Measurements of the response to earthquake motion of an unsaturated soil in Coalinga, California, indicate that this response is linear, within the limits of random errors, for frequencies below 10 Hz and for accelerations up to 0.7 g. The response of the soil site relative to a bedrock site was determined by dividing the amplitude spectrum of the soil site ground motion for a given event by the amplitude spectrum of the bedrock site ground motion for the same event. The relative response of this pair of sites to 23 regional and small local earthquakes is the same in the 1- to 10-Hz band as the response to seven strong motion events, within a random error of a factor of 1.3. At 10 to 11 Hz, the relative response to strong motion is significantly reduced by as much as a factor of 2 from the relative response to the 23 regional and small local earthquakes at the 95 per cent confidence level, suggesting the possibility of nonlinear response. The response of the soil site is a factor of 2 to 3 times larger than that of the bedrock site. The variation in the spectral ratios for strong motion events and for regional and small local events about their respective averages has a standard deviation of about a factor of 2.