Geoscience Australia has produced an Atlas of Australian earthquake scenarios (the Atlas) to support planning and preparedness operations for emergency management agencies. The Atlas provides earthquake scenarios represent realistic “worst-case” events that may impact population centres around Australia. Such scenarios may also support seismic risk assessments for critical infrastructure assets to inform remediation actions that could be taken to improve resilience to rare seismic events in Australia. The Atlas of seismic scenarios uses the underlying science and data of the 2018 National Seismic Hazard Assessment (NSHA18) to identify the magnitudes and epicentre locations of these hypothetical earthquakes. Locations and magnitudes of earthquake scenarios are based upon deaggregation of the NSHA18 hazard model. The USGS ShakeMap software is used to produce ground motion intensity fields with the shaking levels being modified by seismic site conditions mapped at a national scale. Fault sources are incorporated into the Atlas where the magnitude of a given scenario exceeds a threshold magnitude of 6.0 and where the rupture length is likely to be longer than 10 km. If a scenario earthquake is located near a known fault within the Australian Neotectonic Features database, a partial or full-length rupture is modelled along the mapped fault. The Atlas generated two scenarios for each of the160 localities across Australia. The scenarios are based on some of the most likely earthquake magnitude-distance combinations estimated at each site. Output products include shaking contours for a range of intensity measures, including peak acceleration and velocity, as well as response spectral acceleration for 0.3, 1.0 and 3.0 seconds. Also included are raster images and the associated metadata used for generating the scenarios.

Modern geodetic and seismic monitoring tools are enabling the study of moderate-sized earthquake sequences in unprecedented detail. Discrepancies are apparent between the surface deformation envelopes ‘detectable’ using these tools, and ‘visible’ to traditional ground-based methods of observation. As an example, we compare the detectible and visible surface deformation caused by a sequence of earthquakes near Lake Muir in southwest Western Australia in 2018. A shallow MW 5.3 earthquake on the 16th of September 2018 was followed on the 8th of November 2018 by a MW 5.2 event in the same region. Focal mechanisms for the events suggest reverse and strike-slip rupture, respectively. Interferometric Synthetic Aperture Radar (InSAR) analysis of the events suggests that the ruptures are in part spatially coincident and deformed the Earth’s surface over ~ 12 km in an east-west direction and ~ 8 km in a north-south direction. Field mapping, guided by the InSAR results, reveals 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. No surface deformation unique to the second event was identifiable on the ground. New rupture length versus magnitude scaling relationships developed for non-extended cratonic regions as part of this study allow for the distinction between ‘visible’ surface rupture lengths (VSRL) from field-mapping and ‘detectable’ surface rupture lengths (DSRL) from remote sensing techniques such as InSAR, and suggest longer ruptures for a given magnitude than implied by commonly used scaling relationships.

The Earthquake Scenario Selection is an interactive tool for querying, visualising and downloading earthquake scenarios. There are over 160 sites nationally with pre-generated scenarios available. These represent plausible future scenarios that can be used for earthquake risk management and planning (see https://www.ga.gov.au/about/projects/safety/nsha for more details).

One-dimensional shear-wave velocity (VS ) profiles are presented at 50 strong motion sites in New South Wales and Victoria, Australia. The VS profiles are estimated with the spectral analysis of surface waves (SASW) method. The SASW method is a noninvasive method that indirectly estimates the VS at depth from variations in the Rayleigh wave phase velocity at the surface.

