Research Objectives
In this project we are using several million seismic travel times and massively parallel computing to produce the first three-dimensional image of the structure of the entire Earth (crust, mantle, outer core, and inner core). In addition, we simultaneously estimate the topography of the internal boundaries of the Earth, such as the core-mantle boundary. Our results indicate intriguing coherent structure in the lowermost region of the outer core which may be related to the convection process. This structure has never been imaged before and the facilities at NERSC are allowing us to obtain a rigorous inversion of the seismic data.
Computational Approach
We are using an iterative Lanczos algorithm to image the Earth's interior. A critical element of the algorithm is a matrix-vector multiply which is done in parallel on NERSC's T3E.
Accomplishments
We have completed work on our first three-dimensional image of the structure of the entire Earth. The results are described in a paper in press in the Journal of Geophysical Research. An additional paper, submitted to Science, examines an intriguing structure in the Earth's fluid outer core which is symmetric about the equator (see figure). We are in the process of writing up the results of Lanczos recursion approach which is a significant improvement on conventional imaging methods because it enables us to calculate the spatial resolution of the Earth's structure as well as its uncertainty. Current work involves a much more detailed inversion which makes the problem 4 times larger but allows us to estimate fine-scale three-dimensional structures.
Significance
The advent of plate tectonics has brought about an increased interest in the lateral variations of the Earth's material properties. The three-dimensional distribution of density, temperature, and the compressional (P) and shear (S) velocities within the Earth have a direct relationship to it's dynamics. Therefore, fundamental issues related to convection within our planet, such as earthquake generation, resource distribution (petroleum and minerals), and environmental issues, may be answered by studies of the aspherical Earth structure. For example, we image of the outer core, the region in which the Earth's magnetic field is generated. Recent three-dimensional numerical models of convection in the core have generated renewed interest in this region. Our results indicate symmetric structure in the fluid outer core which is compatible with this convection modeling.
Publications
Vasco, D. W., Peterson, J. E., and Majer, E. L. N. d. Resolving seismic anisotropy: Sparse matrix methods for geophysical inverse problems, Geophysics, (in press).
Vasco, D. W., and Johnson, L. R., N. d. Whole Earth structure estimated from seismic arrival times, J. Geophys. Res., (in press).
Vasco, D. W., and Johnson, L. R., N. d. The seismological signature of core dynamics?, Science, submitted.
Map of the seismic velocity variations at the bottom of
the Earth's outer core. Red signifies
slower seismic velocities, and blue signifies faster velocities.