Intellectual Merit: The goal of this research is to advance fundamental understanding of the Earth's convection processes in the mantle through mapping seismic anisotropy in the Bullen transition zone (depth 400-1000 km). This research develops finite-frequency theory to account for wave diffraction effects in multi-mode seismic surface waves, and applies the theory to obtain high-resolution, global anisotropic 3-D structure in the Bullen transition zone. This research addresses the following important questions on mantle dynamics at a global scale including: (1) Is there significant radial anisotropy in the transition zone? How does the anisotropic structure correlate with the isotropic velocity structure? (2) Is the there any significant change in radial anisotropy across (above, within, below) the transition zone? does it support coupling or decoupling of the flow above/below the transition zone? (3) Are there any significant variations in anisotropy in the Bullen transition zone correlated with slabs and plumes? (4) Can a whole-mantle convection model explain the presence/absence and the pattern of radial anisotropy in the transition zone? How does seismic anisotropy in the Bullen transition zone fit into whole (or layered) mantle convection?
This research includes the following main projects: (i) develop 3-D finite-frequency theory for multi-mode surface waves and 2-D boundary sensitivity kernels for mantle transition zone discontinuity depth perturbations, fully account for radial anisotropy in the Earth?s mantle; (ii) build a preliminary global dataset of multi-mode surface-wave measurements; and (iii) diffractional tomography using full 3-D velocity kernels and 2-D boundary kernels to obtain high-resolution global models of seismic anisotropy in the uppermost ~1000 km of the mantle. Broader impacts: The multi-mode surface-wave sensitivity kernels developed in this research can be applied to regional studies. Tomographic models will be made available on-line. The anisotropic model in the Bullen transition zone obtained in this proposed work will provide important references for mantle geodynamical modelings, and provide important seismological constraints on mantle convection hypothesis such as the transition-zone water filter model [Bercovici and Karato, 2003]. The support requested in this project will be used to educate one Ph.D graduate student.
