The tectonic diversity of the North American continent makes it an ideal region to investigate the structure and dynamics of the continental upper mantle. Investigations of timely geophysical questions, such as the relation to geological age of the variations in the lithospheric thickness, the relation of upper-mantle anisotropy to present day asthenospheric flow and past tectonic events, the nature and strength of the lithosphere/asthenosphere coupling and the driving mechanisms of plate motions, are contingent upon obtaining high-resolution 3-D tomographic models of the isotropic and anisotropic mantle structure of the continent.
The first major global tomographic study was made by Professor Dziewonski in early 70's. The idea of this study was that the travel time anomalies observed for many ray paths, criss-crossing the Earth between various points near the Earth's surface and reaching different depths in its interior, could be resolved formally into a three-dimensional (3-D) model. This is now called `seismic tomography', as it conceptually resembles the medical CAT-scan. The early results were reported orally in 1974 and 1975 by Dziewonski and a full report was published in January 1977 (Dziewonski et al., 1977). A more detailed description can be found a statement written by Professor Dziewonski. The motivation for studying 3-D structure of the Earth's interior is that it may offer the best information on the dynamic processes in the deep interior of the Earth. As the seismic wave speeds change with temperature, it is plausible to obtain 3-D snapshots of the convection pattern in the Earth. By performing waveform and travel time inversions using long period seismic records, we have obtained global models for the long wavelength 3-D velocity structure of the Earth.