently explain, since we are uncertain what ULVZs are formed from.

The relationship between ULVZs and convective flows within the Earth’s mantle is also poorly understood. Is ULVZ material a passive substance swept into piles by surrounding mantle flows, or does it act to control large-scale flow structure? For example, observations of ULVZs have been made beneath numerous volcanic hotspots thought to be fed my mantle plumes, including: Hawaii, Iceland, Samoa, Galapagos and the Marquesas. This has led to the hypothesis that these structures could act as plume anchors, potentially controlling the location and long-term stability of hot convective upwellings. Exploring these hypotheses requires an understanding of large-scale flow within the deep mantle close to the CMB around regions containing ULVZs, which is possible using observations of seismic anisotropy (directional wave-speed dependence). All of the major minerals thought to be present in the lowermost mantle are expected to have single-crystal anisotropy. Large scale flow can cause individual crystals to align producing a lattice preferred orientation (LPO) that causes seismically measureable anisotropy.

In this project the student will produce new measurements of seismic anisotropy within and around ULVZs to help us better understand these mysterious features and how they interact with mantle flow. Work will focus on the region containing the previously identified ULVZ beneath Hawaii (Jenkins et al., 2021), with the aim of answering the following questions:

• Is the material forming ULVZs anisotropic – can measurements of seismic anisotropy help identify what material ULVZs are made of?
• What is the wider scale lower mantle flow around the Pacific and what is its relationship with the location/shape of the Hawaiian ULVZ?

This project will suit a numerate geologist or (geo-)physicist, and is primarily computational in character. Candidates must be willing to learn to use, adapt, and extend software written in Matlab, Python and other programming languages. The candidate will also gain expertise in seismic data analysis, the handling of large datasets and the structure and composition of the deep earth.

Click on an image to expand

Image Captions

Figure 1. Cartoon of an ULVZ on the Earth’s core-mantle boundary,Figure 2. Cartoon depicting the splitting of a shear wave into a fast and slow direction when travelling through an anisotropic medium, defined by polarisation direction (φ) and delay time (δt), Figure Courtsey of Ed Garnero,Figure 3. a) ScS data coverage at the CMB (black points) beneath Hawaii from the dataset of Jenkins et al., (2021), b) Potential data coverage at CMB of SKS and SKKS phases recorded at stations in the central Pacific.

Methodology

Materials that are strongly anisotropic cause seismic shear (S) waves travelling through them to become polarised into a fast and slow direction, leading to a difference in measured S arrival times on different components of motion, Figure 2. This phenomena is known as shear wave splitting. Measurements of the difference in arrival times of fast and slow S observations give an indication of the strength of anisotropy while the orientation of fast directions can provide insight into mineral alignment. Such observations can be interpreted in terms of a materials mineralogical composition and flow direction. However to measure deep mantle anisotropy requires both correction for upper mantle anisotropic structure and an understanding of possible lower mantle mineralogy.

The student will assess shear wave splitting in a data set of core-reflected ScS waves that densely sample beneath Hawaii (Jenkins et al., 2021), Figure 3a. To localise structure contained in the lowermost mantle measurements will be made relative to direct S waves that sample similar regions of the upper mantle (Wookey et al., 2005). Shear wave splitting across the wider Pacific will be assessed using core-travelling SKS and SKKS waves, Figure 3b. Upper mantle anisotropy corrections are required to localise lower mantle signals, and will be applied where good constraints prove available. If this is not possible, SKS/SKKS differential measurements (Long, 2009), will be considered instead. Where coincident, multi-directional observations from the two datasets will be jointly analysed (Ford et al., 2015) using a recently compiled library of elastic tensors (Creasy et al., 2020), to test lower mantle and ULVZ compositions and flow regimes.

Measurements will be made using SplitLab software (Wüstefeld et al., 2008), which the student will learn to use with support from deep mantle anisotropy expert collaborators based in Yale University, US.

Project Timeline

Year 1

Literature review, training in seismic data processing techniques, ScS-S anisotropic measurements

Year 2

SKS, SKKS anisotropic measurements. Analysis, assessment and modelling to determine nature and implications of observations. The work from Years 1 & 2 should lead to at least one publication.

Year 3

Combining of ScS and SKS data to better define mineralogy and flow; writing of further manuscript for publication.

Year 3.5

Completion of manuscripts for publication and thesis writing.

Training
& Skills

You will become part of the Geophysics and Geodynamics Research Groups at Durham.

The student will receive training in processing, analysing and modelling seismic data as well as associated essential skills (programming, code development, and usage of high-performance computing systems). Training in a wider range of important skills (e.g. presentation skills, paper/thesis writing) will be provided by the Department of Earth Sciences at Durham University, and the student will also benefit from cross-disciplinary training provided as part of the IAPETUS DTP.

The student will have opportunities to work with collaborators in the University of Yale in the US and will attend national and international scientific meetings to present results. We aim to see all students publish at least two papers in leading scientific journals during their PhD. Upon completion, the student will be well equipped for a career in academia or in a range of industries

References & further reading

Creasy, N., Miyagi, L., & Long, M. D. (2020). A library of elastic tensors for lowermost mantle seismic anisotropy studies and comparison with seismic observations. Geochemistry, Geophysics, Geosystems, 21(4), e2019GC008883.
Ford, H. A., Long, M. D., He, X., & Lynner, C. (2015). Lowermost mantle flow at the eastern edge of the African Large Low Shear Velocity Province. Earth and Planetary Science Letters, 420, 12-22.

Jenkins, J., Mousavi, S., Li, Z., & Cottaar, S. (2021). A high-resolution map of Hawaiian ULVZ morphology from ScS phases. Earth and Planetary Science Letters, 563, 116885.

Long, M. D. (2009). Complex anisotropy in D ″beneath the eastern Pacific from SKS–SKKS splitting discrepancies. Earth and Planetary Science Letters, 283(1-4), 181-189.

Wolf, J., Creasy, N., Pisconti, A., Long, M. D., & Thomas, C. (2019). An investigation of seismic anisotropy in the lowermost mantle beneath Iceland. Geophysical Journal International, 219(Supplement_1), S152-S166.

Wookey, J., Kendall, J. M., & Rümpker, G. (2005). Lowermost mantle anisotropy beneath the north Pacific from differential S—ScS splitting. Geophysical Journal International, 161(3), 829-838.

Wüstefeld, A., Bokelmann, G., Zaroli, C., & Barruol, G. (2008). SplitLab: A shear-wave splitting environment in Matlab. Computers & Geosciences, 34(5), 515-528.

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