<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.

The 20 May 2016 MW 6.1 Petermann earthquake in central Australia generated a 21 km surface rupture with 0.1 to 1 m vertical displacements across a low-relief landscape. No paleo-scarps or potentially analogous topographic features are evident in pre-earthquake Worldview-1 and Worldview-2 satellite data. Two excavations across the surface rupture expose near-surface fault geometry and mixed aeolian-sheetwash sediment faulted only in the 2016 earthquake. A 10.6 ± 0.4 ka optically stimulated luminescence (OSL) age of sheetwash sediment provides a minimum estimate for the period of quiescence prior to 2016 rupture. Seven cosmogenic beryllium-10 (10Be) bedrock erosion rates are derived for samples < 5 km distance from the surface rupture on the hanging-wall and foot-wall, and three from samples 19 to 50 km from the surface rupture. No distinction is found between fault proximal rates (1.3 ± 0.1 to 2.6 ± 0.2 m Myr−1) and distal samples (1.4 ± 0.1 to 2.3 ± 0.2 m Myr−1). The thickness of rock fragments (2–5 cm) coseismically displaced in the Petermann earthquake perturbs the steady-state bedrock erosion rate by only 1 to 3%, less than the erosion rate uncertainty estimated for each sample (7–12%). Using 10Be erosion rates and scarp height measurements we estimate approximately 0.5 to 1 Myr of differential erosion is required to return to pre-earthquake topography. By inference any pre-2016 fault-related topography likely required a similar time for removal. We conclude that the Petermann earthquake was the first on this fault in the last ca. 0.5–1 Myr. Extrapolating single nuclide erosion rates across this timescale introduces large uncertainties, and we cannot resolve whether 2016 represents the first ever surface rupture on this fault, or a > 1 Myr interseismic period. Either option reinforces the importance of including distributed earthquake sources in fault displacement and seismic hazard analyses. <b>Citation: </b>King, T. R., Quigley, M., Clark, D., Zondervan, A., May, J.-H., & Alimanovic, A. (2021). Paleoseismology of the 2016 M-W 6.1 Petermann earthquake source: Implications for intraplate earthquake behaviour and the geomorphic longevity of bedrock fault scarps in a low strain-rate cratonic region. <i>Earth Surface Processes and Landforms</i>, 46(7), 1238–1256.

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.

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.

The 10% in 50 year seismic hazard map is the key output from the 2018 National Seismic Hazard Assessment for Australia (NSHA18) as required for consideration by the Standards Australia earthquake loading committee AS1170.4

Seismic hazard assessments in stable continental regions such as Australia face considerable challenges compared with active tectonic regions. Long earthquake recurrence intervals relative to historical records make forecasting the magnitude, rates and locations of future earthquakes difficult. Similarly, there are few recordings of strong ground motions from moderate-to-large earthquakes to inform development and selection of appropriate ground motion models (GMMs). Through thorough treatment of these epistemic uncertainties, combined with major improvements to the earthquake catalog, a National Seismic Hazard Assessment (NSHA18) of Australia has been undertaken. The resulting hazard levels at the 10% in 50-year probability of exceedance level are in general significantly lower than previous assessments, including hazard factors used in the Australian earthquake loading standard (AS1170.4–2007 [R2018]), demonstrating our evolving understanding of seismic hazard in Australia. The key reasons for the decrease in seismic hazard factors are adjustments to catalog magnitudes for earthquakes in the early instrumental period, and the use of modern ground-motion attenuation models. This article summarizes the development of the NSHA18, explores uncertainties associated with the hazard model, and identifies the dominant factors driving the resulting changes in hazard compared with previous assessments.

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.

