PhD Studentship

Faculty of Environment

Natural Environment Research Council (NERC) funded PhD project

Mapping and Characterization of Ultra-Low Velocity Zones: Deciphering the most Enigmatic

Mapping and Characterization of Ultra-Low Velocity Zones – Deciphering the most Enigmatic Structures of the Deep Earth

Supervisors: Dr Sebastian Rost (s.rost@leeds.ac.uk), Dr Stephen Stackhouse

This fully-funded NERC award is available to UK/EU candidates and, if you are an EU candidate, you should check whether you are eligible for a full award.

Ultra-low velocity zones (ULVZs) are small-scale regions exhibiting strongly reduced seismic velocities at the core-mantle boundary (CMB) and among the most enigmatic structures detected in the deep Earth (Rost, 2013). ULVZ detections show them to be only 5 to 30 km thick and a few hundred kilometres across, but they manifest some remarkable seismic properties. With velocity reductions on the order of 5% to 10% and 10% to 30 for P-wave and S-wave velocities, respectively, and an amazing 10% density increase, they are among the most unusual structures found in the Earth's interior (Rost, 2013; Thorne et al., 2013).

Despite their dramatic property variations they are ill understood and several models of ULVZs that can be separated into two general categories have been put forward: (1) ULVZs are a result of partially molten mantle material – possibly the last remnants of a global magma ocean originating in the moon-forming impact and (2) ULVZs are the result of compositional changes in the mantle material, most likely iron enrichment – with iron either coming from a leaking  outer core or material transported from the surface since the onset of plate tectonics. These structures will affect mantle convection, i.e. plate tectonics and core dynamics and, therefore, the generation of the Earth's magnetic field. Our sampling of the CMB for ULVZs is incomplete which makes any detailed interpretation of their origin and dynamics difficult (McNamara et al., 2010).

The aim of this studentship is to better characterize the location and properties of ULVZs at the CMB. Several probes for ULVZ structure have been developed and can be grouped as (1) core reflections (PcP,ScP,ScS, PKKP) and (2) core diffractions (SP_dKS,P_diff). Each probe has specific trade-offs for velocities, density and ULVZ thickness. Therefore using several probes to study the same area of the CMB will reduce trade-offs in these properties resolved by the different methods. These data will be analysed with respect to the seismic waveforms. Using a large library of synthetic seismic waveforms, cross-correlation techniques can be used to identify the best fitting models for each individual waveform. Using the collection of all possible data fits for the individual probes will reduce trade-offs and will lead to a much better characterization of ULVZ properties and location than is currently possible. This project is novel since few attempts have been made to simultaneously model the waveforms of reflected and diffracted ULVZ probes and even multi-probe studies within one probe group are rare. 

The student will be trained in advanced seismic data analysis and seismic waveform modelling. The student will have the opportunity to attend international and specialist conferences, and might have the opportunity to be involved in seismological fieldwork. In addition they will have the opportunity to teach within the school's diverse undergraduate and postgraduate programs

References:

McNamara, A.K., Garnero, E.J., Rost, S., 2010. Earth Planet. Sci. Lett. 299, 1–9.

Rost, S., 2013. Nature Geosc. 6, 89–90.

Thorne, M.S., Garnero, E.J., Jahnke, G., Igel, H., McNamara, A.K., 2013. Earth Planet. Sci. Lett. 364, 59–67.

For information about how to apply visit:

http://www.see.leeds.ac.uk/admissions-and-study/research-degrees/igt/roststackhouse/