A new probabilistic tsunami hazard assessment for Australia

Geoscience Australia has recently updated the Australian probabilistic tsunami hazard assessment (PTHA). This was last produced in 2008 and has been widely used to support tsunami hazard and risk assessments around Australia, including in NSW. Like the 2008 PTHA, the new PTHA consists of a large, freely available database of pre-computed earthquake-tsunami scenarios with associated rates of occurrence. Wave time series from these tsunami scenarios are stored at approximately 20000 offshore points around Australia. The database can be used to identify earthquake sources driving tsunami hazard in Australia, and provides tsunami wave boundary conditions to drive inundation-scale tsunami hazard studies for risk management applications. Tsunami scenarios in the new PTHA have been extensively tested by comparison with 18 recent earthquake-tsunamis (2006-2016), using measurements from the global network of deep ocean tsunami detection buoys. This observational data (which was largely unavailable for the earlier study) allows earthquake-tsunami generation models to be tested in terms of their capacity to: A) generate tsunamis that 'look like' observations, and; B) represent the natural variability of tsunami wave heights without gross biases. Our results demonstrate the importance of considering natural variations in earthquake rupture size, and spatial heterogeneities in earthquake slip, in order to produce tsunami-scenarios that 'look like' observations. Historically many tsunami hazard studies have neglected these factors, which tends to result in under-estimation of tsunami wave heights and the associated hazard. A particular challenge for tsunami hazard studies is that wave height return periods are very uncertain. Fundamentally this is caused by large uncertainties in the rates of high magnitude earthquakes on most subduction zones. In the new PTHA these uncertainties are quantified using credible intervals, in a manner that retains consistency with both historical earthquake observations, and current knowledge of plate tectonic motions. Abstract presented at the 2018 NSW Coastal Conference