Effects of crust and mantle heterogeneity on PP/P and SS/S amplitude ratios

Long‐period (T > 16 s) PP/P and SS/S amplitude ratios have coherent geographic variations. On average, PP/P is ∼10% higher than predicted by the Preliminary Reference Earth Model (PREM) when PP surface‐reflection points are within continental regions, and ∼10% lower than PREM predictions for oceanic reflection points. Spectral‐element synthetics show that this variation can be attributed mostly to the effect of crustal thickness on the long‐period PP reflection coefficient. The anomalies of SS/S are similar in amplitude but their geographic variation does not obviously correlate with ocean/continent variations. The variation of SS/S determined from spectral‐element waveforms of S and SS for 3‐D models of the crust and mantle is similar to the observed variation of SS/S. This suggests that wave propagation effects are largely responsible for the observed SS/S variation, not only intrinsic attenuation.

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