New constraints on and models of global and plate-scale anisotropic mantle structure

DESCRIPTION: The goal of this project is to advance our knowledge of the large-scale elastic structure of Earth's interior using seismological imaging techniques and a broad range of seismological observations. This research will lead to an improved characterization of the anisotropic velocity structure of the mantle and will thereby help resolve the significant differences that are found between current seismological models. In particular, the research will address the existence and strength of large-scale anisotropy in the transition zone and the lower mantle, the correlation of density and elastic moduli in the deep mantle, and the scaling between shear- and compressional-velocity variations. New measurements of surface-wave overtone dispersion will be collected, which will be diagnostic of the validity of existing models that indicate strong variability in anisotropic fabric in the Pacific asthenosphere. The research will elucidate the relationship between radial and azimuthal anisotropy in the oceanic lithosphere and asthenosphere. Two linked investigations will be pursued. A global study will build on the data sets collected and analysis tools developed in the determination of the recent global model S362ANI (Kustowski et al., 2008). Project research will extend this earlier work in several ways, most importantly by (1) inclusion of normal-mode center frequencies and Q values to constrain better the global average structure, (2) incorporation of published normal-mode splitting functions to resolve transition-zone and lower-mantle velocity heterogeneity and anisotropy, and (3) formulation of the inverse problem using a flexible parameterization of covariation of model parameters (wave speeds, anisotropy, and density) to facilitate model interpretation and investigations of model resolution. A regional study focused on the Pacific will involve the analysis of new and existing seismological data sets derived from permanent and temporary seismic stations within and surrounding the Pacific Basin. Love and Rayleigh wave fundamental-mode and overtone measurements will be collected. Together with several existing travel-time data sets, and incorporating knowledge of crustal thickness and plate age, the surface-wave data will be inverted to determine the radially and azimuthally anisotropic structure of the lithosphere and asthenosphere. Following the approach of Nettles and Dziewonski (2008), a variable-resolution parameterization will be used to embed the detailed model of the shallow mantle beneath the Pacific in the coarser global 3-D model. Analysis and interpretation of the seismological models developed in this research will lead to better-supported inferences regarding the state, composition, and deformation of the Earth's mantle. Tighter constraints on elastic and density heterogeneity in the deep mantle, and their covariation, will inform geophysical models for planetary-scale dynamics. Better-resolved models of the anisotropy of the oceanic lithosphere and asthenosphere will sharpen constraints on possible mineral fabrics and strain geometries, as well as on the potentially ubiquitous presence of melts in the shallow mantle. The new seismological models will be useful for a broad range of geoscientists investigating the composition, evolution, and dynamics of the Earth's deep interior.