Broadband waveform modeling of deep mantle structure

The lowermost 200--300 km of the mantle, known as the D'' region, exhibits some of the strongest seismic heterogeneity in the Earth and plays a crucial role in thermal, chemical and dynamic processes in the mantle. Hot spot volcanism may originate from the deep mantle in regions exhibiting the Earth's most pronounced lateral shear (S)-wave velocity gradients. These strong gradient regions display an improved geographic correlation over S-wave velocities to surface hot spot locations. The origin of hot spot volcanism may also be linked to ultra-low velocity zone (ULVZ) structure at the core-mantle boundary (CMB). Anomalous boundary layer structure at the CMB is investigated using a global set of broadband SKS and SPdKS waves from permanent and portable seismometer arrays. The wave shape and timing of SPdKS data are analyzed relative to SKS, with some SPdKS data showing significant delays and broadening. We produce maps of inferred boundary layer structure from the global data and find evidence for extremely fine-scale heterogeneity where our wave path sampling is the densest. These data are consistent with the hypothesis that ULVZ presence (or absence) correlates with reduced (or average) heterogeneity in the overlying mantle. In order to further constrain deep mantle processes, synthetic seismograms for 3-D mantle models are necessary for comparison with data. We develop a 3-D axi-symmetric finite difference (FD) algorithm to model SH-wave propagation (SHaxi). In order to demonstrate the utility of the SHaxi algorithm we apply the technique to whole mantle models with random heterogeneity applied to the background model producing whole mantle scattering. We also apply SHaxi to model SH -wave propagation through cross-sections of 3-D lower mantle D'' discontinuity models beneath the Cocos Plate derived from recent data analyses. We utilize double-array stacking to assess model predictions of data. 3-D model predictions show waveform variability not observed in 1-D model predictions, demonstrating the importance for the 3-D calculations. An undulating D'' reflector produces a double Scd arrival that may be useful in future studies for distinguishing between D'' volumetric heterogeneity and D'' discontinuity topography.