Canada's 6th Generation seismic hazard model has been developed to generate seismic design values for the 2020 National Building Code of Canada (NBCC2020). The model retains most of the seismic source model from the 5th Generation, but updates the earthquake sources for the deep inslab earthquakes under the Straits of Georgia and adds the Leech River - Devil’s Mountain fault near Victoria. The rates of magnitude ~9 Cascadia earthquakes are also increased to match new paleoseismic information. Two major changes in the ground motion model (GMM) are A) replacement of most of the three-branch representative suite used in 2015 by suites of weighted GMMs, and B) use and adaptation of various GMMs to directly calculate hazard on various site classes with representative Vs30 values, rather than providing hazard values on a reference Class C site and applying F(T) factors as in 2015. Computations are now also being performed with the OpenQuake engine, which has been validated through the replication of the 5th Generation results. Seismic design values (on various Soil Classes) for PGA, and for Sa(T) for T = 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 s are proposed for NBCC2020 mean ground shaking at the 2% in 50-year probability level. The paper discusses chiefly the change in Site Class C values relative to 2015 in terms of the changes in the seismic source model and the GMMs, but the changes in hazard at other site classes that arise from application of the direct-calculation approach are also illustrated.

We present a preliminary probabilistic seismic hazard analysis (PSHA) of a site in the Otway basin, Victoria, Australia, as part of the CO2CRC Otway Project for CO2 storage risk. The study involves estimating the likelihood of future strong earthquake shaking at the site and utilizes three datasets: (1) active faults, (2) historical seismicity, and (3) geodetic surface velocities. Our analysis of geodetic data reveals strain rates at the limit of detectability and not significantly different from zero. Consequently, we do not develop a geodetic-based source model for this Otway model. We construct logic trees to capture epistemic uncertainty in both the fault and seismicity source parameters and in the ground-motion prediction. A new feature for seismic hazard modeling in Australia, and rarely dealt with in low-seismicity regions elsewhere, is the treatment of fault episodicity (long-term activity versus inactivity) in our Otway model. Seismic hazard curves for the combined (fault and distributed seismicity) source model show that hazard is generally low, with peak ground acceleration estimates of less than 0.1g at annual probabilities of 10-3-10-4/yr. Our preliminary analysis therefore indicates that the site is exposed to a low seismic hazard that is consistent with the intraplate tectonic setting of the region and unlikely to pose a significant hazard for CO2 containment and infrastructure.

Seismic hazard models, commonly produced through probabilistic seismic hazard analysis, are used to establish earthquake loading requirements for the built environment. However, there is considerable uncertainty in developing seismic hazard models, which require assumptions on seismicity rates and ground-motion models (GMMs) based on the best evidence available to hazard analysts. This paper explores several area-based tests of long-term seismic hazard forecasts for the Australian continent. ShakeMaps are calculated for all earthquakes of MW 4.25 and greater within approximately 200 km of the Australian coastline using the observed seismicity in the past 50 years (1970-2019). 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. In general, all these seismic hazard models appear to be conservative relative to the observed ground motions that are estimated to have occurred in the last 50 years. To explore aspects of possible prejudice in this study, the variability in ground-motion exceedance was explored using the Next Generation Attenuation-East GMMs developed for the central and eastern United States. The sensitivity of these results is also tested with the interjection of a rare scenario earthquake with an expected regional recurrence of approximately 5,000 - 10,000 years. While these analyses do not provide a robust assessment of the performance of the candidate seismic hazard for any given location, they do provide—to the first order—a guide to the performance of the respective maps at a continental scale. This paper was presented at the Australian Earthquake Engineering Society 2021 Virtual Conference, Nov 25 – 26.

Because all modern ground motion prediction equations (GMPEs) are now calibrated to the moment magnitude scale MW, it is essential that earthquake rates are also expressed in terms of moment magnitudes for probabilistic seismic hazard analyses. However, MW is not routinely estimated for earthquakes in Australia because of the low-to-moderate level of seismicity, coupled with the relatively small number of seismic recording stations. As a result, the Australian seismic catalogue has magnitude measures mainly based on local magnitudes, ML. To homogenise the earthquake catalogue based on a uniform MW, a “reference catalogue” that includes earthquakes with available MW estimates was compiled. This catalogue consists of 240 earthquakes with original MW values between 2.0 and 6.58. This reference catalogue served as the basis for the development of magnitude conversion equations between MW and other magnitude scales: ML, body-wave magnitude mb, and surface-wave magnitude MS. The conversions were evaluated using general orthogonal regression (GOR), which accounts for measurement errors in the x and y variables, and provides a unique solution that can be used interchangeably between magnitude types. The impact of the derived magnitude conversion equations on seismic hazard is explored by generating synthetic earthquake catalogues and computing seismic hazard level at an arbitrary site. The results indicate that we may expect up to 20-40% reduction in PGA hazard, depending on the selection and application process of the magnitude conversion equations. Abstract submitted to and presented at the 2017 Australian Earthquake Engineering Society (AEES) Conference