<div>Forecasting large earthquakes along active faults is of critical importance for seismic hazard assessment. Statistical models of recurrence intervals based on compilations of paleoseismic data provide a forecasting tool. Here we compare five models and use Bayesian model-averaging to produce time-dependent, probabilistic forecasts of large earthquakes along 93 fault segments worldwide. This approach allows better use of the measurement errors associated with paleoseismic records and accounts for the uncertainty around model choice. Our results indicate that although the majority of fault segments (65/93) in the catalogue favour a single best model, 28 benefit from a model-averaging approach. We provide earthquake rupture probabilities for the next 50 years and forecast the occurrence times of the next rupture for all the fault segments. Our findings suggest that there is no universal model for large earthquake recurrence, and an ensemble forecasting approach is desirable when dealing with paleoseismic records with few data points and large measurement errors. <b>Citation:</b> Wang, T., Griffin, J.D., Brenna, M. et al. Earthquake forecasting from paleoseismic records. <i>Nat Commun</i><b> 15</b>, 1944 (2024). https://doi.org/10.1038/s41467-024-46258-z

<div>In mid-2022 two paleoseismic trenches were excavated across the Willunga Fault at Sellicks Hill, ~40 km south of Adelaide, at a location where range front faulting displaces a thick colluvial apron, and flexure in the hanging wall has produced an extensional graben. Vertical separation between time-equivalent surfaces within the Willunga Embayment and uplifted Myponga Basin indicate an average uplift rate of 40 m/Myr since 5 Ma across the Willunga fault at the trench location, equivalent to a slip rate of 57 m/Myr across a 45° dipping fault. </div><div> The field sites preserve evidence for at least 4-5 large earthquake events involving approximately 6.9 m of discrete slip on fault planes since the Mid to Late Pleistocene. If the formation of red soil marker horizons in the trenches are assumed to relate to glacial climatic conditions then a slip-rate of 26-46 m/Myr since the Mid Pleistocene is obtained. These deformation rate estimates do not include folding in the hanging wall of the fault, which is likely to be significant at this site as evidenced by the existence of a pronounced hanging wall anticline. In the coming months, the results of dating analysis will allow quantitative constraint to be placed on earthquake timing and slip-rate, and a structural geological study seeks to assess the proportion of deformation partitioned into folding of the hanging wall.</div><div> The 2022 trenches represent the most recent of ten excavated across this fault. Integration of the 2022 data with those from previous investigations will allow fundamental questions to be addressed, such as whether the Willunga fault ruptures to its entire length, or in a segmented fashion, and whether any segmentation behaviour is reflected in local slip-rate estimates. Thereby we hope to significantly improve our understanding of the hazard that this, and other proximal Quaternary-active faults, pose to the greater Adelaide conurbation and its attendant infrastructure.</div> This paper was presented to the 2022 Australian Earthquake Engineering Society (AEES) Conference 24-25 November (https://aees.org.au/aees-conference-2022/)

Modern geodetic and seismic monitoring tools are enabling study of moderate-sized earthquake sequences in unprecedented detail. Here we use a variety of methods to examine surface deformation caused by a sequence of earthquakes near Lake Muir in Southwest Western Australia in late 2018. A shallow MW 5.3 earthquake near Lake Muir on the 16th of September 2018 was followed on the 8th of November by a MW 5.2 event in the same region. Focal mechanisms produced for the events suggest reverse and strike slip rupture, respectively. Recent improvements in the coverage and observation frequency of the Sentinel-1 Synthetic Aperture Radar (SAR) satellite in Australia allowed for the timely mapping of the surface deformation field relating to both earthquakes in unprecedented detail. Interferometric Synthetic Aperture Radar (InSAR) analysis of the events suggest that the ruptures are in part spatially coincident. Field mapping, guided by the InSAR results, revealed that the first event produced an approximately 3 km long and up to 0.5 m high west-facing surface rupture, consistent with slip on a moderately east-dipping fault. Double difference hypocentre relocation of aftershocks using data from rapidly deployed seismic instrumentation confirms an easterly dipping rupture plane for the first event. The aftershocks are predominantly located at the northern end of the rupture where the InSAR suggests vertical displacement was greatest. The November event resulted from rupture on a NE-trending strike slip fault. Anecdotal evidence from local residents suggests that the southern part of the September rupture was ‘freshened’ during the November event, consistent with InSAR results, which indicate that a NW-SE trending structural element accommodated deformation during both events. Comparison of the InSAR-derived deformation field with surface mapping and UAV-derived digital terrain models (corrected to pre-event LiDAR) revealed a surface deformation envelope consistent with the InSAR for the first event, but could not discern deformation unique to the second event.