Insights from ScS–S measurements on deep mantle attenuation

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, δ t ScS – S ⋆ , sampling the mantle below Central America and below Alaska that we compare to elastic delay times, δ t ScS – 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 δ t ScS – S ⁎ 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 δ t ScS – S ⋆ 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 ( Q μ ≈ 250 ) and a low attenuation layer at its base ( Q μ ≈ 450 ). A comparison of our results with low-frequency normal-model Q models is consistent with frequency-dependent attenuation with Q μ ∝ ω α 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.