Research School of Earth Sciences
Earth Physics
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Research School of Earth Sciences
Earth Physics
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EARTH PHYSICS - SEISMOLOGY 2004
Research Topics:
Research EmphasisThe Earth Physics - Seismology area covers a range of activities designed to exploit the favourable geographic location of Australia relative to the worlds earthquake belts. This is reflected in extensive studies using deployments of portable instrumentation to study the structure of the Australian continent and the mantle beneath. The activities of the group are summarised below, and more detail on different aspects of the work can be found in recent annual reports. The group operates the Warramunga Seismic and Infrasound Research Station (WRA) near Tennant Creek in the Northern Territory. The seismic and infrasound arrays are primary stations in the International Monitoring System for the Comphrensive Nuclear-Test-Ban Treaty (CTBT). The arrays have both been upgraded to meet Treaty standards since 2000, and data is sent by satellite link to the International Data Centre in Vienna, and the National Data Centre at Geoscience Australia in Canberra. The broad-band station CAN at Mt Stromlo is operated in association with the French GEOSCOPE network and a broad-band borehole instrument (WRAB) is supported at the Warramunga Array for the IRIS-IDA network as a contribution to global seismology. The group hosts the Australian National Seismic Imaging Resource (ANSIR) Major National Research Facility which operates both reflection profiling and portable instrumentation for approved projects. Ownership of the reflection component of the Facility was transferred to ANU in mid-2002 and Professor Kennett is now the Director with an Executive Officer for ANSIR supported by Geoscience Australia The group has been able to attract significant external funding from a variety of sources for work relating to Australian structure, seismic wave propagation, inversion for source location and mechanism and the nature of regional phases. SEISMOLOGYa) Seismic Structure of the Lithosphere and Upper Mantle - Kennett, Reading, Heintz, Rawlinson, Fishwick, Saygin, AbdulahThe earthquake belt through Indonesia, New Guinea, Fiji, Tonga and New Zealand provides an excellent set of seismic sources for studying the mantle under Australia, and is occasionally complemented by events from the south on the Australian-Antarctic ridge. In consequence Australia is well placed for a wide variety of studies on the nature and three-dimensional structure of the lithosphere and upper mantle. Portable array experiments have been carried out since the mid 70s, particularly in northern Australia. In the earlier short-period studies the emphasis was on structure in the upper mantle and transition zone. Since 1992 most experiments have used broad-band seismometers and have been used for a range of P and S wave studies. From 1993-1996 a reconnaissance survey of the whole continent was carried out in the SKIPPY experiment. A set of 5-6 month deployments of typically 10 instruments were made, with an interstation spacing of approximately 400 km. Since 1997 a number of more focussed experiments have built on the SKIPPY results so that now more than 120 broad-band sites have been occupied, this provides a very significant data set. The broad band studies have provided a detailed model of the three-dimensional structure for shear waves across the whole continent and the neighbouring oceanic regions and this model continues to be refined using new observations and inversion techniques. The emphasis has been on surface waveform tomography using Rayleigh waves, but some work has been carried out using Love waves that indicates significant variations in polarisation anomalies with depth. Body wave studies provide an independent check on the surface wave results and also allow delineation of attenuation structure for the northern part of the continent, with a significant attenuation beneath the very low-loss lithosphere. Receiver function studies have been carried out for most stations and have been used to significantly enhance knowledge of crustal structure for the continent. Studies in western Australia indicate distinctive crustal structure in the various terranes of the Yilgarn craton. Present experiments are directed at understanding major contrasts in continental structure particularly the transition from the ancient core in western and central Australia with high wavespeeds in the mantle lithosphere to the slower wavespeeds beneath the Phanerozoic belts in the east. In the TIGGER experiment in Tasmania in 2002, a large array of short-period instruments was deployed along with broad-band instruments to elucidate the crustal and upper mantle structure. The Tasman Line experiment starting in 2003 has deployed 20 broadband instruments to bracket the transition from the craton to the younger fold belts. The object is to obtain as high a resolution as possible of the character of the transition as a function of depth. The studies of the Australian continent are complemented by a deployment of 7 broadband instruments in the Australian Antarctic Territory in the SSCUA experiment. Australia and Antarctica were joined in East Gondwanaland, and an object of this work is to try to relate structures in the two continents. c) Seismic tomography: - Kennett, Sambridge, Rawlinson, FishwickThe development of seismic velocity models for