An earthquake catalogue based on the moment magnitude scale is required for calculation of seismic hazard in Australia. However, the estimation of moment magnitudes for small to moderate sized earthquakes is not a routine process at seismic observatories, resulting in a catalogue mainly based on the local magnitude scale for Australia. In this study we explore the application of an automated procedure to estimate moment magnitudes by minimizing the misfit between observed and synthetic displacement spectra. We compile a reference catalogue of 15 earthquakes with moment magnitude values between 3.8 and 5.4 which were based on previous studies. The moment magnitudes were then recalculated and we find that the estimated moment magnitudes are in good agreement with reference values with differences mainly lower than 0.2. However, the reported local magnitudes of the selected events are consistently higher than the reference values with differences between 0.3 and 1.0. The automated procedure will be applied to compute moment magnitudes of the well recorded events in Australia, and to derive a scaling relation between local magnitude and moment magnitude. This abstract was submitted and presented to the 2016 Australian Earthquake Engineering Society Conference (AEES) ( https://aees.org.au/aees-asian-seismological-commission-conferences/)

In plate boundary regions moderate to large earthquakes are often sufficiently frequent that fundamental seismic parameters such as the recurrence intervals of large earthquakes and maximum credible earthquake (Mmax) can be estimated with some degree of confidence. The same is not true for the Stable Continental Regions (SCRs) of the world. Large earthquakes are so infrequent that the data distributions upon which recurrence and Mmax estimates are based are heavily skewed towards magnitudes below Mw5.0, and so require significant extrapolation up to magnitudes for which the most damaging ground-shaking might be expected. The rarity of validating evidence from surface rupturing palaeo-earthquakes typically limits the confidence with which these extrapolated statistical parameters may be applied. Herein we present a new earthquake catalogue containing, in addition to the historic record of seismicity, 150 palaeo-earthquakes derived from 60 palaeo-earthquake features spanning the last > 100 ka of the history of the Precambrian shield and fringing extended margin of southwest Western Australia. From this combined dataset we show that Mmax in non-extended-SCR is M7.25 ± 0.1 and in extended-SCR is M7.65 ± 0.1. We also demonstrate that in the 230,000 km2 area of non-extended-SCR crust, the rate of seismic activity required to build these scarps is one tenth of the contemporary seismicity in the area, consistent with episodic or clustered models describing SCR earthquake recurrence. A dominance in the landscape of earthquake scarps reflecting multiple events suggests that the largest earthquakes are likely to occur on pre-existing faults. We expect these results might apply to most areas of non-extended-SCR worldwide.

We digitize surface rupture maps and compile observational data from 67 publications on ten of eleven historical, surface-rupturing earthquakes in Australia in order to analyze the prevailing characteristics of surface ruptures and other environmental effects in this crystalline basement-dominated intraplate environment. The studied earthquakes occurred between 1968 and 2018, and range in moment magnitude (Mw) from 4.7 to 6.6. All earthquakes involved co-seismic reverse faulting (with varying amounts of strike-slip) on single or multiple (1–6) discrete faults of ≥ 1 km length that are distinguished by orientation and kinematic criteria. Nine of ten earthquakes have surface-rupturing fault orientations that align with prevailing linear anomalies in geophysical (gravity and magnetic) data and bedrock structure (foliations and/or quartz veins and/or intrusive boundaries and/or pre-existing faults), indicating strong control of inherited crustal structure on contemporary faulting. Rupture kinematics are consistent with horizontal shortening driven by regional trajectories of horizontal compressive stress. The lack of precision in seismological data prohibits the assessment of whether surface ruptures project to hypocentral locations via contiguous, planar principal slip zones or whether rupture segmentation occurs between seismogenic depths and the surface. Rupture centroids of 1–4 km in depth indicate predominantly shallow seismic moment release. No studied earthquakes have unambiguous geological evidence for preceding surface-rupturing earthquakes on the same faults and five earthquakes contain evidence of absence of preceding ruptures since the late Pleistocene, collectively highlighting the challenge of using mapped active faults to predict future seismic hazards. Estimated maximum fault slip rates are 0.2–9.1 m Myr−1 with at least one order of uncertainty. New estimates for rupture length, fault dip, and coseismic net slip can be used to improve future iterations of earthquake magnitude—source size—displacement scaling equations. Observed environmental effects include primary surface rupture, secondary fracture/cracks, fissures, rock falls, ground-water anomalies, vegetation damage, sand-blows/liquefaction, displaced rock fragments, and holes from collapsible soil failure, at maximum estimated epicentral distances ranging from 0 to ~250 km. ESI-07 intensity-scale estimates range by ± 3 classes in each earthquake, depending on the effect considered. Comparing Mw-ESI relationships across geologically diverse environments is a fruitful avenue for future research.

