Surface-wave sensitivity to 3-D anelasticity

S U M M A R Y Lateral variations in mantle anelasticity (Q) are important for understanding the Earth’s thermal and chemical structure in the mantle. In the past decades, preliminary global 3-D tomographic Q models have been developed based upon the assumption that traveltime (phase delay) anomalies are due to the Earth’s elastic (velocity) structure whereas amplitude anomalies are dominated by 3-D anelastic (Q) structure. In this paper, we calculate the 3-D finite-frequency sensitivity of fundamental-mode surface-wave phase delays and amplitudes to perturbations in anelasticity (Q). Calculations of Q and velocity sensitivity kernels show that (1) roughly 15–20 per cent of observed phase delays in long-period surface waves can be explained by lateral variations in Q in the upper mantle; and (2) focusing and defocusing effects due to 3-D velocity structure account for a major portion of observed amplitude perturbations in long-period surface waves. The coupling between elastic and anelastic effects in both seismic traveltimes and amplitudes indicates that a joint inversion of 3-D velocity and 3-D Q structure accounting for both anelastic dispersion and associated focusing and defocusing effects is necessary in mapping lateral heterogeneities in the upper mantle.