Radial Qμ structure of the lower mantle from teleseismic body-wave spectra

We have measured 150,000 P and 130,000 S wave spectral ratios up to 0.8 using recordings of 250 deep (focal depth > 200 km) earthquakes from 890 global and regional network stations. We have inverted these data to estimate the attenuation parameters t*(p) and t*(p) for P and S waves and a radial profile of the quality factor Q(mu) for the lower mantle. On average, t*(p) increases by about 0.2 s and t*(s) increases by about 0.7 s between epicentral distances of 30 degrees and 97 degrees. The relatively strong increase of t*(s) (t(s) approximate to 1t*(p)) that intrinsic shear attenuation is the cause of the overall trend in our data. The increase of t*(p) and t*(s) with distance is smaller than predicted by models PREM [12], QL6 [11], and QLM9 [16]. Assuming PREM values for Q in the upper mantle, where the data lack resolving power, the P and S wave spectra are explained best if Q(mu), increases from about 360 at PREM's 670-km discontinuity to 670 in the lowermost mantle. The high values for Q can be reconciled with previously determined values by invoking a frequency-dependence of Q(mu)(omega) that is proportional to omega(0.1). Data that are separated in 'Pacific' and 'circum-Pacific' subsets have slightly different trends. Estimates of eit and t*(s) for the Pacific data, which sample the large low shear-velocity province of the Pacific, are higher than the circum-Pacific estimates. Thus, it appears that the Pacific large low shear velocity province has accompanying low Q(mu). The difference in Q(mu) in the lowermost 1000 km of the mantle beneath the Pacific and beneath the circum-Pacific is at most 17%. Lateral variations of this magnitude are marginally resolvable given the uncertainties of our measurements. (C) 2011 Elsevier B.V. All rights reserved.