Upper mantle anelasticity and its time dependence resolved by GPS in 3-D ocean tide loading displacements

Abstract

Ocean tide loading (OTL) refers to the Earth's deformational response to periodic ocean mass redistributions driven by astronomical tides. This transient deformation likely involves anelasticity, which is associated with energy dissipation and plays a crucial role in understanding Earth's deformation across various temporal scales. However, anelastic contributions to OTL are minor (submillimetre-to-millimetre scale) relative to elastic OTL deformation and are comparable in magnitude to the influence of lateral heterogeneities in crust and upper mantle elasticity. Because of this, detecting and isolating anelastic OTL deformation remains challenging. Here, by analysing high-accuracy (0.2-0.4 mm) Global Positioning System (GPS) data from 255 sites across western Europe, and comparing with OTL forward modelling results from advanced 3-D elastic and anelastic Earth models incorporating sediments, we unambiguously demonstrate anelastic OTL displacements, in both horizontal and vertical directions. Our findings highlight the necessity of incorporating anelasticity into geodynamic models, particularly for processes operating at sub-seismic frequencies, and into a unifying viscoelastic law for modelling Earth deformations across multiple timescales. Furthermore, using GPS observations and realistic 3-D models, we constrain anelastic parameters for Earth's upper mantle. These parameters reveal the weakening or dispersion of upper-mantle shear modulus, by up to 20 % at semi-diurnal tidal frequencies compared to its 1 Hz value in the Preliminary Reference Earth Model (PREM), and constrain the time dependence of this weakening. Our results support the application of OTL observations for constraining Earth's anelasticity in the sub-seismic frequency range.