Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopthat; 1) ground-motion in SEA appears to be consistently lower at almost all frequencies than in thehighly deformed crust of WNA and; 2) low-frequency ground motion is similar to the Atkinson &Boore (1997) model near f = 1 Hz. This is of interest because empirical studies on both the ENA(Atkinson, 2004) and SEA datasets yield similar attenuation near 1 Hz (Figure 2).Figure 4: Comparison of 5% damped RSA for the three North American ground-motion modelsand the stochastic SEA model. Near the seismic source (a), the SEA model is quite similar to theWNA model of Sadigh et al. (1997). With increased distance the SEA model appears to predictlower ground-motions at most frequencies greater than about 2 Hz. For lower frequencies, theSEA model similar to the North American models, particularly at shorter hypocentral distances.Each of the predictive spectral plots assume an earthquake focal depth of 8 km.IMPLICATIONS FOR EARTHQUAKE HAZARDResults from the present study indicate that ground-motion attenuation in SEA is higher thanpreviously assumed in many earthquake hazard and risk studies. Given earthquake hazard is definedas the probability of a certain level of ground-shaking being exceeded at a site within a certain timeperiod, these results have a significant impact of the level of hazard that a structure may be exposedto. We suggest that earthquake hazard in SEA is currently being overestimated using currentground-motion relationships, particularly at frequencies greater than 1 Hz.97
Seismic Hazard in SydneyProceedings of the one day workshopAlthough we have not been able to develop a reliable stochastic ground-motion model for WA atpresent owing to the anomalous nature of the WA dataset, results from empirical studies suggest thatthe attenuation of strong ground-shaking will be lower than in SEA. Consequently, we would expectto see larger ground-motions away from the fault rupture. Comparison of Fourier spectral models byAllen et al. (in press) indicate that at lower-frequency ground-motions ( f < 2 Hz) attenuate less inWA than in ENA. In contrast, they observe lower levels in high-frequency motions relative to lowfrequencymotions. It may be that the use of current attenuation models actually underestimateshazard at low-frequencies in WA. Therefore, low-frequency ground-motions may be slightly higherthan that of ENA models, but with diminished high-frequency motions. This is somewhat consistentto studies by Hao & Gaull (2004), who apply low-pass filters to ENA ground-motion models to fitobserved ground-motion from WA.CONCLUSIONSThis study presents the first ground-motion attenuation models that are appropriate for Australiancrustal conditions. These empirical and stochastic models are based entirely on data recorded inAustralia. We have not distinguished between attenuation models for Palaeozoic (e.g. Lachlan FoldBelt) and Mesozoic (e.g. Sydney and Gippsland Basins) crust, but rather have developed a combinedground-motion model for the whole of southeastern Australia. Developing models that describe thedifferent geological terrains in the region will be the scope of future work.Empirical attenuation models have been developed from two key datasets; one from WA and theother from SEA. In general we observe that ground-motion attenuates less in WA than in SEA,particularly at low frequencies. WA earthquakes, however, appear to have lower high-frequencymotion relative to SEA earthquakes.Given the difficulties associated in constraining reliable source and attenuation parameters for theWA dataset, development of stochastic ground-motion models concentrate on SEA rock sites. Weobserve that near the earthquake source, SEA RSA indicate similar spectral shape to the Sadigh etal. (1997) model developed for WNA with low levels of high-frequency motion (and PGA)compared to the two ENA models. As distance is increased from the source, we observe that allpredictive attenuation models from North America overestimate ground-motion relative to the SEAmodel. These findings have significant implications for earthquake hazard and risk in this region.They suggest that by using North American ground-motion models, we are currently overestimatingearthquake hazard in SEA. Consequently, we may be over-engineering infrastructure for strongground-motion, particularly for frequencies greater than 2 Hz. More work is required to quantifythese results, and also to develop useful models for both rock and soil sites. These models willprovide a means to lower the uncertainty associated with Australian specific hazard and riskanalyses.ACKNOWLEDGEMENTSThe work detailed in this paper was done in collaboration, and with the financial support of theAustralian National Committee on Large Dams (ANCOLD). We particularly acknowledge theefforts of Phil J. Cummins of ANCOLD, coupled with Wayne Peck and Gary Gibson of ES&S togain support from Australian dam owners. We also thank Walt Silva and Nick Gregor of PacificEngineering and Analysis for providing software and support for the development of the stochasticattenuation models.REFERENCESAllen, T.I., Dhu, T., Cummins, P.R. and Schneider, J.F. (in press). Empirical attenuation of groundmotionspectral amplitudes in southwestern Western Australia, Bull. Seism. Soc. Am.98
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