Subducting slabs in the Earth’s transition zone

Background

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 (Figure 1; 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.

Tomographic cross sections through the Central American arc.

Figure 1.  Tomographic cross-sections through the Central American arc, showing P-wave velocity.  Note that in the north (sections A–E), the fast region (believed to be the subducting slab) is arranged much more shallowly and ponds above 1000 km, whilst further south (sections F–J) the slab appears to penetrate quickly into the lower mantle.  In this project, you will study earthquakes from slabs in the transition zone to improve our understanding of these different slab morphologies and dynamics. (Figure 16 of Fukao & Obayashi, 2013.)

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 improve knowledge of 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 to address why such deep earthquakes occur, which is also a largely unexplained phenomenon.

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 (Figure 2; Nowacki et al., 2015).

In this project, you will use a combination of different global seismic methods to make progress on our study of the Earth.  You will use array seismic methods, shear wave splitting, and other measurement techniques to make wholly new observations from raw seismic data.  You will then 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 by your work to models of the chemistry of slabs.  Ultimately, the goal is to 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.

 

Measurement of shear wave splitting from deep earthquakes

Figure 2: Measurement of shear wave splitting from deep earthquakes to infer the anisotropic structure of slabs, and hence their chemical content.  Left panel shows the experimental setup, with rays leaving a slab projected onto a lower hemisphere.  Centre and right panels show on this lower hemisphere the best-fitting (centre) shear planes of bridgmanite and (right) axes of symmetry of Phase D, a hydrous magnesiosilicate phase.  The required layer thickness of such material is shown above.  The thin layer needed for phase D suggests slabs carry significant water and might weaken them.  (Modified from Nowacki et al., 2015.)

Objectives

In this project, you will work with leading researchers at Leeds with backgrounds in seismology 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 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 Seismology, Tectonics and Deep Earth research groups in the Institutes of Geophysics and Tectonics, and Applied Geoscience, interacting daily with not only 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.  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.

References and further reading

  • 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 Mw2 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