Frequency dependence of seismic wave attenuation in the upper mantle beneath the Australian region

S U M M A R Y The differential attenuation for upper-mantle paths beneath northern Australia has been estimated using spectral ratio methods, both in a narrow frequency band near 0.6 Hz to obtain stable estimates for subsequent inversion and also over a broad frequency band up to 3–6 Hz to look at the frequency dependence of attenuation. The frequency dependence of the differential attenuation between Pand S-wave arrivals is estimated using a spectral ratio technique based on a power-law relationship between Q and frequency. This approach isolates the intrinsic Q rather than any scattering component. By exploiting nearly 2000 paths crossing the Australian region, which provide a good coverage of the mantle, the geographical variation of the frequency dependence of attenuation for body waves can be mapped. The differential attenuation (δt∗ sp) between P and S waves in the 0.6 Hz band varies between close to zero for paths wholly within the shield lithosphere to nearly 4 for paths with a long asthenospheric component. A measure of frequency dependence is obtained by fitting the spectral ratio information as a function of frequency using a single power-law relation for the average dependence of the whole path. The estimates of power-law exponents show a strong correlation with qualitative measures of attenuation based on the relative frequency content of P and S. There is a weak frequency dependence in the northwestern part of Australia, especially in the shallow part of the upper mantle where Q is very high. In the eastern part of Australia there is a complex structure in attenuation with a mixture of styles of frequency dependence with exponents approaching 1 for the most attenuative paths.