Geoscience Australia has produced a draft National Seismic Hazard Assessment (NSHA18), together with contributions from the wider Australian seismology community. This paper provides an overview of the provisional peak ground acceleration (PGA) hazard values and discusses rationale for changes in the proposed design values at the 1/500-year annual exceedance probability (AEP) level relative to Standards Australia’s AS1170.4–2007 design maps. The NSHA18 update yields many important advances on its predecessors, including: consistent expression of earthquake magnitudes in moment magnitude; inclusion of epistemic uncertainty through the use of third-party source models; inclusion of a national fault-source model; inclusion of epistemic uncertainty on fault-slip-model magnitude-frequency distributions and earthquake clustering; and the use of modern ground-motion models through a weighted logic tree framework. In general, the 1/500-year AEP seismic hazard values across Australia have decreased relative to the earthquake hazard factors the AS1170.4–2007, in most localities significantly. The key reasons for the decrease in seismic hazard factors are due to: the reduction in the rates of moderate-to-large earthquakes through revision of earthquake magnitudes; the increase in b-values through the conversion of local magnitudes to moment magnitudes, particularly in eastern Australia, and; the use of modern ground-motion attenuation models. Whilst the seismic hazard is generally lower than in the present standard, we observe that the relative proportion of the Australian landmass exceeding given PGA thresholds is consistent with other national hazard models for stable continental regions.

An updated National Seismic Hazard Assessment of Australia was released in 2018 (the NSHA18). This assessment leveraged off advances in earthquake-hazard science in Australia and analogue tectonic regions to offer many improvements over its predecessors. The outcomes of the assessment represent a significant shift in the way national-scale seismic hazard is modelled in Australia, and so challenged long-held notions of seismic hazard amongst the Australian seismological and earthquake engineering community. The NSHA18 is one of the most complex national-scale seismic hazard assessments conducted to date, comprising 19 independent seismic source models (contributed by Geoscience Australia and third-party contributors) with three tectonic region types, each represented by at least six ground motion models each. The NSHA18 applied a classical probabilistic seismic hazard analysis (PSHA) using a weighted logic tree approach, where the model weights were determined through two structured expert elicitation workshops. The response from the participants of these workshops was overwhelmingly positive and the participants appreciated the opportunity to contribute towards the model’s development. Since the model’s publication, Geoscience Australia has been able to reflect on the choices made both through the expert elicitation process and through decisions made by the NSHA18 team. The consequences of those choices on the production of the final seismic hazard model may not have been fully appreciated prior to embarking on the development of the NSHA18, nor during the expert elicitation workshops. The development of the NSHA18 revealed several philosophical challenges in terms of characterising seismic hazard in regions of low seismicity such as Australia. Chief among these are: 1) the inclusion of neotectonic faults, whose rupture characteristics are underexplored and poorly understood; 2) processes for the adjustment and conversion of historical earthquake magnitudes to be consistently expressed in terms of moment magnitude; 3) the relative weighting of different seismic-source classes (i.e., background, regional, smoothed seismicity, etc) for different regions of interest and exceedance probabilities; 4) the assignment of Gutenberg-Richter b-values for most seismic source models based on b-values determined from broad neotectonic domains, and; 5) the characterisation and assignment of ground-motion models used for different tectonic regimes. This paper discusses lessons learned through the development of the NSHA18, identifies successes in the expert elicitation and modelling processes, and explores some of the abovementioned challenges that could be reviewed for future editions of the model. Abstract presented at the 17th World Conference on Earthquake Engineering (17WCEE )

Modern probabilistic seismic hazard assessments rely on earthquake catalogs consistently expressed in terms of moment magnitude, MW. However, MW is still not commonly calculated for small local events by many national networks. The preferred magnitude type calculated for local earthquakes by Australia’s National Earthquake Alerts Centre is local magnitude, ML. For use in seismic hazard forecasts, magnitude conversion equations are often applied to convert ML to MW. Unless these conversions are time-dependent, they commonly assume that ML estimation has been consistent for the observation period. While Australian-specific local magnitude algorithms were developed from the late 1980s and early 1990s, regional, state and university networks did not universally adopt these algorithms, with some authorities continuing to use Californian magnitude algorithms. Californian algorithms are now well-known to overestimate earthquake magnitudes for Australia. Consequently, the national catalogue contains a melange of contributing authorities with their own methods of magnitude estimation. The challenge for the 2018 National Seismic Hazard Assessment of Australia was to develop a catalog of earthquakes with consistent local magnitudes, which could then be converted to MW. A method was developed that corrects magnitudes using the difference between the original (inappropriate) magnitude formula and the Australian-specific corrections at a distance determined by the nearest recording station likely to have recorded the earthquake. These corrections have roughly halved the rates of ML 4.5 earthquakes in the Australian catalogue. To address ongoing challenges for catalog improvement, Geoscience Australia is digitising printed and hand-written observations preserved on earthquake data sheets. Once complete, this information will provide a valuable resource that will allow for further interrogation of pre-digital data and enable refinement of historical catalogs. Presented at the 2019 Seismological Society of America Conference, Seattle in the special session on “Seismology BC(d)E: Seismology Before the Current (digital) Era”