We apply a recently developed method based on the instantaneous frequency to analyze broadband seismic data recorded by the transportable USArray. We measure in the frequency band [0.018–0.2] Hz about 700 high-quality differential ScS–S anelastic delay times, View the MathML source, sampling the mantle below Central America and below Alaska that we compare to elastic delay times, δtScS–S, obtained by cross-correlating the S and ScS signals. We confirm that the instantaneous frequency matching method is more robust than the classical spectral ratio method. By a series of careful analyses of the effects of signal-to-noise ratio, source mechanism characteristics and possible phase interferences on measurements of differential anelastic delay times, we demonstrate that in order to obtain accurate values of View the MathML source the seismic records must be rigorously selected. In spite of the limited number of data that satisfy our quality criteria, we recover, using an additional stacking procedure, a clear dependence of View the MathML source on the epicentral distance in the two regions. The absence of correlation between the obtained anelastic and elastic delay-times indicates a complex compositional-thermal origin of the attenuation structure, or effects of scattering by small scale structure, in accordance with possible presence of subducted material. The regional 1-D inversions of our measurements indicate a non-uniform lower mantle attenuation structure: a zone with high attenuation in the mid-lower mantle (Qμ≈250) and a low attenuation layer at its base (Qμ≈450). A comparison of our results with low-frequency normal-model Q models is consistent with frequency-dependent attenuation with Qμ∝ωα and α=0.1–0.2 (i.e. less attenuation at higher frequencies), although possible effects of lateral variations in Q in the deep mantle add some uncertainty to these values.
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