The Geological Survey of Western Australia, in collaboration with the Australian National University, Macquarie University, the Department of Fire and Emergency Services and Geoscience Australia has just installed the first seismometers of an array across the South West Seismic Zone of Western Australia. This region is one of the most seismically active areas of Australia having experienced over 2000 small (between ML 2 to 3) earthquakes since the year 2000. Many smaller events are also noted by the local people who often hear them coming. Yes – hear them coming – this area is known for its “noisy” earthquakes. Most of these earthquakes occur in swarms rather than main shock-aftershock sequences (Dent, 2015). This means that the region experiences a lot of small earthquakes, all much the same size and which occur in a similar area. These swarms can be active for years. The hazard associated with these seismic events is relatively small. However, in the past six decades this region has also hosted five of the nine surface rupturing earthquakes in Australia, most notably; Meckering (M 6.5) in 1968 from which there are photos of the bends in the railway lines (Fig 1a) and faulting of 2-3 m in height across the fields (Fig 1b) (Gordon and Lewis 1980; Johnston and White 2018, Clark and Edwards 2018); Calingiri (M5.9) in 1970 and Lake Muir (M5.6), which was felt by a lot of people across Western Australia just two years ago (Clark et al. 2020). Despite the high rates of seismicity, seismic monitoring in the region remains relatively sparse. To overcome this lack of instrumentation, the consortium of institutions mentioned above, came together for an ARC Linkage project to put in place a temporary network- the South West Australia Network (SWAN) - to improve the monitoring and detection capabilities in this area. This project will see a total of twenty-five broadband seismometers deployed across the Southwest of Western Australia for a period of approximately 2 years (Fig 2a and b). This temporary array will enable the detection and location of smaller-magnitude earthquakes which can be used to improve the crustal velocity models which in turn enables more accurate earthquake locations and helps the understanding of the crustal structure of this part of Australia. Better velocity models also enable better magnitude calculation methods, which improve the knowledge about recurrence of earthquakes of a certain magnitude. From a seismic hazard point of view, this data has the potential to assist in the development of improved methods for modelling how shaking intensity varies as it propagates through the earth’s crust from the earthquake source. Overall, this information will feed into an improved understanding of the earthquake hazard in the Southwest region of Western Australia. For local communities, it will provide an improved situational awareness following significant earthquakes. More broadly, the improved understanding of the seismicity of the Southwest of Western Australia will enhance emergency response capabilities, and inform building codes and mitigation initiatives, which are the best methods we have to minimise the earthquake risks to communities. Data will be released through AusPASS, the Australian Passive Seismic Server two years after the last data has been collected.

Geoscience Australia is currently drafting a new Australian Earthquake Hazard Map (or more correctly a series of maps) using modern methods and models. Among other applications, the map is a key component of Australia’s earthquake loading code AS1170.4. In this paper we provide a brief history of national earthquake hazard models in Australia, with a focus on the map used in AS1170.4, and provide an overview of the proposed changes for the new maps. The revision takes advantage of significant improvements in both the data sets and models used for earthquake hazard assessment in Australia since the original map was produced. These include:  Earthquake observations up to and including 2010  Improved methods of declustering earthquake catalogues and calculating earthquake recurrence  Ground-motion prediction equations (i.e. attenuation equations) based on response spectral acceleration rather than peak ground velocity, peak ground acceleration or intensity-based relations.  Revised earthquake source zones  Improved maximum magnitude earthquake estimates based on palaeoseismology  The use of open source software for undertaking probabilistic seismic hazard assessment which promotes testability and repeatability The following papers in this series will address in more detail the changes to the earthquake catalogue, earthquake recurrence and ground motion prediction equations proposed for use in the draft map. The draft hazard maps themselves are presented in the final paper.

The local magnitude ML 5.4 (MW 5.1) Moe earthquake on 19 June 2012 that occurred within the Australian stable continental region was the largest seismic event for the state of Victoria for more than 30 years. Seismic networks in the southeast Australian region yielded many high-quality recordings of the moderate-magnitude earthquake mainshock and its largest aftershock (ML 4.4; MW 4.3) at a hypocentral range of 10 to 480 km. The source and attenuation characteristics of the earthquake sequence are analyzed. Almost 15,000 felt reports were received following the main shock, which tripped a number of coal-fired power generators in the region, amounting to the loss of approximately 1955 megawatts of generation capacity. The attenuation of macroseismic intensities are shown to mimic the attenuation shape of Eastern North America (ENA) models, but require an inter-event bias to reduce predicted intensities. Further instrumental ground-motion recordings are compared to ground-motion models (GMMs) considered applicable for the southeastern Australian (SEA) region. Some GMMs developed for ENA and for SEA provide reasonable estimates of the recorded ground motions of spectral acceleration within epicentral distances of approximately 100 km. The mean weighted of the Next Generation Attenuation-East GMM suite, recently developed for stable ENA, performs relatively poorly for the 2012 Moe earthquake sequence, particularly for short-period accelerations.

