Sydney, NSW Chapter

Estimation of design earthquake ground motions in Australia

Dr Paul Somerville


MCE ground motions are often defined as those having a return period of 10,000 years, as estimated from a site-specific probabilistic seismic hazard analysis. However, in some cases the MCE is defined by “deterministic” ground motions that are defined as the “maximum possible” ground motions that could occur at the site. In practice, these “deterministic” ground motions consist of spectra for either the median ground motion level or the 84th percentile ground motion level of a scenario earthquake defined by a maximum earthquake magnitude and a closest distance. In high seismic hazard regions such as New Zealand, the probabilistic ground motions at a 10,000 year return period may exceed the scenario-based ground motion, even at the 84th percentile level, and so the use of the “deterministic” approach may be unconservative. In contrast, in low seismic hazard regions such as Australia, the probabilistic ground motions at a 10,000 year return period may be significantly lower than the scenario-based ground motion, even at the median level, and so the use of the “deterministic” approach may be overconservative.

Computationally, the scenario-based approach is simpler to apply than the probabilistic approach, since it only requires specification of a single earthquake scenario. However, in practice, the specification of the earthquake source model for the scenario-based approach is much more difficult because it is extremely sensitive to the selection of the maximum magnitude and the location of the controlling earthquake source, which are especially difficult to identify in regions of low seismicity.

When nonlinear time history analysis is used for design or analysis, the design response spectrum needs to be represented by ground motion time histories. Earthquake events that dominate the hazard in the period range of importance for the structure for the specified annual probability are identified through the process of deaggregation of the probabilistic response spectrum. This results in one or more earthquake scenarios, each having a specified magnitude and distance. Currently, the probabilistic uniform hazard response spectrum (UHS) is usually used as the target spectrum for scaling or spectral matching of the time histories. However, at low probabilities, this response spectrum is too “broadband” (i.e. large over a range of periods that is unrealistically broad), and envelopes a more appropriate response spectrum, called the conditional mean spectrum (CMS). Current ground motion prediction equations for Australia are reviewed, and the importance of measuring or estimating the near-surface shear wave velocity in the foundations of the structure for use in calculating near-surface ground motion amplification effects is noted.

About Dr Paul Somerville

Dr Paul Somerville was born in Armidale, NSW, Australia and received his B.Sc. degree in Geophysics from the University of New England. He received his M.Sc. and Ph.D. degrees in Geophysics at the University of British Columbia in Vancouver, Canada. He spent two years as a Visiting Research Fellow at the Earthquake Research Institute, University of Tokyo, and has been involved with Japanese colleagues in engineering seismology research for his whole career.

Paul is Principal Seismologist for AECOM in Los Angeles, where he does research and development on earthquake source and strong ground motion prediction models. He has applied these skills in the design and analysis of major buildings, bridges, dams and power generation facilities in many countries, including Australia, New Zealand, the United States and Japan. Paul is also Chief Geoscientist at Risk Frontiers, Macquarie University, which is sponsored by the insurance industry in Australia and New Zealand.
Paul is currently President the Australian Earthquake Engineering Society (AEES), and is currently a member of a committee that is updating the ANCOLD seismic guidelines.

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