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

Lateral variations in anelasticity (Q) provide important constraints complementary to 3-D wave speed variations in mapping 3-D thermal and compositional structures in the mantle. In present-day joint tomographic inversions of global velocity and anelasticity (Q) structure, 3-D anelastic dispersion effects on surface waves have been ignored. In this work, we quantify the effects of 3-D Q structure on surface wave phase delays by simulating wave propagation in 3-D wave speed and 3-D Q models using a spectral element method (SEM). We compare phase delays caused by 3-D Q structure and those caused by 3-D velocity structure. Our results show that (1) roughly 15-20 per cent of the observed phase delays (traveltimes) in long-period surface waves are due to 3-D Q structure; this implies that neglecting 3-D anelastic dispersion effects can lead to biased velocity models in seismic tomography; (2) the effects of Q perturbations on surface wave phase delays are frequency dependent as a result of local S-wave anelastic dispersion, frequency-dependent depth sensitivity of surface waves as well as the 3-D distribution of Q anomalies. In our numerical experiments, the significance of 3-D anelastic dispersion increases with wave period, and the frequency dependence is most apparent in the period range between 60 and 150 s and becomes weaker at 150-200 s and (3) assuming a thermal origin, anelastic delays caused by 'hot' anomalies (or advances caused by 'cold' anomalies) are correlated with elastic delays (or advances), but their relation is not linear: the ratio between anelastic and elastic delays (or advances) becomes larger for 'hotter' anomalies than for 'colder' anomalies.