This Geoscience Australia Record contains technical data and input files that, when used with the Global Earthquake Model’s (GEM’s) OpenQuake-engine probabilistic seismic hazard analysis software (Pagani et al., 2014), will enable end users to explore and reproduce the 2018 National Seismic Hazard Assessment (NSHA18) of Australia (Allen et al., 2018a). This report describes the NSHA18 input data only and does not discuss the scientific rationale behind the model development. These details are provided in Allen et al. (2018a) and references therein.

Located within an intraplate setting, continental Australia has a relatively low rate of seismicity compared with its surrounding plate boundary regions. However, the plate boundaries to the north and east of Australia host significant earthquakes that can impact Australia. Large plate boundary earthquakes have historically generated damaging ground shaking in northern Australia, including Darwin. Large submarine earthquakes have historically generated tsunami impacting the coastline of Australia. Previous studies of tsunami hazard in Australia have focussed on the threat from major subduction zones such as the Sunda and Kermadec Arcs. Although still subject to uncertainty, our understanding of the location, geometry and convergence rates of these subduction zones is established by global tectonic models. Conversely, actively deforming regions in central and eastern Indonesia, the Papua New Guinea region and the Macquarie Ridge region are less well defined, with deformation being more continuous and less easily partitioned onto discrete known structures. A number of recently published geological, geodetic and seismological studies are providing new insights into present-day active tectonics of these regions, providing a basis for updating earthquake source models for earthquake and tsunami hazard assessment. This report details updates to earthquake source models in active tectonic regions along the Australian plate boundary, with a primary focus on regions to the north of Australia, and a subsidiary focus on the Puyesgur-Macquarie Ridge-Hjort plate boundary south of New Zealand. The motivation for updating these source models is threefold: 1. To update regional source models for the 2018 revision of the Australian probabilistic tsunami hazard assessment (PTHA18); 2. To update regional source models for the 2018 revision of the Australian national seismic hazard assessment (NSHA18); and 3. To provide an updated database of earthquake source models for tsunami hazard assessment in central and eastern Indonesia, in support of work funded through the Department of Foreign Affairs and Trade (DFAT) DMInnovation program.

One of the key challenges in assessing earthquake hazard in Australia is understanding the attenuation of ground-motion through the stable continental crust. There are now a handful of ground-motion models (GMMs) that have been developed specifically to estimate ground-motions from Australian earthquakes. These GMMs, in addition to models developed outside Australia, are considered in the 2018 National Seismic Hazard Assessment (NSHA18; Allen et al., 2017). In order to assess the suitability of candidate GMMs for use in the Australian context, ground-motion data forom small-to-moderate Australian earthquakes have been gathered. Both qualitative and quantitative ranking techniques (e.g., Scherbaum et al., 2009) have been applied to determine the suitability of candidate GMMs for use in the NSHA18. This report provides a summary of these ranking techniques and provides a discussion on the utility of these methods for use in seismic hazard assessments in Australia; in particular for the NSHA18. The information supplied herein was provided to participants of the Ground-Motion Characterisation Expert Elicitation workshop, held at Geoscience Australia on 9 March 2017 (Griffin et al., 2018).

