Layered or Not? Regional seismic structure of the Earth’s outer core

Earth’s core is the source of our global magnetic field, but its remote nature means that observation of its structure and dynamics is very challenging. Seismic waves that travel through the core provide information on the core’s structure and changes in the Earth’s magnetic field reflect the core’s dynamics.

However, there is a general incompatibility of these two main geophysical observables for the structure and dynamics of the outer core.

The geomagnetic observations indicate that the flowing liquid iron is turbulent and well mixed throughout the outer core. Seismic observations on the other hand show evidence for a stable layer at the top of the outer core that is not participating in the convection of the core. Such a layer would have a considerable impact on our understanding of how the geodynamo works and how heat is transferred out of the core.

Dynamic simulations of RILs
Figure 1: An example of regional stratification from geodynamic models (Mound et al., 2019). (a) Numerical model with CMB heat-flux variations leads to two non-convection regions (green) that correspond to known mantle structures supressing heat flux from ths core. Strong convection and radial motion can be seen in the equatorial cross section through a core convection model elsewhere in the core (temperature field). (b) Time-averaged radial temperature profiles (dT/dr) in the top half of the outer core. Stratification is indicated by positive dT/dr close to the CMB. Regional profiles for Africa (blue dashed) indicating a stratified region beneath the LLSVP and the Americas (dashed blue) indicating a non-stratified region are shown. Global average shown as solid red. Global averages might show convection, non-stratified regions potentially explaining the globally averaged seismic observations.

 

Recently, we argued for a new model based on numerical core convection simulations (Mound et al., 2019), proposing a model that is potentially able to unify the geomagnetic and seismological observations.  This model argues for a regional stratification of the outer core where some regions, controlled by the structure of the mantle, are not fully convectively mixed (Figure 1). This represents a fundamental paradigm shift in our view of the structure of the Earth’s core that is able to solve the incompatibility between the geomagnetic and seismic observation. This new model requires a new look at seismic data to see if this a core model with regional layers is directly supported by the seismic data.

 

Wider impact

Stratified layers in the outer core have important implications for the long-term evolution of the Earth’s deep interior, physical and chemical interactions between the core and mantle, and the dynamics of the outer core and the global magnetic field (Figure 2). Failure to properly understand core stratification will affect our ability to predict the development of the magnetic field on different time-scales; e.g., the structure of the paleomagnetic field and the mechanism by which reversals of the geomagnetic field happen, and the present pattern of secular variations, changes in the Earth’s magnetic field with periods of a year or more, and hence medium-term geomagnetic forecasting for space-weather predictions. Seismic evidence for the regional stratified layers will have an impact on our overall understanding of Earth’s dynamics and the interpretation of a wide range of geophysical observations.

 

Aim of the project

Seismic studies (e.g. Helffrich and Kaneshima, 2010; Kaneshima, 2018; Wu and Irving, 2020) observe a deviation of the seismic velocity at the top of the outer core from established 1D Earth models (e.g. PREM (Dziewonski and Anderson, 1981)). These observations have been interpreted as evidence for the existence of a global stratified layer at the top of the core although the evidence is not conclusive (e.g. Alexandrakis and Eaton, 2010; Irving et al., 2018).

This project will revisit the seismic observations and will test if they are compatible with the hypothesis of regional stratification. We will exploit the dense seismic networks that are available globally and will use array processing (e.g. Wu and Irving, 2020; Rost and Thomas, 2002) to isolate the seismic phases sampling the outer core and will consider regional analysis in areas of the Earth where we expect regional stratification and regions where stratification is unlikely. We aim to carefully consider mantle contributions to the seismic data that might influence the resolution of stratification. We will use the seismic data to better constrain geodynamo models and predict the seismic signal from existing geodynamo models with and without stratification.Objectives

This novel project will test the recently proposed hypothesis (Mound et al., 2019) that the outer core may be only partially stratified, a model that has not been considered so far when analysing seismic data sampling the outer core. We will use a combination of new seismic data analysis, geodynamic modelling and synthetic seismic wave propagation to detect regional stratification. This project is now possible due to the global distribution of high-quality broadband seismometers that allow regional rather than global analysis of the seismic wavefield, improvements of our understanding of core dynamics and material properties at high-temperature and pressure.

Probes to sample the outer core
Figure 2: Cartoon showing the effect of a stratified layer (A-B) and imaging methods (1-3). A stratified layer (orange) screens the core’s interior: motion inferred for the outermost core is not indicative of the bulk core (B), whereas it is in (A).
The geophysical methods that are used to constrain the stratified layer properties are (1) geomagnetic waves, (2) geomagnetic flux expulsion and (3) seismic travel time analysis. This project will focus on a combination of geophysical methods and mainly rely on seismic data.

