Attenuation near Anza, California

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 d κ / 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 Q i , the frequency-dependent component of Q , in the few kilometers immediately below a station and d κ / dr due to a more regional Q i 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.