The Effect of Wavefront Healing on Estimates of S-Wave and P-Wave Heterogeneity in D”
Abstract:Large Low-Shear Velocity Provinces (LLSVPs) form the dominant degree-2 heterogeneity in the lower mantle and cause the largest perturbations of teleseismic body-wave traveltimes and normal-mode frequencies. The ratio R = dlnVS/dlnVP of perturbations in the S-wave velocity and P-wave velocity has been estimated to be higher than 2–3 for LLSVPs. This and other observations support the hypothesis that LLSVPs represent compositionally distinct piles in the lower mantle.
R in the lowermost mantle is primarily estimated using diffracted P waves (Pd) and diffracted S waves (Sd). These long-period body waves have different Fresnel zones. Our preliminary analysis of Pd and Sd traveltimes using spectral-element method synthetics for long-wavelength models of wave speed heterogeneity in D” indicate that ray-theoretical interpretations of Pd and Sd delay times may bias estimates of R. We discuss the effect of wave front healing on Pd and Sd delay times, compare our simulations with 15,000 Pd and Sd delay times and the splitting of Stoneley modes with peak sensitivity in D”, and evaluate the purely thermal and thermo-chemical interpretations of LLSVPs.