abstract

As a consequence of the VELA-Uniform program, it has been possible to contour Pn and P-wave maximum amplitude patterns over the United States from a number of well-recorded seismic events in the unified magnitude range of 4 and 5 occurring both within and without North America. The seismogeological effects reported by Pasechnik for Eurasia appear to be present within the United States, for there are marked and persistent deviations from the conventional curves showing attenuation of signal amplitude with distance. In general, the United States west of a line from Glacier National Park to El Paso produces lower seismic amplitudes for teleseismic events of a given magnitude than would be expected. Two other areas of comparatively low signal strength are tentatively located in the Wichita-Ouachita Mountain belt and the eastern portion of the Appalachian Mountains. Higher than usual signal amplitudes were found in the Dakotas, southwest Texas, and the northern Lake Michigan region. It is suggested that regions of high signal anomalies are typical of areas with deep sedimentary basins or of high upper mantle velocities, while regions with low signal levels tend to be typical of areas with considerable tectonic activity and low upper-mantle velocities.

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