J. Ritsema $^{a}$ , L.A. Rivera $^{b}$, J. Tromp $^{a}$, D. Komatitsch $^{a}$, H.-J. van Heijst $^{a}$
$^{a}$ Seismological Laboratory, California Institute of Technology, Pasadena, USA $^{b}$ Institut de Physique du Globe, Universit\'{e} Louis Pasteur, Strasbourg, France
Long-period (T$>$16 s) PP/P and SS/S amplitude ratios have coherent geographic variations. On average, PP/P is $\sim$10\% higher than predicted by the PREM when PP surface-reflection points are within continental regions, and $\sim$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.