Amplitudes of the vertical component of ground motion caused by Rayleigh waves with periods of about 20 sec were measured over an array consisting of the 25 stations of the WWSSN in the United States; the Rayleigh waves were produced by shocks in selected regions of the Pacific and one area along the Mid-Atlantic Ridge. Amplitudes were plotted as a function of azimuth from epicenter to station and on outline maps centered on the epicenters. Patterns of amplitude measured by the array are highly sensitive to epicentral location; a pattern appears to change continuously with change in epicenter. The amplitudes generally show variations of a factor of 10 or more within the array. These striking fluctuations of amplitude appear to be caused by focusing due to lateral variations in phase velocity; they cannot be explained by other factors such as an asymmetrical radiation pattern at the source. Analysis based on a simple model of a horizontal variation in phase velocity yields theoretical amplitude patterns that explain the size and character of the most prominent variations in amplitude. The most marked fluctuations in amplitude here presented can be explained as the effect of regions of horizontally varying phase velocity that coincide with obvious features in the Pacific, notably, the island of Hawaii.

Amplitudes of long-period P waves appear to vary about one-fourth as much as those of Rayleigh waves in the array.

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