Tectonic oceanic plates are usually subducted beneath another plate and sink deep into the Earth, eventually being mixed back into the rest of the mantle. However, a slab’s history after subduction can be 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 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 and influence the degree to which mantle convection is layered.
This project will investigate how slabs travel through the mantle by examining earthquakes occurring in the transition zone and above. This seismicity—the occurrence, timing, location and focal mechanism of these events—can tell us the state of stress in the slab and relate this to the viscosity contrast between the slab and lower mantle. It will also help explain why such deep earthquakes occur, which is 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).
-- Project plan
In this project, you will use array seismic methods, shear wave splitting, and other techniques to make wholly new observations from raw seismic data. You will invert these data for earthquake locations using novel non-linear techniques. You will test hypotheses for the seismic velocity structure near the earthquakes, including anisotropy. Armed with this information, you will compare the structure and kinematics implied to models of the chemistry of slabs. Ultimately, you will test if slabs with different subduction styles show different chemistry and dynamics. In this way, we can potentially identify why subduction style varies and predict the future of stagnating slabs.
Depending on your background and interest, project objectives could include:
1. Examine seismicity in the transition zone by searching for seismicity using new autmatic low-signal-to-noise methods (Shi et al., 2018).
2. Gather measurements of shear wave splitting around deep earthquakes, and invert for the anisotropic elastic structure 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.
-- Applicant suitability
You will have an interest in the fundamental way the Earth behaves, and a passion for interrogating datasets by both using existing computations seismic techniques and developing enw ones. You will need to be able to collate seismic data, process it with available tools, develop new tools, and model your data using existing workflows. Candidates will usually be armed with undergraduate training in geophysics, physics, geology, applied mathematics or a similar quantitative science. Programming experience is advantageous but not essential.
-- Training environment
Leeds hosts the UK’s largest group of researchers studying the Earth’s deep interior. 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 not only with your supervisors, but other senior colleagues, postdoctoral researchers and fellow PhD students. In this project you will be trained in many transferrable scientific skills, including analysis of large datasets, high-performance computer modelling, probabilistic inversion of geophysical data, and the dynamic communication of your ideas.
-- How to apply
For details see: https://panorama-dtp.ac.uk/how-to-apply
• Connolly, J.A.D., 2009. The geodynamic equation of state: What and how. Geochemistry, Geophysics, Geosystems. https://doi.org/10.1029/2009GC002540
• Fan, W., S.S. Wei, D. Tian, J.J. McGuire, and D.A. Wiens, 2019. Complex and Diverse Rupture Processes of the 2018 Mw 8.2 and Mw 7.9 Tonga‐Fiji Deep Earthquakes. Geophysical Research Letters, 46, pp. 2434–2448. https://doi.org/10.1029/2018GL080997
• Fukao, Y. and M. Obayashi, 2013. Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity. Journal of Geophysical Research: Solid Earth, 118, pp. 5920–5938. https://doi.org/10.1002/2013JB010466
• Nowacki, A., J.-M. Kendall, J. Wookey and A. Pemberton, 2015. Mid-mantle anisotropy in subduction zones and deep water transport. Geochemistry, Geophysics, Geosystems, 16, pp. 1-21. http://doi.org/10.1002/2014GC005667
• Shi, P., A. Nowacki, S. Rost, D.A. Angus, 2018. Automated seismic waveform location using Multichannel Coherency Migration (MCM)–II. Application to induced and volcano-tectonic seismicity. Geophysical Journal International, 216, pp. 1608–1632. https://doi.org/10.1093/gji/ggy507