The project will consist of different objectives each forming a sub-project, which can be varied dependent on the interests of the student.

We will initially aim to resolve seismic evidence for regional stratification. We will employ new seismic data from globally distributed seismic networks and array processing techniques to identify and characterize the body waves sampling the uppermost outer core (e.g. Kaneshima 2018, Wu and Irving, 2020). Regional stacking will allow us to focus on potentially stratified and non-stratified regions.  To gain better control of the seismic velocity structure, Bayesian modeling of the traveltimes and slownesses will be used.Seismic waves sampling the core travel through the mantle and will be influenced by the mantle velocity heterogeneity. Using seismic wave propagation through complex 3D velocity fields we can control their influence on the resolved core structure.

Using existing geodynamo models allows us to predict how seismic waves “see” the proposed structures. We will convert the temperature and compositional fields from existing geodynamo simulations and use seismic wave propagation methods to link the seismic observations to the expected seismic structure.

If the seismic observations are incompatible with regional stratification this will better constrain the parameter space for core convection and geodynamo models and inform our future understanding of core dynamics.

 

Impact of Research and Publications

The project is designed to test recent hypotheses on outer core structure and to develop a better understanding of the structure and dynamics of the outer core in a multi-disciplinary approach. It will use new data and combine several processing techniques to test these hypotheses using the expertise of the supervisors from a variety of disciplines. It will be structured into several work packages with each package aiming for publications in international journals. The detection (or non-detection) of regional layering in the outer core will change our understanding of the evolution and dynamics of Earth’s core and the generation of the geomagnetic field. The work will be presented at national and international workshops and conferences.

 

Student requirements

This project is suitable for students interested in seismic data analysis, numerical modelling of seismic wave propagation and numerical modelling of core structures and the structure of the Earth’s deep interior. Relevant undergraduate backgrounds include Geophysics, Geology, Physics, Natural Sciences and Applied Mathematics.

 

Fieldwork

This project will mainly use existing datasets held at international data centres. Opportunities for active fieldwork participation might arise as part of other fieldwork-oriented projects or teaching within the School of Earth and Environment. There will be opportunities to collaborate and visit with international researchers working on similar problems.

 

An excellent Training and Research Environment

The Deep Earth Research Group (http://www.see.leeds.ac.uk/research/igt/deep-earth-research) at the University of Leeds consists of researchers in seismology, core dynamics, magneto-hydrodynamics and high-pressure mineral-physics. The group is one of the largest grouping of scientists interested in deep Earth structure and dynamics in the world. The research group is part of the Institute of Geophysics and Tectonics (IGT) with about 25 permanent staff working in a wide variety of solid Earth geoscience disciplines including Tectonophysics, Geodynamics, Petrology, Structural Geology, Seismology, Petrology, Mineral-Physics, Remote Sensing and Geochemistry (http://www.see.leeds.ac.uk/research/igt/). The successful candidate will have the opportunity to interact with internationally leading specialists in these areas and will have the opportunity to present the research work at national and international workshops and conferences.

 

 

References/Further Reading

Alexandrakis, C., & Eaton, D. W. (2010). Precise seismic-wave velocity atop Earth’s core: No evidence for outer-core stratification. Physics of the Earth and Planetary Interiors, 180(1–2), 59–65.

Helffrich, G. & Kaneshima, S., 2013. Causes and consequences of outer core stratification. Physics of the Earth and Planetary Interiors, 223, pp.2–7.

Irving, J. C. E., Cottaar, S., & Lekic, V. (2018). Seismically determined elastic parameters for Earth’s outer core. Science Advances, 4(6), eaar2538. https://doi.org/10.1126/sciadv.aar2538

Kaneshima, S., 2018. Array analyses of SmKS waves and the stratification of Earth’s outermost core. Phys. Earth Planet. Inter. 276, 234–246. https://doi.org/10.1016/J.PEPI.2017.03.006

Mound, J., Davies, C., Rost, S., Aurnou, J., 2019. Regional stratification at the top of Earth’s core due to core–mantle boundary heat flux variations. Nat. Geosci. 12, 575–580. https://doi.org/10.1038/s41561-019-0381-z

Rost, S., & Thomas, C. (2002). Array seismology: Methods and applications. Review of Geophysics, 40(3), 1008.

Wu, W., & Irving, J. C. E. (2020). Array-Based Iterative Measurements of SmKS Travel Times and Their Constraints on Outermost Core Structure. Journal of Geophysical Research: Solid Earth, 125(3), e2019JB018162. https://doi.org/10.1029/2019JB018162