On GPS-based Ocean Tidal Loading Displacements and Their Potential to Constrain Mechanisms of Anelasticity

Ocean tidal loading (OTL) displacements obtained from fine-tuning GPS data analyses have recently been reported with a remarkable increase in both quantity and quality. Kinematic precise point positioning (PPP-GPS) data analysis techniques, in particular, can now reliably infer periodic vertical displacements down to the 0.2 mm level. Such accuracy, together with good spatial coverage of coastal areas subjected to high tidal range at the principal lunar semi-diurnal frequency, is reviving interest in a long-standing goal of Earth-Tide research: constraining Earth properties in-situ, in the one and two cycle(s) per day range. Thus, the time is appropriate to review the status of research on anelasticity in seismology, geodynamics and mineral physics in order to identify which findings are relevant to tidal energy absorption via loading in the microHz band. For a given network of GPS stations, these findings call for a careful consideration of: the spatial distribution of tidal loading energy, both in depth and laterally; the constitution of the rock formations being deformed; and the ambient thermodynamics states. The focus here is on the dissipation factor 1/Q and its frequency dependence, taken as a measure of anelasticity. Particular attention is paid to expected mechanisms of dissipation, in the context of the traditional absorption band model. While modelling a 1/Q (f) depth-distribution from surface data is an inherently non-uniqueness problem, it is possible to construct a set of forward models and use GPS-based OTL displacements (especially their phases) to validate which models are realistic, at least, thus far, for fluid-saturated, fractured rock formations.