Seismological modelling of global earth structure
Upper mantle discontinuities from PP- and SS-precursors The 410km discontinuity is examined using two global datasets of precursors to the PP and SS phases. The precursor amplitudes are used to constrain the impedance contrast, and examine lateral variations in the reflection coefficient of the discontinuity. P- and S-wave reflection amplitudes vary over different scale lengths, which could be due to the presence of melt, water or other chemical heterogeneities in the transition zone. Models for 410-topography are also derived, and the relationship between topography and transition zone seismic velocity anomalies examined. A moderate negative correlation exists between long wavelength 410-topography and transition zone velocity anomalies. However, when shorter wavelengths are included the relationship becomes more complex, and in some regions positive correlations are seen. This suggests that long wavelength variation of discontinuity topography and seismic velocity, is due to thermal effects. However, at shorter wavelengths the influence of chemical heterogeneities becomes important. Different spectra for thermal and chemical heterogeneity suggests that chemical anomalies can survive in convecting mantle. Lower mantle reflectors and S-wave scattering The lowermost mantle is investigated using a phase stripping technique and two migration methods: a backprojection, and a scheme with weights based on the Generalised Radon Transform. Resolution of the results is tested by migrating synthetic datasets. In some regions the results can be simulated using fairly simple distributions of point scatterers, but elsewhere the results require more complicated structures. The results identify several important properties of the D" region including reflectors within the D" region and a complex pattern of positive and negative scattering potentials near the core-mantle boundary. The results also show the presence of an intermittent D" discontinuity which is not a continuous, nor a global, feature. This suggests that the D" discontinuity is caused by either localised structures, such as thermal or chemical heterogeneity, or a global boundary with a variable impedance contrast.