Understanding Earthquakes in Australia
The Newcastle earthquake
A moderate earthquake with a magnitude of 5.6 shook Newcastle on 28 December 1989. The earthquake claimed 13 lives and injured more than 160 people. Buildings were highly vulnerable to horizontal shaking due to widespread corrosion of steel frames, which had resulted from prolonged exposure to oceanic spray and corrosive particulates from a large steel works. The combination of old buildings, inadequate maintenance, brittle unreinforced masonry materials and poor foundations caused over A$800M in insured losses. This devastating event showed that Australian communities are vulnerable to earthquake hazards and need to be better prepared.
The need for an Australia-specific ground motion model
A range of earthquake ground motion models have been developed by different agencies for their regions of interest. In the past, Geoscience Australia has applied models developed in other settings, such as Eastern-North America. However, differences in factors such as regolith composition mean that uncertainties were high. In response to this Geoscience Australia has recently developed an earthquake ground motion model for Australia based on Australian data to ensure better modelling results.
Numerical models of ground motion are important in stable continental regions such as Australia because the infrequency of instrumental earthquake records leads to large uncertainties in predictive ground-motion models for large earthquakes.
Geoscience Australia research has resulted in significant improvements in earthquake hazard analyses for Australia by:
- capturing high-quality Australian earthquake ground-motion data
- developing improved numerical simulation techniques
- developing the first national-scale Australian site-response model.
This allowed for new ground-motion prediction equations to be derived for the South-Eastern Australian crust. Predicting the level of ground-shaking at a given distance from an earthquake rupture depends on three key elements:
- the magnitude and frequency content of the earthquake source which is estimated from the recorded seismograms
- how earthquake energy decays as it propagates through the Earth's crust which is modelled using ground-motion prediction equations
- how near-surface regolith (the layer of weathered rock, unconsolidated sediments and/or soils that overlies fresh bedrock) modifies the observed ground motions, as represented by a site-response model.
Regolith can contribute significantly to the modification of earthquake ground-motions. Therefore, modelling and predicting the potential impact of earthquakes on the built environment also requires an understanding of how the regolith behaves during an earthquake.
A first generation national-scale site classification map applying a modified US National Earthquake Hazard Reduction Program schema was developed for Australia (Figure 1). The map uses surficial geology and other geoscientific data at a variety of scales to identify and group regolith materials into classes likely to exhibit a similar response to earthquake ground-shaking.
Comparing ground motion models using the Newcastle 1989 earthquake as a scenario
New ground-motion prediction equations integrated with the first site-response model (regolith) for Australia can refine estimates of earthquake ground-shaking, providing the potential to rapidly assess earthquake impact for disaster response. The research highlights:
- differences in calculated 'hazard on rock' using an Eastern-North America (ENA) ground-motion model versus the new South-Eastern Australian (SEA) ground-motion model
- the significance of resolving earthquake hazard with and without the incorporation of site response (regolith) information.
Specifically, SEA is fairly similar to ENA for distances less than 100km from a fault. Figure 2 demonstrates capabilities before and after the development of Australian-specific ground-motion prediction equations and the national site response model for a scenario earthquake in the Newcastle region. The SEA equations predict significantly lower ground-motions than those produced using the ENA model and also demonstrate the significance of incorporating regolith site response into an earthquake hazard assessment. Furthermore, the addition of modelled site response information significantly enhances our ability to predict spatial variation in strong ground-shaking, a key factor in understanding and modelling the distribution of damage and loss.
The benefits of Australian models to emergency managers
The application of a national-scale site response model to ground-shaking allows for significantly refined estimates of earthquake hazard and risk. Derivatives of this work will aid emergency managers in disaster planning, and may have implications for revision of the Australian Building Code and earthquake loading standard. They also offer significant potential as applications in decision-support tools for rapid post-event assessment of earthquake-affected areas.
Toro GR, Abrahamson NA & Schneider JF. 1997. Model of strong ground motions from earthquakes in central and eastern North America: best estimates and uncertainties. Seismological Research Letters 68(1):41 - 57.
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