Chapter 28: Seismic properties of the crust and uppermost mantle of the conterminous United States and adjacent Canada

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Abstract
Seismic refraction profiles for the conterminous United States and adjacent Canada have been compiled from published and unpublished sources. The crustal models derived from these profiles were used to compile data on upper-mantle seismic velocity (P n ), crustal thickness (H c ) and average seismic velocity of the crystalline crust (V̅). These data indicate continent-wide averages of P n = 8.02 km/sec, H c = 36.1 km, and V̅ = 6.44 km/sec. Comparison of compressional wave parameters with shear-wave date derived from surface-wave dispersion models at 95 North American locations indicates an average value for Poisson’s ratio of 0.258 for the crust and 0.270 for the uppermost mantle. Contour maps illustrating lateral variations in crustal thickness, upper-mantle velocity, and average seismic velocity of the crystalline crust show a number of features correlative with geologic and tectonic provinces. Comparison of the distribution of seismic parameters with a smoothed free-air anomaly map indicates that a complicated mechanism of isostatic compensation exists for the North American continent involving both lateral density changes in the crust and upper mantle and variations in the thickness of the crust. Several features on the seismic contour maps also are correlative with regional magnetic anomalies.

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