| Using high quality digital broadband waveforms, I present constraints on mantle structure in three geographic regions: the lower mantle beneath the central Pacific and Central America and the upper mantle beneath northeast Africa.; Highly variable ScS travel times, amplitudes, waveforms, and shear wave splitting indicate that the lowermost mantle beneath the Pacific is heterogeneous both laterally and radially. Beneath the central Pacific, ScSH-SH differential travel times are larger than predicted by PREM due to delayed ScS arrivals and reveal a spatial trend in D″ of decreasing shear velocity to the northeast. ScS splitting reveals the presence of lower mantle anisotropy and spatial variations in the fast polarization direction. These trends suggest lateral gradients in shear flow that may be related to the root of the Hawaiian plume. Stacking of S and P waveforms reveals the presence of a discontinuity 230 km above the core-mantle boundary with a 1.7% increase in shear velocity and a 0.75% increase in compressional velocity. Beneath Central America, positive and negative ScSH-SH residuals are measured. Negative residuals are segregated to the western portion of the region and correspond to a lateral decrease in D″ shear velocity southward. Positive residuals are located in the east due to fast S arrivals which may be caused by mid-mantle heterogeneity. Stacking of shear waveforms shows evidence for two discontinuities above the CMB in the northern subset of data, with no evidence for the presence of any discontinuity in the southern region.; Complete waveform modelling of 3-component seismograms provides constraints upon the crust and upper mantle structure beneath northeast Africa. The best-fit model for the Nubian shield contains a thick 38 km two-layered crust with Vp = 6.3 and 7.0 km/s respectively and lid Vp = 7.9 km/s; this correlates well with modelled structure beneath the Arabian shield. Modelling the Red Sea rift zone shows evidence for a 17 km thick crust with Vp = 6.5–6.7 km/s underlain by anomalously low upper mantle velocities (Vp = 7.7 km/s) and 3–4% upper mantle anisotropy (SH > SV) that points to a fast anisotropic polarization axis parallel to the rift's spreading direction. |
