THE 2023 UPDATE TO THE AUSTRALIAN NATIONAL SEISMIC HAZARD ASSESSMENT (NSHA23) - PHILOSOPHY AND MODEL CHANGES

The Australian National Seismic Hazard Assessment (NSHA) was updated in 2023 to coincide with the revision of the Australian Standard for the structural design actions for earthquake (AS1170.4). This updates the 2018 NSHA (NSHA18), which applied globally accepted, best-practice probabilistic approaches and yielded considerably lower hazard estimates at the 1/500 annual exceedance probability relative to previous national assessments and the AS1170.4–2007 seismic design values. Whilst the science underpinning the NSHA18 has withstood on-going peer review and critique, Geoscience Australia has reflected on this model and has advocated for adjustments to modelling choices where warranted. The use of structured expert elicitation was a major advance in the development of the NSHA18 and promoted ownership of the model amongst the Australian seismological community. However, some consequences of the modelling choices made during that process were not fully appreciated at the time. The 2023 NSHA (NSHA23) was intended to be a modest update to the 2018 model and again is characterised through structured expert elicitation. However, there are some key changes to the earthquake catalogue, which include: 1) the inclusion of events from mid-2017 through to the end of 2022; 2) the recalculation of local magnitudes for recent events (2010 onwards) to resolve recently discovered inconsistent observatory practice by different Australian monitoring agencies and errors with station metadata, and; 3) the subsequent revision of local magnitude to moment magnitude conversions. All seismic source models in the updated NSHA23 that rely on the updated earthquake catalogue (pre-instrumental and instrumental) are updated to reflect these changes. Incremental updates to the fault-source model and changes to the relative weights between different source-model class types have also been implemented. Another significant advance is the augmentation of the Australian Ground-Motion Database with new and legacy data. High-quality data acquired from Australian earthquakes since 2018 were used to enable more informed choices for the ground-motion characterisation model. In summary, the 2023 updates to the instrumented earthquake catalogue have led to reduced earthquake rates and seismic hazard. This change is counteracted using an updated suite of ground-motion models. Adjustments to weights in the source characterisation model yield spatially variable changes in hazard, most commonly leading to modest increases in hazard.

Presented at the 2024 18th World Conference on Earthquake Engineering, Milan, Italy