The seismic noise spectrum in the frequency range 0.5 to 31.5 cycles per second is presented graphically for a number of sites over a wide geographical range. Except for a small anomalous effect near 2 or 3 cycles per second, the ground particle motion curves are observed to decrease smoothly with increasing frequency at a rate approximately proportional to the second power of frequency. The curves steepen at frequencies below 1 cycle per second.

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