Two methods of analysis have been applied to the El Centro differential array data from the 1979 Imperial Valley earthquake. Spectral ratios of array averages were developed as one way of estimating the spatial averaging of ground motion. This spatial averaging will result in a reduction in the high-frequency motion experienced by a large rigid foundation that is assumed to move as a single unit. As an alternative, covariance measurements in several frequency bands were made to illustrate how the fraction of the power propagating as single traveling wave decreases with increasing distance between stations. Base averaging reduction factors determined from these analyses show that reductions of 10 to 30 per cent would be expected for this particular earthquake for foundations of about 50 m in dimension and for frequencies above 20 Hz. The effect is frequency-dependent and does not show any significant reduction at frequencies below 5 Hz. For this particular situation, at a distance from the rupture surface of about 5 km, the reduction is more pronounced for horizontal components than for vertical components.