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Research Activities 2009

Earth Physics

 

Click on the links below to read the 2009 Earth Physics research highlights or Click HERE to download the PDF version


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Introduction

The Research School of Earth Sciences includes substantial activities in geophysics. The main research themes are Geodynamics, Geodesy, Geophysical Fluid Dynamics, Mathematical Geophysics and Seismology. These span observational, theoretical, laboratory, computational and data oriented studies, all directed towards understanding the structure and physical processes in the earth’s interior, the crust or the earth’s fluid envelope.

In 2009 Professor M. Sambridge was awarded the 2009 Price medal from the Royal astronomical Society of London for achievements in Earth Science. Dr M. Roderick was awarded the Australasian Science Prize by the journal Australasian Science for his work on the Earth’s hydrologic cycle and showing how pan evaporation rates in Australia have decreased over time with global warming. Professor R. Griffiths joined the Australian Research Council College of Experts (Physics, Chemistry, Earth Science panel).

RSES continues to take a major role in the National Cooperative Research Infrastructure Strategy (NCRIS): “Structure and Evolution of the Australian Continent”, which is managed through ‘AuScope’. RSES hosts activities in Earth Imaging through support of portable instrumentation and transects, Geospatial through gravity measurements and testing of portable equipment for satellite laser ranging, and Simulation & Modelling through ‘pPlates’ software for tectonic reconstruction. As a linked activity between three AuScope components (Imaging, Geospatial and Access and Interoperability), the Terrawulf II cluster computer at RSES (Centre for Advanced Data Inference) provides capability in geophysical inversion and the computation reduction of observational data. RSES also continues the management of the Warramunga Seismic and Infrasonic Research Station near Tennant Creek in the Northern Territory, as a primary station in the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty Organisation.

Research in geodynamics and geodesy has focused on measuring deformation of the Earth from both terrestrial and space-based observations. Estimates of total water storage from the GRACE space gravity mission were compared to terrestrial estimates of surface, soil and ground water in a study of the multi-year drought in the Murray-Darling Basin. The finding that a significant component of water loss had come from groundwater reserves generated considerable media interest. Other results were a demonstration of the non-stationary nature of geophysical signals observed by GRACE, that positive gravity anomalies in Enderby Land (Antarctica) are not related to glacial isostatic adjustment, and improvements in GPS analysis strategies that lead to a better agreement between GPS- and GRACE-based surface deformations than reported in previous studies. Terrestrial gravity measurements have been made using an absolute gravimeter and tidal meters in order to quantify the surface deformations caused by ocean tide loading.

In geophysical fluid dynamics, laboratory experiments have been used to examine the three-dimensional flow in mantle subduction zones, and the interaction of ascending mantle plumes with subduction zones, with a view to explaining the history of volcanism in the Columbia River Basalts, the Lava High Plains and Yellowstone hotspot of the northwest USA. Modelling of the combined chemical and thermal evolution of the Earth’s mantle has been extended to Venus' mantle to test ideas about the operation of plate tectonics on Venus and whether the 'basalt barrier' mechanism can explain the outburst of volcanism that completely resurfaced Venus about 500 Myr ago. Another highlight is an explanation of the energetics of the global meridional overturning circulation of the oceans, which shows that energy supplied to irreversible turbulent mixing from the winds and tides (or other sources) must be in balance with the available potential energy supplied by the surface buoyancy fluxes. Hence the energetics indicate that the rate of overturning is governed by both the buoyancy fluxes and the mixing rate, as previously argued on the basis of dynamical considerations. Numerical solutions from a general circulation model were also found to be different depending on whether it was run in the usual hydrostatic and low-resolution mode, or in non- hydrostatic mode with an extremely high resolution resolving the vertical convective motions. In closely related work aimed at understanding both turbulent mixing and the global circulation, new experiments were carried out to obtain additional information about the nature of mixing in exchange flows over topographic sills. The results indicate that the proportion of energy input that goes into raising the potential energy by mixing is in the range 5-10%, efficiencies much smaller than the 20% often assumed in analyses of global ocean energy balances. Work also continues in high-resolution modelling of flow in the Southern Ocean, designed to determine the dynamics driving the circulation in this region. The latest results give a clearer indication of the likely response of the Southern Ocean to climate change.

In parallel with the gravity satellite research, and the ocean studies, we conduct detailed studies on the hydrologic cycle. Of principal interest here is the development of a theoretical framework that can provide physical understanding of the possible changes in the hydrologic cycle with global warming, at both global and local scales. One highlight this year was the publication of “The Global Water Atlas” (by ANU ePress), which documents model predictions contributed by international climate modelling groups to the 2007 4th Assessment Report of the Intergovernmental Panel on Climate Change.

Activities in seismology in 2009 included extensive field-based deployments of seismic instrument arrays, data analysis and theoretical development, for studies of Earth structure from the crust to the core. Much of the activity centred on the WOMBAT experiment, a rolling-array deployment that has been in operation since 1998 and is currently focused on achieving high resolution imaging of the crust and upper mantle beneath south-eastern Australia, including the Flinders ranges and the Murray Basin. The results show little evidence for the Palaeozoic building blocks of the southeast Australian continent that had been inferred from geological mapping. By utilizing differential PcP-P times from WOMBAT, high resolution imaging of the core-mantle boundary is also possible and current efforts are directed at mapping variations in core-mantle boundary geometry and D" velocity. New global observations of waves reflected from the Earth’s inner and outer core (PKiKP and PcP waves), originating from earthquakes and nuclear explosions, were used to place bounds on density ratio between the inner and outer cores. Other arrays were deployed in the region around Mt Isa in Queensland and between the Eyre Peninsula in southern Australia and Tennant Creek in northern Australia, in order to examine the transition between the northern and southern Australian cratons. The new Seismic Data Centre (SDC) now provides easy access to all current and past seismic data collected by RSES in a variety of user-friendly formats. Model structures for the inner core were re-examined using our Antarctic permanent stations as well as the temporary SSCUA deployment operated by the RSES during 2002-2005, and provide evidence against a proposed cylindrical structure in the outer core tangent to the inner core in the southern hemisphere.



Research Projects