Seismic Hazard Assessment in Australia: Can Structured Expert Elicitation Achieve Consensus in the “Land of the Fair Go”?

The 2018 National Seismic Hazard Assessment of Australia incorporated 19 alternative seismic-source models. The diversity of these models demonstrates the deep epistemic uncertainty that exists with regards to how best to characterize intraplate seismicity. A complex logic tree was developed to incorporate the alternative models into a single hazard model. Similarly, a diverse range of ground-motion models were proposed for use and incorporated using a logic tree. Expert opinion was drawn upon to weight the alternative logic tree branches through a structured expert elicitation process. This process aims to transparently and reproducibly characterize the community distribution of expert estimates for unknown parameters and thereby quantify the epistemic uncertainty around estimates of seismic hazard in Australia. We achieve a multi-model rational consensus where each model, and each expert, is, in accordance with the Australian cultural myth of egalitarianism, given a ‘fair go’. Yet despite this process, we find that the results are not universally accepted. A key issue is a contested boundary between what is scientifically reducible and what remains epistemologically uncertain, with a particular focus on the earthquake catalog. Furthermore, a reduction, on average, of 72% for the 10% in 50 years probability of exceedance peak ground acceleration levels compared with those underpinning existing building design standards, challenges the choice of metrics upon which design codes are based. As questions of epistemic uncertainty are quantified or resolved, changes in our understanding of how the hazard behaves should inform dialogue between scientists, engineers and policy makers, and a re-appraisal of the metrics used to inform risk management decisions of societal importance.