Using seismic waves to investigate tectonic plates as they sink into the deep Earth

Summary

In this PhD project, you will study deep-focus earthquakes in the Earth’s transition zone to understand how subducting slabs make their way from the surface into the deep mantle. You will use seismic observations and modelling, combined with mineral physics data, to tackle one of the longest-standing problems in the study of the deep Earth.

Overview

The ultimate fate of tectonic oceanic plates is usually to be subducted beneath another plate and sink into the Earth, eventually to be mixed back in with the rest of the mantle. However, a slab’s history between the surface and the deep can be various and complicated. Whilst all known slabs sink quickly to the transition zone (410–660 km beneath the surface), hereafter there are significant differences between slabs (Fukao & Obayahi, 2013). Some like the Farallon plate apparently travel unimpeded to the core–mantle boundary, whilst many like the Honshu and Bonin slabs ‘stagnate’ at 660 km, or even pond deeper still at 1000 km such as the Mariana slab. The reason for these differences is as yet unclear. It also remains uncertain whether or not these ponded slabs will eventually sink once again, or be entrained into the transition zone and upper mantle. These questions are of fundamental importance to the history of the Earth, since slabs cycle material back into the deep interior. Were stagnation the norm, it would be easier to maintain chemical differences between the upper and lower mantles; whilst if fast sinking is more usual, the two layers would be much more efficiently stirred together.

This project will investigate how slabs travel through the mantle, and what material they carry, by examining earthquakes occurring in the transition zone and above. By examining seismicity—the occurrence, timing, location and focal mechanism of these events—it will be possible to understand the state of stress in the slab and relate this, for example, to the viscosity contrast between the slab and lower mantle. It will also help answer why such deep earthquakes occur, something also largely unexplained.

The project will also use deep earthquakes to study the structure of the slab. Careful measurement and modelling of the waveforms created by deep events can reveal the structure around the event (e.g., Fan et al., 2019). This includes inferring the presence of anisotropy (Nowacki et al., 2015).

In this project, you will use a combination of seismic methods to make progress on our study of the Earth. You will use array seismic methods, shear wave splitting, and other techniques to make new observations from raw seismic data. You will invert these data for earthquake locations using non-linear techniques. You will test hypotheses for the seismic structure near the earthquakes, including anisotropy. With this information, you will compare the structure and kinematics implied by your work to models of slab chemistry. Ultimately, you will test if slabs with different subduction styles show different chemistry and dynamics. In this way, we may identify why subduction style varies and predict the future of stagnating slabs.

Objectives

In this project, you will work with leading researchers at Leeds (Andy Nowacki and Tim Craig) and Oxford (Andrew Walker) with expertise in seismology, geodynamics and mineral physics. Initially, work will focus on measuring seismicity and seismic anisotropy, but a number of objectives could be pursued depending on your own interests and background:

1. Examine seismicity in the transition zone by searching for seismicity using new low-signal-to-noise methods for automatically detecting earthquakes (Shi et al., 2018).
2. Expand and improve upon an existing set of measurements of shear wave splitting from the vicinity of deep earthquakes, and invert for the anisotropic elastic structure around slabs in the transition zone.
3. Use high-frequency full-waveform modelling of deep earthquakes to infer elastic structure. Coupled with (2), infer the chemistry of slabs by comparison with data.
4. Use thermodynamic mineral physics databases (e.g., Connolly, 2009) along with hypotheses for the chemistry of slabs to construct candidate 3D velocity models around slabs. Test the synthetic seismograms these produce against data.

Impact of this work

Two coupled long-standing questions which you will address in this project—why deep earthquakes occur and where slabs go—are of critical importance in understanding the Earth’s development. Hence it is expected that successful candidates will be in a strong position to publish their work in high-impact journals in a number of articles. Global interest in the work will also allow for travel to international conferences to present your findings.

Applicant suitability

This seismic, deep Earth PhD project would suit candidates with an interest in the fundamental way in which the Earth behaves, and a passion for interrogating datasets by both using existing computational seismic techniques and possibly developing new ones. You will need to be able to collate seismic data, process it using available tools, develop new tools where appropriate, and model your data using existing modelling workflows. Candidates will usually be armed with undergraduate training in areas such as geophysics, physics, geology, applied mathematics and similar branches of quantitative science. Programming experience is advantageous but not essential.

Training environment

You will be a key member of a team of researchers across the School of Earth and Environment tackling fundamental problems in the study of the solid Earth. You will be part of the Seismology, Tectonics and Deep Earth research groups in the Institutes of Geophysics and Tectonics, and Applied Geosciences, interacting daily with not only your supervisors, but other senior colleagues, postdoctoral researchers and fellow PhD students. You will also work with Dr Walker at the University of Oxford and travel there for project meetings to work with Dr Walker. In this project, you will be trained in many transferable scientific skills, including analysis of large datasets, high-performance computer modelling, probabilistic inversion of data, and the dynamic communication of your ideas. Study for a PhD in the Seismic and Deep Earth groups involves international travel to conferences and the possibility of seismic fieldwork. As a member of the Panorama Doctoral Training Programme, you will received tailored training alongside a cohort of other postgraduate researchers.