Tidal Loading and Asthenospheric Anelasticity

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The Earth's surface oscillates on timescales of a few hours, both horizontally and vertically, by up to several centimetres because it deforms under the weight of the oceans which is regularly redistributed by the ocean tides. These 'ocean tide loading' deformations are too small, slow, and spatially smooth to be apparent to us humans, but they can be detected by precise satellite positioning techniques such as GPS. This allows us to investigate the Earth's rheology (deformational behaviour in response to forces) at time scales intermediate between the frequencies of seismic vibrations following earthquakes (seconds to minutes) and the Chandler wobble (an almost-regular rotational movement at near-yearly timescales). Because the ocean tides are similar over spatial scales of a few tens to hundreds of kilometres, ocean tide loading tells us most about the Earth's behaviour within the few hundred kilometres nearest the surface (corresponding to its crust and uppermost mantle). An important question is whether the Earth behaves perfectly elastically (like a rubber ball, as it does over very short seismic timescales), or if it behaves anelastically (i.e. not perfectly elastically; more like a wet sponge ball or an under-inflated football, that exhibits a time delay after the removal of the force before it returns to its original form). The way in which the Earth's behaviour changes from elastic to anelastic (or even more fluid-like over geological timescales) is not just scientifically interesting in itself, but it affects how we can infer other aspects of its behaviour from geodetic measurements of Earth's shape. The ocean tides are the only regular, well-known, phenomena that affect the Earth at these depths, and allow us to model its behaviour so we can later understand other less-regular and therefore less-tractable phenomena. Thus, the regular ocean tide forcing of the Earth's deformation, dominantly at semi-diurnal (roughly 12-hour) and diurnal (roughly 24-hour) periods, provides a way to understand Earth's behaviour in ways we could not before the advent of GPS and which are now important to the way we use geodesy to study earthquake recurrence, sea level rise, and other geohazards.Precise GPS geodesy allows us to measure ocean tide loading deformations with hitherto unsurpassed accuracy and spatial coverage (as we recently demonstrated for the dominant 'M2' tidal constituent in western Europe). However, GPS is problematic at certain tidal and near-annual frequencies corresponding to the GPS satellites' orbital and geometry repeat periods. New developments in multi-GNSS (Global Navigation Satellite Systems: GPS, GLONASS, Beidou, and Galileo) positioning offer a way around this obstacle. We will use multi-GNSS data to observe the tidal harmonic motions of the Earth's surface and infer the degree of anelastic deformation of the solid Earth over the full range of semi-diurnal and diurnal tidal timescales. Our observations will allow us to investigate the behaviour of the soft 'asthenosphere' layer of the Earth, in the uppermost mantle, at this poorly-studied timescale, which will have implications for (e.g.) the understanding of slow slip events and short-term postseismic relaxation in subduction zones (where the largest earthquakes occur). In addition to these more "blue-sky" aspects, improved forward models (resulting from our work) of the Earth's near-instantaneous response to surface mass loads will have immediate practical consequences for users measuring key climate change variables, e.g. GRACE satellite measurements of water and ice mass transfer, and GNSS measurements of tide gauge vertical land motion to correct sea level change observations.