I focus on resolving the structure of the deep Earth to better understand its dynamic evolution. My tools are observational seismology and forward waveform modelling. I investigate structures in the mantle and core across a range of spatial scales using often using seismic arrays of various sizes.
Small, kilometre-scale heterogeneity distributed throughout the mantle is responsible for much of the energy that we observe in the high-frequency seismic wavefield. The distribution of this heterogeneity is likely controlled by the patterns of mantle convection. The scattering of seismic body wave energy by discrete volumes with sharply contrasting elastic parameters (bulk and shear moduli and density) is responsible for the long codas of energy that often follow the arrivals of major phases (see figure 1). The direct wave makes investigation of the coda waves challenging. However, in for some ray geometries (such as PKPdf, the wave that samples the Earth’s mantle, outer and inner cores), scattered seismic waves can arrive as precursors the direct wave, thus can be more easily identified. Seismic arrays, collections of closely spaced seismometers, allow resolution of the direction of the incoming energy. By identifying scattered seismic energy with seismic arrays I am able to precisely determine the direction, and thus the location in the Earth of the heterogeneity responsible for the scattering. With this approach I have mapped volumetric heterogeneity throughout the mantle under South Africa (Frost et al., 2013). Through analysis of the frequency content of the scattered energy I resolved the size of the heterogeneity to be around 3-7 km, and determined that heterogeneity within 300 km of the Core-Mantle Boundary (CMB) is more concentrated around the edges of large, convective mantle structure (the LLSVPs) (Frost et al., 2017a). I further mapped scattering heterogeneities within the mantle from the CMB to the surface, finding that heterogeneities are more common close to mantle features likely associated with convection (Frost et al., 2017b).
The iron core occupies only 15% of the volume of the Earth. yet supplies a significant proportion of the heat budget, and is the source of the magnetic field. Interactions across the CMB may affect the composition of the Earth’s lower mantle, and modulate the pattern of the magnetic field. Meanwhile, the inner core affects the pattern of convection in the outer core, and thus likely the magnetic field. The growth of the inner core is recorded in it’s crystal structure, which may manifest as seismically resolvable velocity variations.