Data from the Anza array in southern California have been analyzed to study attenuation. The data set includes records from 68 earthquakes, each of which recorded at an average of 4 to 5 three-component stations. Total Q was estimated from spectral amplitudes observed from similar-sized sources at a wide range of distances from single stations. Also, two attenuation-related parameters were obtained using a parametric model for the seismic acceleration spectrum. These data are consistent with the model that the high-frequency acceleration spectrum is described by exponential decay for frequencies between 15 and 100 Hz. The decay parameter, κ, is observed as a function of hypocentral distance, site, and source characteristics. A comparison of κ(r) at Anza with results from the Imperial Valley, California, shows that dκ/dr is similar in the two regions, but that κ(0) is significantly smaller at Anza. This supports the interpretation that κ is an attenuation effect, with κ(0) reflecting Qi, the frequency-dependent component of Q, in the few kilometers immediately below a station and dκ/dr due to a more regional Qi structure at depth. The extrapolated zero-frequency intercepts from these exponential decay fits to the spectra fall-off more rapidly than geometrical spreading, indicating that there is also a significant frequency-dependent conribution to Q in the region.

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