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.