Albuquerque Seismological Laboratory
Upper Mantle Shear Velocity Structure of North America
This abstract is published as:
- D. Alsina, R.L. Woodward, and R. Snieder, Upper mantle shear velocity structure of North America, EOS Trans. AGU, 76, 422, 1995.
We have obtained a model of the upper mantle shear velocity structure of North America by using a linearized scattering theory to invert a large number of direct arriving Rayleigh wave waveforms. The model covers the contiguous United States as well as Mexico and most of Central America. We parameterize the shear velocity variations in three layers (25-100 km, 100-200 km, and 200-300 km). The lateral resolution for most areas of the model is on the order of several hundred kilometers.
The largest and most striking feature of the model is the rapid transition, roughly aligned along the Rocky Mountain Front, between the high-velocity shield in the northeastern U.S. and the low velocities which characterize the western U.S. Numerous other smaller scale features are also visible in the model. For example, the model shows a high velocity anomaly beneath the state of Washington which could be explained as the subducting Juan de Fuca plate. Elongate low velocity structures, which may be related to the presence of magnetism and partial melt, are seen beneath the eastern Snake River Plain and Yellowstone Plateau and beneath the Trans-Mexican Volcanic Belt. The most distinctive low velocity region is located beneath the southwestern U.S. and northwestern Mexico, extending roughly from the Mendocino to the Rivera triple junctions. This low velocity feature roughly corresponds to the portion of the continental margin where subduction is not occurring. A more poorly resolved low velocity feature is seen beneath the southeastern U.S.
The key aspect of our inversion procedure is the use of linearized scattering theory. The scattering formalism allows the inversion to use both the phase and amplitude of the surface wave waveforms and, most importantly, accommodates sharp lateral variations in the velocity structure such as those seen in the model.