A four-level downhole array of three-component instruments was established on the southwest shore of San Francisco Bay to monitor the effect of the sediments on low-amplitude seismic ground motion. The deepest instrument is at a depth of 186 m—2 m below the top of the Franciscan bedrock. Earthquake data from regional distances (29 km ≦ Δ ≦ 485 km) over a wide range of azimuths are compared with the predictions of a simple plane-layered model with material properties independently determined. Spectral ratios between the surface and bedrock, computed for the one horizontal component of motion that was analyzed, agree rather well with the model predictions; the model predicts the frequencies of the first three peaks within 10 per cent in most cases and the height of the peaks within 50 per cent in most cases. Surface time histories computed from the theoretical model predict the time variations of amplitude and frequency content reasonably well, but correlations of individual cycles cannot be made between observed and predicted traces.