the Earth's interior depends on our ability to invert different classes of seismic data and current studies exploit both seismic travel times and long-period seismic waveforms for regional studies. Studies using travel times exploit the joint use of P and S wave data to obtain images on both global and regional scales. An effective parameterisation is in terms of bulk-sound speed and shear wavespeed to isolate the influence of the bulk and shear modulus. Joint global inversions using 2x2 degree cells have demonstrated the dominance of S structure in the mid mantle, so that the narrow slab-like features in P wave images are controlled by S structure. At a regional scale variations in the balance of the bulk-sound and shear wavespeed have a strong correlation with the age of subducting lithosphere. Lithosphere older than around 85 Ma normally displays stronger shear anomalies. A research focus is on the improvement of methods for extracting 3-D seismic shear structure using the records of fundamental and higher mode surface waves. The path-by-path inversions have been improved by using multiple starting models so that better estimates can be made for model error. A three-stage approach to 3-D model construction via the intermediary of multi-mode phase speed maps as a function of frequency provides the opportunity to include both ray-tracing corrections and the effects of finite frequency propagation. c) Seismic Wave Propagation - Kennett, Procko, Tarlowski Current research is directed towards understanding of the influence of three-dimensional structures on the seismic wavefield on both small scales through stochastic methods and larger scales through direct numerical simulation. Numerical simulation of wave propagation through the Australian region is planned using the Spectral Element method as a means of testing analysis algorithms and waveform inversion methods.
d) Mathematical Geophysics - Sambridge, Rawlinson, Brodie, Kennett Research in mathematical geophysics is primarily focused on developing new methodologies for mathematical and data analysis problems in seismology, geodynamics and geophysical inverse theory. The ability to extract reliable information on Earth structure from seismic data, e.g. travel times or digital waveforms, or other classes of data, depends on methods of inverse theory. Fully non-linear (stochastic) inversion methods can provide valuable insight into the character of the solution. Genetic algorithms and variants are of particular interest. A new method of data space exploration termed the Neighbourhood Algorithm (NA) has been developed and has had a number of applications in different areas of seismology. A number of links have arisen with other groups in the School in the development of new data analysis tools; one example is in separating mixtures of different age components in geochronology. The study of mathematical aspects of data analysis supports the observation work of the group. Strong links also exist with other areas in the school, notably with the geodynamics group where the common interest is in the development of new computational techniques for geodynamic modelling, which involves the solution of partial differential equations using finite difference and finite element techniques in two and three dimensions. Significant progress has been made in the application of these techniques to unstructured numerical grids, an approach which also has potential for studies of seismic wave propagation.
GEOMAGNETISM- Lilley, BartonThe Research School has made wide contributions to geomagnetism over the fifty years of its existence. Paleomagnetism and fundamental rock magnetism were pursued vigourously when geophysics was first established. These studies led to such important developments as the recognition of "continental drift" of the Australian continent over geologic time, and, in association with radiometric age-dating, to some of the first determinations of the history of the reversals of the Earth's magnetic field (the "geomagnetic polarity time-scale"). Present research in geomagnetism concentrates on magnetic fields generated by electromagnetic induction as measured at the surface of the Earth, and in its oceans. The source fields used as the powerful natural ones provided by ionospheric and magnetospheric processes. The techniques of magnetotellurics and geomagnetic depth-sounding are applied to determining the electrical conductivity structure of the Australian continent, and its surrounding seafloors. In view of present rapid developments in aeromagnetic mapping, especially in Australia, interest is also being taken at present in the fundamental interactions of electromagnetic induction in the Earth with the techniques of aeromagnetic surveying. A different class of source fields which is also being explored are those arising from motional induction by ocean currents, when electrically-conducting seawater move in the steady magnetic field of the Earth. High-sensitivity magnetometers, combined with the techniques of modern marine science, allow the observation of such motional induction phenomena in a way which has only recently become possible.
Questions about the topics in this Report to Brian Kennett: Brian.Kennett@.anu.edu.au
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Page last updated: 2004 December 21 Please direct all enquiries to: earthphysics.admin@anu.edu.au Page authorised by: Coordinator, Earth Physics, RSES |
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