Determining the nature of middle and lower mantle heterogeneity using array analyses of teleseismic data
Vanacore, Elizabeth A.
Doctor of Philosophy
The structure of the Earth's lower mantle has key implications to the distribution of potential mantle reservoirs and the dynamics of mantle convection. To characterize the nature of mantle heterogeneity, three teleseismic studies are combined in this thesis to expand the observational knowledge of middle mantle and lower mantle features. Array stacking techniques (linear, phase weight, Nth root, common-conversion point) are applied to P-wave coda data from West Pacific and South American subduction zones to search for middle mantle discontinuities and scatterers using S to P converted waveforms. In the West Pacific, the mid-mantle reflectors associated with a high velocity anomaly (HVA) are observed dipping towards the western edge of the study region at depths from 930km to 1080km. In contrast, scatterers beneath the middle mantle of South America occur in three depth families at ∼830-950km, ∼1020-1150km, and ∼1450-1550km, primarily sample the edge of HVA's or occur in regions where no velocity anomaly is present with a distinct geometry. Where mineral phase changes within the slab are a viable solution for scatterers/reflectors associated with HVA's, scatterers without geometry occurring in unperturbed mantle as observed in South America are likely due to small scale heterogeneity in the lower mantle. The final study relates to searching for the root of a deep plume source beneath the Galapagos Islands using analysis of SKS and SKKS waveforms. 248 Residual differential travel times of SKKS-SKS are measured and combined with a statistical analysis to reveal a region of positive residual differential travel times in the northeast portion of the sampled region. First order modeling of three potential low velocity structures, an ultra low velocity zone (ULVZ), a plume conduit, and a D" structure, are used as a diagnostic tool. We find that the most likely cause for the observed SKKS-SKS travel time residuals is a low velocity structure within the D" layer. To determine the presence of lower mantle anisotropy, amplitude ratios of the radial and transverse component of SKS and SKKS waveform are calculated and compared. Regions with significant differences in anisotropy are interpreted as the edge of the flow field of a hypothetical mantle upwelling.