<p>As part of the 2018 National Seismic Hazard Assessment (NSHA), we compiled the geographic information system (GIS) dataset to enable end-users to view and interrogate the NSHA18 outputs on a spatially enabled platform. It is intended to ensure the NSHA18 outputs are openly available, discoverable and accessible to both internal and external users. <p>This geospatial product is derived from the dataset generated through the development of the NSHA18 and contains uniform probability hazard maps for a 10% and 2% chance of exceedance in 50 years. These maps are calculated for peak ground acceleration (PGA) and a range of response spectral periods, Sa(T), for T = 0.1, 0.2, 0.3, 0.5, 1.0, 2.0 and 4.0 s. Additionally, hazard curves for each ground-motion intensity measure as well as uniform hazard spectra at the nominated exceedance probabilities are calculated for key localities.

Damaging earthquakes in Australia and other regions characterised by low seismicity are considered low probability but high consequence events. Uncertainties in modelling earthquake occurrence rates and ground motions for damaging earthquakes in these regions pose unique challenges to forecasting seismic hazard, including the use of this information as a reliable benchmark to improve seismic safety within our communities. Key challenges for assessing seismic hazards in these regions are explored, including: the completeness and continuity of earthquake catalogues; the identification and characterisation of neotectonic faults; the difficulties in characterising earthquake ground motions; the uncertainties in earthquake source modelling, and the use of modern earthquake hazard information to support the development of future building provisions. Geoscience Australia recently released its 2018 National Seismic Hazard Assessment (NSHA18). Results from the NSHA18 indicate significantly lower seismic hazard across almost all Australian localities at the 1/500 annual exceedance probability level relative to the factors adopted for the current Australian Standard AS1170.4–2007 (R2018). These new hazard estimates have challenged notions of seismic hazard in Australia in terms of the recurrence of damaging ground motions. Consequently, this raises the question of whether current practices in probabilistic seismic hazard analysis (PSHA) deliver the outcomes required to protect communities and infrastructure assets in low-seismicity regions, such as Australia. This manuscript explores a range of measures that could be undertaken to update and modernise the Australian earthquake loading standard, in light of these modern seismic hazard estimates, including the use of alternate ground-motion exceedance probabilities for assigning seismic demands for ordinary-use structures. The estimation of seismic hazard at any location is an uncertain science, particularly in low-seismicity regions. However, as our knowledge of the physical characteristics of earthquakes improve, our estimates of the hazard will converge more closely to the actual – but unknowable – (time independent) hazard. Understanding the uncertainties in the estimation of seismic hazard is also of key importance, and new software and approaches allow hazard modellers to better understand and quantify this uncertainty. It is therefore prudent to regularly update the estimates of the seismic demands in our building codes using the best available evidence-based methods and models.

<p>The 2018 Australian Probabilistic Tsunami Hazard Assessment (PTHA18) was developed by Geoscience Australia to better understand Australia’s tsunami hazard due to earthquakes in the Pacific and Indian Oceans. The PTHA18 contains over a million hypothetical earthquake-tsunami scenarios, with associated return periods which are constrained using historical earthquake data and long-term plate tectonic motions. The tsunami propagation is modelled globally for 36 hours, and results are stored at thousands of sites in deep waters offshore of Australia. Average Return Interval (ARI) estimates are also provided, along with a representation of the associated uncertainties. ARI uncertainties tend to be large because of fundamental limitations in current scientific knowledge regarding the frequency of large earthquakes on global subduction zones. <p>The PTHA18 provides a nationally consistent basis for earthquake-tsunami scenario design, as required for inundation hazard assessments. The results and source-code are also freely available. The current paper aims to provide a short and accessible introduction to the PTHA18 methodology and results, while deliberately limiting technical details which are covered extensively in the associated technical report and code repository.