Continental deformation in Australia due to the geocenter motion, seasonal water cycles and the 2010-11 La Niña from GPS and GRACE

As atmosphere, ocean, ice, and terrestrial water are redistributed, the center of mass of the Earth's system moves and the accompanying loading yields global surface deformation. In Australia, when GPS surface displacements were corrected for local mass change (hydrology, atmosphere, and ocean) with GRACE data, the residual GPS data identify a peculiar seasonal mode of continental deformation. The entire continent coherently shifts northwest by 1 mm and the southeastern part is uplifted, while the northwestern part is subsided by 2-3 mm during southern Summer and the opposite patterns of deformation are observed during southern Winter. Such characteristic deformation could be understood as a result of the Earth's elastic response to globally-averaged surface mass load, generally heavier in Europe during southern Summer and in the south Pacific Ocean during southern Winter. It was found that such deformation is even larger than local hydrology-induced loading effects in horizontal motion over Australia. A simple method of determining locations of the center of mass was developed by combining GPS and GRACE data; the latter being insensitive to the center of mass but sufficiently accurate to remove the local loading effects in GPS data. The coordinates of the center of mass estimated by inversion of the Australian GPS dataset and GRACE agree with the geocenter motions determined by satellite tracking analysis. When the geocenter motion is properly modelled and removed, the GPS displacement data revealed the load deformation induced by local hydrology, atmosphere, and ocean mass changes. I also present the other mode of the continental deformation associated with seasonal and inter-annual water cycles, particularly before and after the 2010-2011 La Niña event in Australia.