The effects of 3-D anelasticity (Q) structure on surface wave amplitudes

SUMMARY

We investigate the effects of lateral perturbations in velocity and anelasticity (Q) on surface wave amplitudes based upon wave propagation simulations in 3-D earth models using a spectral element method (SEM). We construct 3-D Q models based upon a wave speed model S20RTS using a set of reasonable mineralogical parameters assuming lateral variations in both wave speed and anelasticity are due to temperature perturbations. We measure and compare amplitude perturbations of surface waves caused by 3-D wave speed (elastic) structures and those caused by 3-D anelastic (Q) structures at a period range of 50–200 s. The measurements show that influence of 3-D wave speed structures on amplitudes is comparable to that of 3-D Q structures at short period (50 s), but becomes dominant at longer periods. In ray theoretical framework, surface wave amplitudes can be decomposed into three terms—elastic focusing, anelastic attenuation and anelastic focusing—which depend, respectively, upon the roughness of phase velocity perturbations (), perturbations in anelasticity (δ ln Q⁻¹) and the roughness of perturbations in anelasticity (). Ray-theoretical calculations confirm the relative importance of 3-D Q and 3-D wave speed in perturbing surface wave amplitudes and show that anelastic focusing effects, which have been ignored in present-day tomographic studies, have a more significant effect than attenuation. Therefore, the effects of 3-D Q structures can be ‘counter-intuitive’ at long period for surface waves travelling through a low-Q region may experience amplitude increase.