Models of the Frequency Dependence of Q in the Mantle Underlying Tectonic Areas of North America, Eurasia and Eastern Pacific

摘要

The results of wide-band data analyses for the determination of frequency dependent Q models of te mantle under the tectonic areas of Eurasia, North America and the Eastern Pacific are presented. For tectonic areas of Eurasia, the spectra of short period P waves from Kazakh explosions are used to estimate t* in the 1 to 8 Hz range. Amplitude ratios of long period multiple ScS phases were used to estimate attenuation in the 0.3 to 0.9 Hz band. Under tectonic Eurasia, these analyses gave t* sub ScS of approx. 3.9 + or - 0.4 sec. Thus, for Eurasia, t*(f( for tectonic paths is approximately parallel to, but approximately 0.25 sce (for t* sub p) greater than t*(f) under the shield. For North America, th amplitudes of long period multiple ScS phases were analyzed for two type of paths. Multiple ScS amplitudes recorded in the Northeastern US for events in the Aleutians (paths across the Canadian shield) and similar data recorded in the Southeastern US for a Central American event. These data analyses show that the attenuation in the long period band in North America varied in the same sense regionally as shown in previous work in the short period band. For Eastern Pacific the upper mantle attenuation appears to be the highest of all regions studies with t* uniformly higher than in the two 'tectonic' continental areas discussed above. These findings support the general idea that t*(f) curves for teleseismic paths crossing the upper mantle in various areas of the world generally do not converge towards the long period end of the seismic spectrum as implied by some 'absorption band shift' models. Instead, the t*(f) curves tend to run quasi-parallel throughout the seismic frequency band.