No abstract available

The wide-angle reflection seismic survey coincident with regional transect through Northern Yilgarn focused on the Leonora-Laverton Tectonic Zone, Western Australia was carried out to supplement deep seismic reflection studies. The major objectives were to collect high-density refraction information for offsets up to 60 km, to carry out a comparative study of near-vertical and wide-angle recordings of vibroseis energy at various offsets within the Leonora-Laverton tectonic zone and to obtain velocity information for upper crust. The survey deployed 120 short period recorders with spacing of 500 m. Acquisition parameters used for wide-angle reflection experiment were selected to fit into conventional reflection survey. The same vibrations were recorded in both surveys simultaneously. The major challenge in processing the Vibroseis data is to manage the huge volume of data. The processing of data includes several steps: sorting into receiver and source gathers, cross-correlation with reference sweeps and summing original seismic traces to form single source point traces, producing seismograms from individual traces and finally creating seismic record section from separate seismograms. The major step in processing and interpretation of data is to analyse recorded wave fields on the basis of seismological criteria prior to seismic velocity modelling. Seismic velocity models developed by using forward and inverse travel modelling software will supplement geological interpretations for this complex region and allow an estimation of its crustal composition.

Crustal-scale seismic surveys mostly collect data along single profiles, and the data processing has an underlying assumption that the data have imaged 2D structure striking at right angles to the seismic profile. However even small amounts of out-of-plane topography on a reflector can result in reflections that do not map the reflector shape accurately. Out of plane energy will migrate within the plane of the section to an apparent depth (represented as two-way-time TWT) that is greater than the depth of the reflection point out of the plane of the section. It will fall within the plane of the section at depths less than, equal to or greater than the intersection of the reflector with the plane of the section, depending on the amount of topographic relief on the reflector out of the plane of the section, and the offset of the topographic relief from the plane of the section. Reflectors that are a single surface can therefore be manifested in the seismic section as a band of several reflections, rather than a single reflection. The top of the band of reflections may not represent the position of the reflector in the plane of the section. More complex reflectors that have a finite thickness because they are made up of several to many anastomosing reflectors caused by altered and anisotropic rock embedded in protolith, will appear as laterally short reflections within a laterally continuous reflection band. Examples of such reflectors would be shear zones, the Moho in some places, and rock with compositional layering. With increasing topographic relief, the top of the reflection band for both single- and multi-layer reflectors will be a poor indicator of the top of the reflector in the Earth. The bottom of the reflection band will always be a poor indicator of the bottom of the reflector. Because out-of-plane energy can arrive at TWTs that are different from those of the reflector in the plane of the section, out-of-plane energy has the potential to interfere constructively or destructively with the in-plane energy. In synthetic data calculated for a simple model assuming one layer and topographic relief of 250m over wavelengths of 4-5 km, similar to that imaged in a real sub-horizontal detachment, amplitudes ranged up to 2.6 times the expected amplitude for the layer. A model with anastomosing layers built to resemble a thick shear zone rather than a discrete fault surface allowed tuning between layers. The effects of out-of-plane energy when combined with the effects of tuning caused amplitudes up to 3.1 times those expected. Larger amplitudes could be achieved if model was contrived. The results indicate that care must be taken when calculating impedance contrasts using real data. The highest amplitude reflections are likely to yield overestimates the true impedance contrast.

Deep-seismic reflection data across the Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia have provided information on the region?s crustal architecture and on several of its highly mineralised regions. The seismic reflection data has imaged several prominent crustal scaled features, including an eastward thickening of the crust across the northeastern Yilgarn Craton, the subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many other smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed as the Laverton structure. In all the major structures a wedge-geometry is formed at the intersection of the east-dipping shear and a low-angle shear zone within its hanging wall. This wedge-geometry forms a suitable trap where the upward and/or sub-horizontal moving fluids were focused into the wedge's apex and then distributed and deposited into the surrounding complexly deformed greenstones.

This paper describes the methods used to define earthquake source zones and calculate their recurrence parameters (a, b, Mmax). These values, along with the ground motion relations, effectively define the final hazard map. Definition of source zones is a highly subjective process, relying on seismology and geology to provide some quantitative guidance. Similarly the determination of Mmax is often subjective. Whilst the calculation of a and b is quantitative, the assumptions inherent in the available methods need to be considered when choosing the most appropriate one. For the new map we have maximised quantitative input into the definition of zones and their parameters. The temporal and spatial Poisson statistical properties of Australia's seismicity, along with models of intra-plate seismicity based on results from neotectonic, geodetic and computer modelling studies of stable continental crust, suggest a multi-layer source zonation model is required to account for the seismicity. Accordingly we propose a three layer model consisting of three large background seismicity zones covering 100% of the continent, 25 regional scale source zones covering ~50% of the continent, and 44 hotspot zones covering 2% of the continent. A new algorithm was developed to calculate a and b. This algorithm was designed to minimise the problems with both the maximum likelihood method (which is sensitive to the effects of varying magnitude completeness at small magnitudes) and the least squares regression method (which is sensitive to the presence of outlier large magnitude earthquakes). This enabled fully automated calculation of a and b parameters for all sources zones. The assignment of Mmax for the zones was based on the results of a statistical analysis of neotectonic fault scarps.

The year was another marginally below average one with respect to the frequency and magnitude of earthquakes in Australia; there were two of magnitude 5 or more but no large earthquakes of magnitude 6 or more. It was an above average year for major earthquakes worldwide with one great earthquake of maptude M 8.2 and 16 of magnitude M 7.0 or more.

Seismic activity in the region around Australia results in a significant tsunami hazard to the coastal areas of Australia. Hence seismicity is monitored in real time by Geoscience Australia (GA), which uses a network of permanent broadband seismometers. Although seismic moment tensor (MT) solutions are routinely determined using 1-D structural models of Earth, we have recently demonstrated that a 3-D model of the Australian continent developed using full waveform tomography significantly improves the determination of MT solutions of earthquakes from tectonically active regions. A complete-waveform, time-domain MT inversion method has been developed using a point-source approximation. We present a suite of synthetic tests using first a 1-D and then a 3-D structural model. We study the feasibility of deploying 3-D versus 1-D Earth structure for the inversion of seismic data and we argue for the advantages of using the 3-D structural model. The 3-D model is superior to the 1-D model, as a number of sensitivity tests show. Current work is focused on a real time automated MT inversion system in Australia relying on Australian and other international stations.

Legacy product - no abstract available

In the Cities Project Perth, GA is developing a multi-hazard risk assessment of the Perth metropolitan area. One of the main objectives of the Project is to develop an earthquake hazard and risk model for this area. The hazard model is developed using informaiton about earthquakes, the geology and the local soil characteristics of the region. As part of this process, we will develop several earthquake scenarios with events originating at specific locations in the region. To define an appropriate model of seismicity, the Cities Project initiated an expert debate to discuss and to formulate geological and seismological parameters for the Southwest Seismic Zone and the area around Perth. The debate was followed by a workshop in December 2002 in Canberra where the seismicity model of SW WA was ultimately defined. Further discussion was held following the workshop, culminating in an agreed seismicity model for the region.

Legacy product - no abstract available