The 2018 National Seismic Hazard Assessment (NSHA18) aims to provide the most up-to-date and comprehensive understanding of seismic hazard in Australia. As such, NSHA18 includes a range of alternative models for characterising seismic sources and ground motions proposed by members of the Australia earthquake hazard community. The final hazard assessment is a weighted combination of alternative models. This report describes the use of a structured expert elicitation methodology (the ‘Classical Model’) to weight the alternative models and presents the complete results of this process. Seismic hazard assessments are inherently uncertain due to the long return periods of damaging earthquakes relative to the time period of human observation. This is especially the case for low-seismicity regions such as Australia. Despite this uncertainty, there is a demand for estimates of seismic hazard to underpin a range of decision making aimed at reducing the impacts of earthquakes to society. In the face of uncertainty, experts will propose alternative models for the distribution of earthquake occurrence in space, time and magnitude (i.e. seismic source characterisation), and how ground shaking is propagated through the crust (i.e. ground motion characterisation). In most cases, there is insufficient data to independently and quantitatively determine a ‘best’ model. Therefore it is unreasonable to expect, or force, experts to agree on a single consensus model. Instead, seismic hazard assessments should capture the variability in expert opinion, while allowing that not all experts are equally adept. Logic trees, with branches representing mutually exclusive models weighted by expert opinion, can be used to model this uncertainty in seismic hazard assessment. The resulting hazard assessment thereby captures the range of plausible uncertainty given current knowledge of earthquake occurrence in Australia. For the NSHA18, experts were invited to contribute peer-reviewed seismic source models for consideration, resulting in 16 seismic source models being proposed. Each of these models requires values to be assigned to uncertain parameters such as the maximum magnitude earthquake expected. Similarly, up to 20 published ground motion models were identified as being appropriate for characterising ground motions for different tectonic regions in Australia. To weight these models, 17 experts in seismic hazard assessment, representative of the collective expertise of the Australian earthquake hazard community, were invited to two workshops held at Geoscience Australia in March 2017. At these workshops, the experts each assigned weights to alternative models representing their degree of belief that a particular model is the ‘true’ model. The experts were calibrated through a series of questions that tested their knowledge of the subject and ability to assess the limits to their knowledge. These workshops resulted in calibrated weights used to parameterise the final seismic source model and ground motion model logic trees for NSHA18. Through use of a structured expert elicitation methodology these weights have been determined in a transparent and reproducible manner drawing on the full depth of expertise and experience within the Australia earthquake hazard community. Such methodologies have application to a range of uncertain problems beyond the case of seismic hazard assessment presented here.

People in Australia are surprised to learn that hundreds of earthquakes occur below our feet every year. The majority are too small to feel, let alone cause any damage. Despite this, we are not immune to large earthquakes.

<div>The 1 March 1954 earthquake in South Australia is the most damaging earthquake to impact the densely populated Adelaide region since European settlement. Previous interpretations have associated the event with the Eden-Burnside Fault zone, with a presumed epicentre near Darlington. Surprisingly, comparing macroseismic intensities from the 1954 earthquake with similar modern observational datasets suggests the 1954 event was perhaps larger than previously thought. We assess the validity of this observation by reviewing available macroseismic and instrumental data. We observe damaging shaking extending east from Adelaide into the Adelaide Hills, but without a well-defined locus of higher intensities. The limited teleseismic observations lead us to further speculate that the 1954 earthquake could have been deeper and/or associated with a higher-than-normal stress drop. These new findings question the conventionally assumed location for the 1954 earthquake. Our work highlights the potential seismic hazards faced by large urban centres in Australia such as Adelaide.</div> This paper was presented to the 2022 Australian Earthquake Engineering Society (AEES) Conference 24-25 November (https://aees.org.au/aees-conference-2022/)

The geological structure of southwest Australia comprises a rich, complex record of Precambrian cratonization and Phanerozoic continental breakup. Despite the stable continental cratonic geologic history, over the past five decades the southwest of Western Australia has been the most seismically active region in continental Australia though the reason for this activity is not yet well understood. The Southwest Australia Seismic Network (SWAN) is a temporary broadband network of 27 stations that was designed to both record local earthquakes for seismic hazard applications and provide the opportunity to dramatically improve the rendering of 3-D seismic structure in the crust and mantle lithosphere. Such seismic data are essential for better characterization of the location, depth and attenuation of the regional earthquakes, and hence understanding of earthquake hazard. During the deployment of these 27 broadband instruments, a significant earthquake swarm occurred that included three earthquakes with local magnitude (MLa) ≥ 4.0, and the network was supplemented by an additional six short-term nodal seismometers at 10 separate sites in early 2022, as a rapid deployment to monitor this swarm activity. The SWAN experiment has been continuously recording since late 2020 and will continue into 2023. These data are archived at the FDSN recognized Australian Passive Seismic (AusPass) Data center under network code 2P and will be publicly available in 2025. <b>Citation:</b> Meghan S. Miller, Robert Pickle, Ruth Murdie, Huaiyu Yuan, Trevor I. Allen, Klaus Gessner, Brain L. N. Kennett, Justin Whitney; Southwest Australia Seismic Network (SWAN): Recording Earthquakes in Australia’s Most Active Seismic Zone. <i>Seismological Research Letters </i><b>2023</b>;; 94 (2A): 999–1011. doi: https://doi.org/10.1785/0220220323

