The first tomographic images of P wave variations in the Earth's lower mantle obtained with finite-frequency theory (as opposed to classical ray theory) show a significant enhancement of important features at depth, especially those of smaller scale that are susceptible to effects of wavefront healing, that are neglected in the classical ray theoretical inversion methods. In particular, the compressional velocity anomalies show that a limited number of plumes (Hawaii, Tahiti, Easter Island, Cape Verde) clearly originate deep in the lower mantle, some clearly do not (Iceland, Afar), while the jury is still out on others. The investigators will continue efforts to improve tomographic images using these new and innovative methods. This will include adding short period P waves to the present long period data set, extending efforts to S waves, and continuing a recently begun program to measure systematically the arrival times and amplitudes of P and S waves for a range of frequencies. The investigators will also extend finite-frequency theory to complicated 3D models so it can be used for shallower investigations such as USArray. They plan to extend Radiative Transfer theory such that it can predict scattering energy from tomographic models, and predict observables that can be used to constrain such models. This will be initiated with the inversion of amplitudes. Finally, they propose to investigate finite frequency effects in waveform inversions for the Australian upper mantle using a spectral element approach, in an effort to resolve discrepancies between various models of heterogeneity and anisotropy.
Preliminary results seem already to have resolved the major controversy about the existence of deep mantle plumes. The investigators expect to throw new light on the puzzling negative correlation between compressibility and shear modulus variations in the lower mantle, on the existence of a lower mantle layering, and on the origin of mantle plumes. In addition, several students and postdocs will be educated to use the new tomographic methods.
