Data from the Anza array in southern California have been analyzed to yield a model for the depth dependence of attenuation. The result is obtained from a formal inversion of the distance dependence of the spectral decay parameter, κ, observed from sources at a wide range of distances from single stations.
The inversion procedure assumes constant Qi in plane layers and finds models which are as nearly constant with depth as possible. We find that the data cannot be explained by a model in which Qi is constant with depth and that the data generally require three-layer models. The resulting models typically give Qi for P waves between 300 and 1000 in the top 5 km, rising to 1000 to 3000 at greater depths, and decreasing to 700 to 1000 around 12 km depth. Qi for S waves is slightly higher in most cases. Because this depth dependence of Qi is generally correlated with the depths of earthquake epicenters, we suggest that Qi may be due to a pressure and temperature-controlled intrinsic attenuation mechanism.