Q and Its Effect on Short Period P Waves from Explosions in Centra Asia

A detailed model for the attenuation of high frequency (1-8Hz) P waves is developed for the paths from the Soviet test site in eastern Kazakhstan to the sites of the four 20-element U K arrays. These short period arrays (Scotland, India, Canada and Australia) have operated in an essentially constant configuration since the mid-1960's and are well suited to high frequency spectral analyses. Event P wave spectra are computed by summing the power spectra (corrected for the noise power) from individual array elements, and the interpretation is based on very smooth average path spectra obtained by stacking spectra from many similar events. Effects of source differences, especially corner frequency variations, can be seen and taken into account. The attenuation model includes contributions from both intrinsic absorption and scattering. The absorption Q dominates at low frequencies and is strongly dependent on frequency on this band. The preferred model has t* = 0.6 seconds at long period and frequency dependence characterized by a half-amplitude value of 0.05 to 0.1 seconds. The scattering is represented by an essentially frequency-independent t* of about 0.1 seconds and has an important effect above 2.5 Hz. Differences in the phase spectrum for these two mechanisms for attenuation are important. A key conclusion is that regional attenuation variations are not presented very well by fitting frequency-independent t* operators to P wave spectra in the 0.5 - 3.0 Hz band. Source spectrum variations can have a large biasing effect, as can the effect of frequency dependence of Q.