title: Vertical land motion of the Australian plate
Creators: name: Riddell, AR
Publisher: name: University of Tasmania
Year: 2021

description: The surface deformation field on Earth represents an integrated response to a complex array of geophysical processes that occur over different temporal and spatial scales. High-accuracy measurements of surface deformation can therefore inform our understanding of processes ranging from past and present mass loading (e.g. water redistribution between land and ocean, atmospheric pressure, continental surface and groundwater storage, non-tidal ocean, glacial isostatic adjustment), crustal dynamics (e.g. tectonic motion, elastic and viscoelastic relaxation), as well as subsurface and internal processes (e.g. geocentre motion, mantle convection). The vertical component of surface deformation is both particularly important, yet problematic to observe. Vertical deformation informs our understanding of processes that often present a natural hazard to society (e.g. earthquake, tsunamis and volcanic eruptions), and are critical to enable quantitative predictions of regional sea level change which poses an increasing concern to coastal and island communities. Geodetic time series are used to monitor the deformation of the solid Earth, but their accurate interpretation depends entirely on the stability and accuracy of the underlying reference frame. This thesis undertakes a homogeneous reprocessing of Global Positioning System (GPS) data to serve as the foundation to aid in the understanding of reference frame accuracy and stability, as well as to investigate the present-day deformation of the Australian continent. With a focus on the vertical coordinate component at GPS site locations installed throughout the Australian AuScope network, space geodetic time series are utilised alongside modelled estimates of mass surface transport, glacial isostatic adjustment, coseismic offsets and postseismic deformation. Through the use of time series and spectral analysis, noise characterisation and spatiotemporal filtering, three key contributions are made to improving knowledge regarding vertical deformation across the Australian continent. First, this thesis explores the temporal variation of the motion of the reference frame origin and its deviation from an assumption of long-term linear motion. The realisation of the International Terrestrial Reference Frame (ITRF) origin is reliant on a single geodetic technique, Satellite Laser Ranging (SLR), which is sensitive to the movement of the Earth’s centre of mass. A comparison of the network translations calculated from surface mass transport models with those observed by SLR finds that unaccounted for variability and temporal correlation exists. Non-linear trends and coloured noise are identified in the SLR translations, which increases uncertainties in the rates by a factor of five (upper bound) compared to a linear and whitenoise-only assumption. The uncertainty of the ITRF2014 SLR Z rate is reduced by 27% since the previous model of ITRF2008 with the consideration of a powerlaw and white noise model and short-term non-linear motion. The attainment of a stable reference frame with increasing accuracy to serve as the foundation for all geospatial activity presents an ongoing challenge for the Earth Science community. Second, the disagreement between large-scale geophysical theory and present-day observations of land motion across Australia is investigated. Recent but sparsely distributed GPS measurements suggest that the vertical motion of the Australian continent at permanent GPS sites is between 0 and -2 mm/yr. Current understanding of the Earth’s rheology and the knowledge that Australia is located in the far field of past ice sheet loading suggests that the expected vertical motion of the crust should be close to zero. GPS data from more than 100 new Australian Global Navigation Satellite System (GNSS) stations are processed, capitalising on the recent national investment in geodetic infrastructure. The investigation of large-scale geophysical models and GPS observations of vertical land motion in Australia reveals that the velocities disagree at the millimetre per year level. The novel use of spatiotemporal filtering, multivariate regression, and modelling short-term geophysical effects is shown to reduce the uncertainty of vertical velocity estimates at Australian GPS sites by 35%. It is shown that despite the level of noise in the filtered time series remaining too high to isolate land motion from glacial isostatic adjustment (the solid Earth response to ice-ocean mass changes), significant insight is gained into the spatial and temporal evolution of Australian GPS sites and the surface motion they monitor. The El-Niño Southern Oscillation is identified as a driver of spatially coherent motion following the removal of surface mass transport as common mode deformation. Third, the elastic and viscoelastic response of the crust within the Australian plate is investigated using several large earthquakes along the Indo-Australian plate boundary and a recent intraplate earthquake. Postseismic motion of the Australian continent has been comprehensively studied in the horizontal direction, but there is a lack of studies that investigate the vertical component. Earthquake events in the far-field of AuScope GNSS sites are identified, selecting events that are not yet present within publicly available deformation models. Ongoing far-field postseismic deformation of the Australian continent from large plate-boundary earthquakes is supported by evidence from forward viscoelastic models and GPS observations. Evidence of changes in vertical velocity up to several mm/yr are observed in the GPS time series following large earthquakes. Models constrained by earthquake rupture parameters as far as 2000 km from the Australian continent suggest that coseismic deformation occurs in the vertical direction by up to 3 mm and postseismic deformation up to 4 mm/yr over 2004.9-2010.0 (with the magnitude of both varying spatially and by earthquake). Modelled postseismic deformation due to an historic earthquake in 1979 to the south of New Zealand indicates ongoing present-day deformation in Australia. Far-field deformation from intraplate and large plate boundary earthquakes can be observed in Australian GPS time series and provides information on how the Earth responds to changing stress fields. Each of these advances presents a novel incremental advance to our understanding of surface deformation across the Australian plate. The inclusion of coloured noise and non-linear transients in the definition of geocentre motion is important to accurately assess frame uncertainty and its contribution to interpretation of geophysical processes. The surface deformations modelled and observed in this thesis push the limit of signal to noise present in current space geodetic analyses and available geophysical models. New insights in the spatial patterns of the surface deformation field informs our understanding of relevant processes and highlights an ever-increasing demand for accurate position and velocity estimates