Long-period P waves recorded in the central United States at distances of 20 to 110 degrees for earthquakes of magnitude 6 and greater indicate that the amplitude of the ground motion is very similar at all stations in the area. The crust is uniform in this region, but the sedimentary section varies both in thickness and in character. Direction of approach, as determined by the ratio of the amplitudes of the horizontal components of motion, does not differ significantly from the great-circle path in the majority of cases studied.
Apparent angles of incidence cannot be satisfactorily explained by a half-space model, because calculations based on such a model require a larger value of the P-wave velocity for the shorter period waves. However, calculations based upon Haskell's model of a crustal layer over a half-space give a satisfactory fit to the observed data.