Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopearthquakes. The source and attenuation coefficients evaluated in these studies, however, are used askey inputs to stochastic methods discussed below.Figure 1: Comparison of empirical Fourier displacement spectra at hypocentral distances of(a) 1, (b) 10, (c) 50 and (d) 100 km for an earthquake of M 4.5. Note that for all three models,source spectra (i.e. R = 1 km) converge at low-frequency indicating that estimates of M areconsistent (a). In figures (b-d) we observe the effect of lower rates of attenuation in WA.Figure 2: Comparison of Fourierdisplacement spectra at a frequency of 1.0Hz for hypocentral distances to 200 km foran earthquake of M 4.5. It can be observedthat attenuation in SEA and ENA are verysimilar for R < 70 km. Beyond this distancewe observe higher rates of ground-motionattenuation in SEA.95
Seismic Hazard in SydneyProceedings of the one day workshopFigure 3: Residuals of modelled SEA ground-motions plotted against hypocentral distance at1.6 and 5 Hz. These data are compared with the Atkinson (2004a,b) attenuation model. Atdistances beyond the ENA postcritical distance of R = 70 km, the Atkinson (2004a,b) modeloverestimates ground-motions in southeastern Australia.STOCHASTIC GROUND-MOTION MODELSThe stochastic method is particularly useful for simulating earthquake ground-motions in regions ofthe world where recordings from large damaging earthquakes are simply not available (Hanks &McGuire, 1981; Boore, 2003). The method relies on an initial earthquake source model, based uponresults from empirical studies.We use software developed by Pacific Engineering & Analysis, El Cerrito, USA (Silva, 1987), tosimulate ground-motions for a series of magnitude-distance bins. To accommodate uncertaintyowing to the unpredictable nature of future events, model parameters are randomised about the base(mean) value a number of times for each magnitude-distance bin. For each randomisation, 5%damped Response Spectral Acceleration (RSA) are calculated. Subsequent regression analysis yieldsa stochastic ground-motion model that also incorporates estimates of uncertainty in the predictedground-motion. Since the initial model parameters are based on ground-motion recordings on rock,our default attenuation model will be for rock sites.Figure 4 shows the stochastic SEA rock ground-motion model compared to three North Americanmodels. Two of these were developed for the stable crust of ENA (Atkinson & Boore, 1997; Toroet al., 1997) and the other for seismically active Californian crust of WNA (Sadigh et al., 1997).Near-source RSA indicate that the SEA rock ground-motion models have similar spectral shape tothe Sadigh et al. (1997) model with low levels of high-frequency motion (and PGA) compared to thetwo ENA models (Figure 4a). As distance is increased from the source, we observe that allpredictive attenuation models from North America overestimate ground-motion relative to the SEAmodel (Figures 4b-c), particularly at higher frequencies ( f > 2 Hz). Two key points to consider is96
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Landslides in the Sydney Basin - Geoscience Australia

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