<div>This paper explores several area-based tests of long-term seismic hazard forecasts for the Australian continent. Using the observed seismicity, ShakeMaps are calculated for earthquakes that are expected to have generated moderate-to-high levels of ground shaking within continental Australia in the past 50 years (January 1972 through December 2021). A “composite ShakeMap” is generated that extracts the maximum peak ground acceleration “observed” in this 50-year period for any site within the continent. The fractional exceedance area of this composite map is compared with four generations of Australian seismic hazard maps for a 10% probability of exceedance in 50 years (~1/500 annual exceedance probability) developed since 1990. </div><div>In general, all these models appear to forecast higher seismic hazard relative to the ground motions that are estimated to have occurred in the last 50 years, with the most recent hazard model yielding a fractional exceedance area most similar to the target 1/500 annual exceedance probability. The sensitivity of these results to various modeling assumptions was tested by exploring an alternative ground-motion characterization model that forecasts higher overall ground-shaking intensities.&nbsp;The sensitivity of these results is also tested with the interjection of a rare scenario earthquake with an expected regional recurrence of approximately 8,700 years. While these area-based analyses do not provide a robust assessment of the performance of the candidate seismic hazard for any specific location given the limited independent data, they do provide—to the first order—a guide to the performance of the respective maps at a continental scale</div> <b>Citation:</b> Allen TI, Ghasemi H, Griffin JD. Exploring Australian hazard map exceedance using an Atlas of historical ShakeMaps. <i>Earthquake Spectra</i>. 2023;39(2):985-1006. doi:10.1177/87552930231151977

<div>Geoscience Australia, together with contributions from the wider Australian seismology community, have produced the 2023 National Seismic Hazard Assessment (NSHA23), intended for inclusion into the 2024 revision of Standards Australia’s Structural design actions, part 4: Earthquake actions in Australia, AS1170.4–2007 (Standards Australia, 2018). This Standard is prepared by sub-committee BD-006-11, General Design Requirements and Loading on Structures of Standards Australia. </div><div>This Geoscience Australia Record provides the technical overview for the development of the NSHA23. Time-independent, ground-motion values with the mean value of the target exceedance probability are calculated for the geometric mean of the horizontal peak ground acceleration (PGA) and spectral accelerations, <em>Sa</em> (<em>T</em>), for eleven oscillator periods <em>T</em>&nbsp;= 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.7, 1.0, 1.5, 2.0 and 3.0 s. Maps illustrating the spatial distribution of ground-motion hazard are calculated using a 12.5-km national grid spacing (over 100,000 sites). Hazard curves and uniform-hazard spectra are also calculated for key localities. Maps of PGA, in addition to <em>Sa </em>(0.2&nbsp;s) and <em>Sa </em>(1.0&nbsp;s) are presented for a 10% (Figure 1‑1) and 2% probability of exceedance in 50 years. These exceedance probabilities refer to 1/475 and 1/2475 annual exceedance probability (AEP), respectively. Ground-motion values with a given probability of exceedance in the investigation time are calculated for each grid point on a national scale, while uniform-hazard spectra (UHS) have been calculated specifically for AS1170.4 city localities and additional sites for two probability levels: 10%, and 2% probability of exceedance in 50 years. </div><div>The NSHA23 has used the 2018 National Seismic Hazard Assessment (NSHA18) as a foundation and has built upon the previous assessment through several key updates and revisions to model components. Whilst the NSHA23 was intended to be a modest update to the 2018 model, there was considerable effort placed into updating several model components, including: 1) updating and extending the earthquake catalogue (Allen<em> et al.</em>, in press); 2) updating the fault-source model (Clark, 2023; Allen<em> et al.</em>, 2024, in press); 3) the augmentation of the Australian Ground-Motion Database (Ghasemi and Allen, 2021, 2023) with new and legacy data for ground-motion model (GMM) evaluation and weighting; and 4) review and revision of the seismic-source and ground-motion characterisations model logic trees through expert elicitation. </div><div>For the first time, the NSHA23 calculates hazard considering different site classes, assuming varying time-averaged shear-wave velocities in the upper 30 m of the crust (i.e., <em>VS</em>30): 150, 270, 450, 760 and 1,100 m/s. It is important to note that many localities across Australia lie within sedimentary basins and sites may be subject to significant ground-motion amplification owing to basin resonance effects. Whilst the calculation of hazard for different site conditions is a significant advance, there is no explicit modelling of basin resonance effects. Consequently, users of the NSHA23 should use caution and ensure they are aware of any local site conditions that may modify the earthquake ground motions that have been calculated through this assessment. Further work is required to fully characterise the probabilistic seismic site response of major Australian urban centres that lie within deep sedimentary basins (e.g., Adelaide and Perth) where earthquake ground motions could be significantly modified by local geological structure. </div><div>Sensitivity tests demonstrate that there are minor changes in the mean PGA hazard (mostly decreases) relative to the NSHA18 due to the NSHA23 seismic-source characterisation model (SSCM). However, these decreases due to the SSCM are more than offset due to changes in the ground-motion characterisation model (GMCM), resulting in a net increase in hazard over the range of exceedance probabilities considered. The most significant changes in hazard occurred in the City of Darwin, Northern Territory. This change in hazard is almost exclusively due to the use of the new Allen (2022) GMM, which forecasts significantly higher short-period ground motions than the GMMs which contributed to the NSHA18 GMCM. Considering all localities, the mean (plus and minus one standard deviation) percentage increase for the NSHA23 relative to the NSHA18 for mean PGA at the 10% chance of exceedance in 50 years is 25.8% ± 33.5%.&nbsp;Whilst this may seem like a rather significant change, when the hazard difference is considered for the same probability level across all sites, the mean difference in PGA hazard is only 0.008 ± 0.011 g.</div>