seismology
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Legacy product - no abstract available
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Tomographic images of Southeast Asia and Australia were created by inverting the travel-times of the Rayleigh wave Green's functions retrieved from cross-correlations of the ambient seismic noise. The travel-times of the Green's functions are inverted with a nonlinear two dimensional inversion scheme to map the seismic velocity perturbations of the Earth. Continuous records from the vertical components of 187 permanent broad-band seismic stations operated from 2007 to 2008 are processed. We limit our picks only for Green's functions with interstation separation between 1o and 60o. This ensures that only wide scale anomalies are included in the tomographic inversions. By employing a nonlinear wavefront tracker for the forward problem, we avoid the artefacts of the deviations from the great circle path assumptions for very long interstation paths. We conduct dispersion measurements of group velocities between 6 and 50 seconds by narrowly filtering the envelopes of the extracted Green's functions. The Rayleigh waves for the selected periods sample the Earth from upper-crust (~9 km) to uppermost mantle (~90 km). The tomographic images reveal heterogeneous structure of Australia marking major sediment deposits on shallow layers and the high-velocity structure of the Western Australia cratons composed of ancient Archaean and Proterozoic blocks. Low velocity zones in deeper layers correlate well with the areas of high heat flow and agree with the results of recent surface wave tomographic studies. The Sunda Arc is characterized by prominent low-velocity zones located below the western tip of Java, Java Sea, and Banda Sea for longer periods.
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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.
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Legacy product - no abstract available
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Geoscience Australia has more than 50 years experience in the acquisition of deep crustal onshore seismic data, beginning with analogue low-fold explosive data, progressing through digital explosive data, and finally, in the last 12 years, moving into the digital vibroseis era. Over the years, shot data in a variety of formats has been recovered from a variety of media, both in-house and by external contractors. Processing through to final stack stage was used as a QC tool for transcription of some of the older analogue surveys, and proved so successful that the reprocessed data was released for interpretation. In other cases, more recent digital explosive surveys have benefited from reprocessing using modern processing algorithms. Key modules in Paradigm Geophysical's Disco/Focus software used by Geoscience Australia for reprocessing old data include refraction statics, spectral equalisation, stacking velocity analysis, surface consistent automatic residual statics and coherency enhancement. Coherency enhancement is commonly carried out on both NMO corrected shots and stack sections, with several iterations of NMO and autostatics. With the irregular offset distribution and low fold of legacy explosive data, dip moveout (DMO) correction is not possible, but due to the shorter spreads is not as critical as for modern high fold vibroseis data. Nevertheless, 'poor man's DMO' has proved successful in the shallow section, by the simple expedient of omitting 25% of the traces with the longest offsets.
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Mmax estimates for the Australian stable continental region (SCR) derived from palaeoseismicity data
The inventory of over 200 fault scarps captured in GA's Australian neotectonics database has been used to estimate the maximum magnitude earthquake (Mmax) across the Stable Continental Regions (SCRs) of Australia. This was done by first grouping the scarps according to the spatial divisions described in the recently published neotectonics domain model and calculating the 75th percentile scarp length for each domain. The mean Mmax was then found by averaging the maximum magnitudes predicted from a range of different published relations. Results range between Mw 7.0-7.5±0.2. This suggests that potentially catastrophic earthquakes are possible Australia-wide. These data can form the basis for future seismic hazard assessments, including those for building design codes, both in Australia and analogous SCRs worldwide.
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Legacy product - no abstract available
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Legacy product - no abstract available
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Legacy product - no abstract available
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Legacy product - no abstract available