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Measurements of spectral density levels of shortperiod seismic noise are obtained for points distributed throughout the continental United States and for a few non-North American locations. Noise amplitudes and gradients in the far field correlate on a regional basis with major physiographic provinces, as revealed by iso-particle-velocity contouring. The ensemble of space and time samples of noise is examined to illustrate the average spectrum and dispersion for three orthogonal components of ground-particle velocity in the range 0.25 to 100 cps. Probability distributions of noise amplitudes based on a collection of space averages and time averages are presented for several frequencies. The ratio of horizontal to vertical noise amplitude (H/Z) exceeds the theoretical value for simple Rayleigh waves and shows a frequency-dependent variation with regional geologic environment. (Author)