<p>The hazard factors in every version of AS 1170.4 since 1993 have been based on a seismic hazard map published in 1991. In this paper I statistically test the validity of that 1991 map. <p>Two methods are used to calculate the hazard for 24+ sites across Australia. Firstly, for each site I calculate how many standard deviations (?1) separate the 1991 hazard value from the calculated PSHA value. Secondly, the magnitude frequency distribution (MFD; i.e. a and b values) is adjusted so that the calculated hazard matches the 1991 hazard value. The number of standard deviations (?2) in the MFD that separate the adjusted MFD differs from the best estimate MFD is subsequently calculated. The first method was applied using four seismic source models (AUS6, DIM-AUS, NSHM13 and these combined), while the second method used NSHM13 only. The average number of standard deviations was calculated from the best 20 of the 24 sites. These statistics are considered a test the validity of the 1991 map. The two methods using five models in total all give similar results. The 1991 map is found, on average, to overestimate the hazard by 3 standard deviations. This suggests that the 1991 map is best described as a 95th+ percentile map. <p>Practitioners using this map, whether for setting building standards or assessing insurance exposure, need to be conscious that the seismic design values are not scientifically valid relative to modern mean probabilistic seismic hazard assessments.

<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>

The 2018 National Seismic Hazard Assessment (NSHA18) is a flagship Geoscience Australia product, used to support the decisions of the Australian Building Codes Board and Standards Australia to ensure buildings and infrastructure are built to withstand seismic events in Australia. It is also important for the insurance sector and provides a baseline for setting national reinsurance premiumsguides the level of reinsurance premiums. The National Seismic Hazard Assessment Earthquake Epicentre Catalogue (NSHA18-Cat) of historical earthquakes is the authoritative catalogue underpinning the NSHA18. The NSHA18-Cat is compiled from Australian and international sources and combines the highest quality epicentres and magnitudes for the assessment of earthquake hazard in Australia. For the first time in an Australian national seismic hazard assessment, earthquake magnitudes are uniformly expressed in the moment magnitude MW scale, using Australian-specific magnitude conversion equations appropriate for several common magnitude types. The magnitude harmonisation represents a significant advance in our ability to represent earthquake hazard in a uniform manner throughout the country. Key points and advances on the NSHA18-Cat include: - The addition of almost three decades worth of additional earthquake data gathered by seismic networks across the Australian continent, relative to hazard assessments from the early 1990s. - The use of the International Seismological Centre-Global Earthquake Model Catalogue (Version 5) for regional plate boundary source zones; - An improved methodology for revising local magnitudes due to the historical use of inappropriate magnitude attenuation formulae using a consistent and objective methodology; - The development of conversion equations from original magnitude types to MW specific for the Australian earthquake catalogue. This ensures consistency between rates of earthquake recurrence and ground-motion models in hazard calculations; - The development of new magnitude completeness models in terms of MW. The combination of these new data and advances demonstrates global best practice and evidence based science for undertaking national-scale earthquake hazard assessments. The earthquake epicentre solutions are provided in simple comma separated value and shapefile formats and are attached to this report.