title: Probing the asthenosphere beneath the Australian region with surface GPS/GNSS
Creators: name: Matviichuk, B
Publisher: name: University of Tasmania
Year: 2022

description: The gravitational potential of celestial bodies, for example the Moon and the Sun, generates oscillating displacements of the Earth’s surface, otherwise known as solid Earth Body Tides (SEBT). The magnitude of SEBT displacements reach heights of 40 cm and above. The changing gravitational potential over the surface of the earth at any given time induces the redistribution of the atmospheric and global ocean masses creating atmospheric and ocean tide loading displacements (ATLD and OTLD). The latter, OTLD, generally affects coastal areas with displacements up to 10 cm that decrease with distance inland. The periodical displacements affect geodetic observations (e.g., GNSS, SLR, VLBI, DORIS) used in scientific applications requiring geophysical interpretation (e.g., sea level rise, glacial isostatic adjustment) and corrections need to be applied to obtain unbiased measurements and thus a meaningful interpretation. The energy associated with the SEBT is distributed pseudo-evenly over the Earth’s interior while the loading-associated energy acts mainly on the outer shell – the Lithosphere and Upper Mantle. The Earth’s rheological response to both ATLD and OTLD phenomena affects, in most cases, the elastic structure to depths of approximately 100 km. The inversion of observed tidal displacements may be used to infer the rheological properties of the lithosphere and asthenosphere at tidal frequencies – these may then be used to constrain geophysical models of the Upper Mantle. Improvements in these models are required in order to more accurately monitor elastic stresses, predict isostatic adjustment and stress relief events such as earthquakes. Making use of the Global Positioning System (GPS) measurements to estimate tidal deformation of the Earth is now a well established approach. Expanding this to also include non-GPS Global Navigation Satellite Systems (GNSS) such as Russian GLObal Navigation Satellite System (GLONASS) is a current area of activity in the geodetic community. The use of GNSS more broadly to study tidal deformations of the Earth is the focus of this thesis. National GNSS networks are essential for a wide variety of applications; these range from the geodetic to construction and agriculture, and due to an increasing need for improved spatial coverage are being actively expanded. GNSS sites observe tidal displacement directly, and a dense network of sites can enable the computation of tidal displacement at high spatial resolution. There are known limitations of GPS-only estimates of tidal deformation to infer the elastic structure of the Earth. A key limitation relates to the GPS orbital and constellation repeat periods. The GLONASS constellation has a different orbital configuration with constellation repeat periods well away from solar-related tidal frequencies. Thus, GLONASS observations, in theory, should have less systematic error at frequencies problematic with GPS and important to tidal modelling. This thesis develops our understanding of the GNSS estimation of OTLD and its application to inferences of geophysical properties through the analysis of three GNSS datasets: a subset of coastal stations in the United Kingdom, and sites across the New Zealand and Australia. The datasets span different spatial scales and tectonic conditions, but all have vast coastal areas that experience large tidal displacements. The variations between these datasets enable an assessment of the sensitivity to a wide range of conditions. In all cases, GNSS observations are compared to OTLD models after subtraction of modelled SEBT. The thesis commences with an assessment of the performance of the GLONASS constellation in observing tidal loading displacements using previously published GPS-only results from western Europe as a baseline for the comparison. Combining GPS and GLONASS constellation observations improved the GPS-only geodetic timeseries, performing comparably for constituents M\(_2\), N\(_2\), O\(_1\), P\(_1\) and Q\(_1\) to GPS-only with ambiguities resolved. The residuals of K\(_2\) and K\(_1\) constituents (GNSS-observed minus model) were improved with GPS+GLONASS but were shown to still be biased. The GLONASS S\(_2\) constituent estimates were shown to have an elevation cutoff angle dependency while GPS estimates possessed a constant bias in the case of floating ambiguities solutions. Ambiguity resolution was demonstrated to substantially reduce the observed GPS bias. Next, M\(_2\) OTLD were analysed nearby an active tectonic margin using sites from the national geodetic network of New Zealand. Application of an anelastic dissipation correction, and varying water density and compressibility substantially improved the agreement between the various models and observed OTLD. Despite this, some regional spatiallycoherent unmodelled residual signals remain in the North Island with significant magnitudes of up to 0.3mm. These show substantial variation in phase over ~100km in the region producing the sharp change of the residual tidal signals between the Taupo Volcanic Zone and the East coast in the North Island. The residuals likely highlight the deficiencies of current models of Earth structure that do not model lateral variations in the rheological structure forced largely by ocean tide loading with negligible unmodelled SEBT. Finally, the continental scale observations of M\(_2\) and O\(_1\) constituents from sites within the Australian national GNSS network were analysed using the advancements made and lessons learned from the previous two analyses. The scale of the studied region enabled the identification of residual tidal fields that could be associated with inconsistencies in the analysed GNSS orbit and clock products and centre-of-mass biases associated with global ocean tide models. Each regional assessment undertaken in this thesis contributes to a better understanding of tidal phenomena and the way tides interact with the solid Earth, as well as our ability to observe them using space geodetic techniques. The addition of the dissipation, spatial water density and compressibility corrections was demonstrated to significantly reduce the residual OTLD. Further reduction, however, is limited by the inconsistency of the observed displacements when using different satellite products (e.g., ~0.2mm for M\(_2\)) and ignored lateral variations in the Earth’s rheology. Multi-GNSS ambiguity resolution will contribute to the unraveling of this inconsistency and enable reliable geophysical interpretation of multiple tidal constituents to further increase the understanding of the Earth’s interior processes and enhance both Earth and ocean tide models that have global implications