Anelasticity from seismic to tidal timescales: Theory and observations

Abstract

We examine strain energy dissipation of Earth's normal modes and body tides in order to compare these observations with predictions from an experimental model of intrinsic dissipation. For this comparison we employ a recently developed self-consistent treatment of modes and tides, that includes the separation of dynamical processes (self-gravity and inertia) from intrinsic attenuation. We select a set of normal modes and tides with similar depth sampling centered on the lower mantle, without contributions from the core. This dataset ranges from 7 min to 18.6 yr, or about 6 decades in frequency. The results show that the attenuation of the higher-frequency modes defines a broad plateau in dissipation (inferred experimentally to occur at the transition from elastic to anelastic behavior) whereas the attenuation of lower-frequency tides falls on a frequency-dependent absorption band. This reconciles published, seemingly contradictory models of dissipation for modes versus tides: the absorption band for modes terminates at periods where tidal models infer its onset. Microphysically, the experimental data can be explained by elastically accommodated grain boundary sliding, producing a broad plateau at short periods, transitioning to diffusionally assisted grain boundary sliding at longer periods, corresponding to an absorption band. Extrapolation of the fit to the experimental data with parameters adopted for the lower mantle can reconcile the seismological and geodetic observations.