We calculated the spectral decay parameter (κ) in the southern Basin and Range province, using records from earthquakes located near the Pitáycachi fault. We found that for a given distance and recording site the values of κ vary considerably, possibly as a result of the lateral heterogeneity of the crust. Individual values of κ do not show a clear trend with magnitude, nor with the average values of κ. The average values of κ tend to increase with distance up to about 70–80 km and then to decrease between 80 and 100 km, suggesting higher S-wave attenuation at shallow depths and lower attenuation for deeper paths. We analyzed the possible dependence of κ on earthquake size by determining empirical curves that describe the behavior of κ with distance and magnitude. Our results indicate that κ is independent of earthquake size within the magnitude range (M<3.5) of the events analyzed. The nonparametric curves also provided site-specific estimates of κ near the surface (κ0) for the stations and groups of stations analyzed. We found that the average κ0 value of 0.04 sec in northeastern Sonora is similar to values of κ0 reported in other regions. We conclude that κ0 depends not only on the rock type but also on the degree of fracturing and erosion of the rocks near the recording site.

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