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

Abstract

We have measured 150,000 P and 130,000S wave spectral ratios up to 0.8Hz using recordings of 250 deep (focal depth >200km) earthquakes from 890 global and regional network stations. We have inverted these data to estimate the attenuation parameters tP⁎ and tS⁎ for P and S waves and a radial profile of the quality factor Qμ for the lower mantle. On average, tP⁎ increases by about 0.2s and tS⁎ increases by about 0.7s between epicentral distances of 30° and 97°. The relatively strong increase of tS⁎ (ts≈4tP⁎) suggests that intrinsic shear attenuation is the cause of the overall trend in our data. The increase of tP⁎ and tS⁎ with distance is smaller than predicted by models PREM [12], QL6 [11], and QLM9 [16]. Assuming PREM values for Qμ in the upper mantle, where the data lack resolving power, the P and S wave spectra are explained best if Qμ increases from about 360 at PREM's 670-km discontinuity to 670 in the lowermost mantle. The high values for Qμ can be reconciled with previously determined values by invoking a frequency-dependence of Qμ(ω) that is proportional to ω0.1. Data that are separated in ‘Pacific’ and ‘circum-Pacific’ subsets have slightly different trends. Estimates of tP⁎ and tS⁎ 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 Qμ. The difference in Qμ in the lowermost 1000km 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.