Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopGround-motion attenuation modelling insoutheastern AustraliaTREVOR ALLEN, PHIL R. CUMMINS, TREVOR DHURISK RESEARCH GROUP, GEOSCIENCE AUSTRALIAABSTRACTGround-motion attenuation models have been derived for the southeastern Australian crust. Thesemodels employ both empirical and stochastic methods and are the first spectral ground-motionmodels to be based entirely on Australian ground-motion data. In the past, these studies have beenhampered by a lack of quality ground-motion data given Australia’s relatively low levels ofseismicity.Data recorded in the Palaeozoic crust of southeastern Australia (SEA) have been employed to deriveempirical ground-motion attenuation models for the region. Empirical ground-motion models arederived for each of these datasets. These empirical models suggest that SEA has similar near-sourceattenuation with eastern North America for hypocentral distances less than 70 km.Stochastic methods are employed to simulate ground-motions for larger earthquakes in regionswhere recordings from real events are not available. These models are largely based upon source andattenuation parameters derived from empirical studies. Spectral ground-motion predicted by thesemodels are generally lower for SEA than ground-motion predicted by both eastern and westernNorth American models, particularly at high-frequency ( f > 2 Hz). Results from this study havesignificant implications for earthquake hazard and risk in Australia. They suggest that we arecurrently overestimating earthquake hazard in SEA. Furthermore, they suggest that we cannotsimply rely on North American ground-motion models to predict earthquake ground-motionsin Australia.INTRODUCTIONIt has long been recognised by engineers and seismologists alike, that the traditional measure ofearthquake ground-motion, peak ground acceleration (PGA), is not a particularly good indicator forestimating structural damage (e.g. Hanks & McGuire, 1981). For this reason, frequency-dependentmodels are favoured to assess the degree of vulnerability a structure may be exposed to inearthquake hazard and risk studies. These models are referred to as ground-motion attenuationmodels.Earthquake hazard and risk assessments in Australia typically adopt ground-motion models fromother stable continental regions such as eastern North America (ENA) (e.g. Dhu and Jones, 2002;Jones et al., 2005). However, there has been very little analysis undertaken to show whether ENAmodels (e.g. Atkinson & Boore, 1997; Toro et al., 1997) are applicable to Australian conditions.Moreover, there has been some conjecture that ground-motion in some parts of Australia resemblesthat of active tectonic regions and may be better described by models such as Sadigh et al. (1997),developed for western North America (WNA). Uncertainties in the choice of attenuation modelcould potentially lead to undesirable outcomes, such as unrealistically high (or low) loadingstandards in the design and construction of critical infrastructure (e.g. large dams, power stations,hospitals, etc.). Furthermore, the choice of attenuation model has been demonstrated to have asignificant impact on risk estimates in Australia (e.g. Patchett et al., 2005).93
Seismic Hazard in SydneyProceedings of the one day workshopAttenuation relations appropriate for the Australian crust have, in the past, been difficult to quantifyowing to a lack of ground-motion data from moderate-to-large local earthquakes. GeoscienceAustralia (GA), in association with Environmental Systems and Services (ES&S) and the AustralianNational Committee on Large Dams (ANCOLD) have collaborated to assemble an Australianground-motion database suitable for attenuation studies. The key focus of this project is to improvefuture earthquake hazard and risk assessments in Australia.The present paper describes the development of the first spectral ground-motion attenuation modelsappropriate for the Australian crust. These models are based upon data recorded in the Palaeozoiccrust of southeastern Australia (SEA) including some data recoded within younger the Sydney andGippsland Basins.AUSTRALIAN GROUND-MOTION DATADue to the development of much of the nation’s infrastructure and higher than average earthquakeactivity, the seismograph network in SEA is well developed. Earthquake data used were recordedand located by the ES&S Seismology Research Centre, Melbourne, who have monitored earthquakeactivity in the region since the mid 1970’s. Most of the earthquakes selected (80 in total) were welllocated,having well-constrained focal depths. The events occurred from 1993 to 2004 and momentmagnitudes range from 2.0 M 4.7. Data in this region has a good spatial distribution (toapproximately 700 km) and comprises some 1220 ground-motion records.EMPIRICAL GROUND-MOTION MODELSThe development of empirical ground-motion attenuation models for Australia largely follows themethods adopted by Atkinson (2004a) for ENA. Detailed analysis of the SEA dataset indicates thatthe decay of Fourier spectral amplitudes can be described by a trilinear geometrical attenuationmodel. The subsequent decay of spectral amplitudes can be approximated by the coefficient of R -1.3(where R is hypocentral distance) within 90 km of the seismic source. From 90 to 160 km, the SEAdataset indicates a zone whereby the seismic coda appear to be affected by crustal reflections andrefractions. In this distance range, geometrical attenuation is approximately R +0.1 . Beyond 160 km,low-frequency seismic energy (i.e. f = 1 Hz) attenuates rapidly with R, having a geometricalattenuation coefficient of R -1.6 . Note that at present, we have not distinguished between attenuationmodels for Palaeozoic (e.g. Lachlan Fold Belt) and Mesozoic (e.g. Sydney and Gippsland Basins)crust, but rather have developed a combined ground-motion model. Developing models that describethe different geological terrains in southeastern Australia will be the scope of future work.Fourier ground-motion models for SEA, WA (Allen et al., in press) and ENA (Atkinson, 2004a,b)are compared for hypocentral distances at 1, 10, 50 and 100 km for an earthquake of M 4.5 (Figure1). In general, we observe that SEA and ENA models are relatively consistent since the geometricalattenuation is the essentially the same over this distance range [geometrical attenuation for ENA isR -1.3 for R < 70 km (Atkinson, 2004a)]. Owing to the higher geometrical attenuation observed inSEA for R > 70 km, the models differ considerably at larger distances (Figure 2). Low-frequencyspectral amplitudes calculated for the WA model are observed to be significantly higher withincreasing source-receiver distance than both SEA and ENA. Comparing spectral amplituderesiduals (log observed less predicted) for the Atkinson’s (2004a,b) ENA model furtherdemonstrates the higher attenuation in SEA beyond the ENA postcritical distance of 70 km (Figure3).Given that we have no quality ground-motion records for earthquakes greater than M 4.7, theseempirical models cannot be used directly to predict ground-motion for larger, potentially damaging94
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Landslides in the Sydney Basin - Geoscience Australia

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