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.

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.

Geoscience Australia, together with contributors from the wider Australian seismology community, have produced a draft National Seismic Hazard Assessment (NSHA18), recommended for inclusion in the 2018 update of Standards Australia’s Structural design actions, part 4: Earthquake actions in Australia, AS1170.4–2007 (Standards Australia, 2007). This Standard is prepared by Subcommittee BD-006-11, General Design Requirements and Loading on Structures of Standards Australia. The provisional seismic hazard values presented in this report have been submitted to comply with Standards Australia’s public comment and publication timelines. This report provides a brief overview of provisional mean peak ground acceleration values (equivalent to the seismic hazard factor Z in AS1170.4) and the approaches used. The hazard values are calculated on rock sites (AS1170.4 Site Class Be) for a probability of exceedance of 10% in 50 years (0.0021 per annum). Continued refinement of these values will occur throughout, and in response to, the first public comment period. While only minor changes are expected, the final NSHA18 will be completed prior to Standard Australia’s planned second public comment period (likely in late 2017). The NSHA18 update yields many important advances on its predecessors, including: • calculation in a full probabilistic framework (e.g., Cornell, 1968) using the Global Earthquake Model Foundation’s OpenQuake-engine (Pagani et al., 2014); • consistent expression of earthquake magnitudes in terms of moment magnitude, MW; • inclusion of epistemic uncertainty through the use of third-party source models contributed by the Australian seismology community; • inclusion of epistemic uncertainty on magnitude-frequency distributions; • inclusion of a national fault-source model based on the Australian Neotectonic Features database (Clark et al., 2012; Clark et al., 2016); • inclusion of epistemic uncertainty on fault-slip-model magnitude-frequency distributions and earthquake clustering; and • use of modern ground-motion models.

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 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 )

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.

The Philippine archipalego is tectonically complex and seismically hazardous, yet few seismic hazard assessments have provided national coverage. This paper presents an updated probabilistic seismic hazard analysis for the nation. Active shallow crustal seismicity is modeled by faults and gridded point sources accounting for spatially variable occurrence rates. Subduction interfaces are modelled with faults of complex geometry. Intraslab seismicity is modeled by ruptures filling the slab volume. Source geometries and earthquake rates are derived from seismicity catalogs, geophysical datasets, and historic-to-paleoseismic constraints on fault slip rates. The ground motion characterization includes models designed for global use, with partial constraint by residual analysis. Shallow crustal faulting near metropolitan Manila, Davao, and Cebu dominates shaking hazard. In a few places, peak ground acceleration with 10% probability of exceedance in 50 years on rock reaches 1.0 g. The results of this study may assist in calculating the design base shear in the National Structural Code of the Philippines.

Since the publication of the Global Seismic Hazard Assessment Project (GSHAP) hazard map in 1999, Australia has stood out as a region of high earthquake hazard among its stable continental region (SCR) peers. The hazard map underpinning the GSHAP traces its lineage back to the 1990 assessment of Gaull and others. This map was modified through a process of expert judgement in response to significant Australian earthquakes (notably the MW 6.2, 6.3 and 6.6 1988 Tennant Creek sequence and the deadly 1989 MW 5.4 Newcastle earthquake). The modified map, developed in 1991 (McCue and others, 1993), underpins Standards Australia’s structural design actions to this day (AS1170.4–2007). But does this assessment make sense with our current understanding of earthquake processes in SCRs? Geoscience Australia (GA) have embarked to update the seismic hazard model for Australia through the National Seismic Hazard Assessment (NSHA18) project. Members of the Australian seismological community were solicited to contribute alternative seismic source models for consideration as inputs to the updated Australian NSHA18. This process not only allowed for the consideration of epistemic uncertainty in the hazard model in a more comprehensive and transparent manner, but also provides the community as a whole ownership of the final model. The 3rd party source models were assessed through an expert elicitation process that weighed the opinion of each expert based on their knowledge and ability to judge relevant uncertainties. In total, 19 independent seismic source models (including regional and background area sources, smoothed seismicity and seismotectonic sources) were considered in the complete source model. To ensure a scientifically rigorous, transparent and quality product, GA also established a Scientific Advisory Panel to provide valuable and ongoing feedback during the development of the NSHA18. The NSHA18 update yields many important advances on its predecessors, including: calculation in a full probabilistic framework using the OpenQuake-engine; consistent expression of earthquake magnitudes in terms of MW; inclusion of epistemic uncertainty through the use of third-party source models; inclusion of a national fault-source model based on the Australian Neotectonic Features database; inclusion of epistemic uncertainty on fault occurrence models and earthquake clustering; and the use of modern ground-motion models. The preliminary NSHA18 design values are significantly lower than those in the current (1991-era) AS1170.4–2007 map at the 10% in 50-year probability level. However, draft values at lower probabilities (i.e., 2% in 50-years) are entirely consistent (in terms of the percentage land mass exceeding different PGA thresholds) with other SCRs with low strain rates (e.g. the central & eastern United States). The large reduction in seismic hazard at the 10% in 50-year probability level has led to much consternation amongst the building code committee in terms of whether the new draft design values will allow enough resilience to seismic loads. This process underscores the challenges in developing national-scale PSHAs in slowly deforming regions, where 10% in 50-year probability level may not adequately capture the maximum considered earthquake ground motions. Consequently, a robust discussion is required is amongst the Australian building code committee (including hazard practitioners) to determine alternative hazard and/or risk objectives that could be considered for future standards. Presented at the Probabilistic Seismic Hazard Assessment (PSHA) Workshop 2017, Lenzburg, Switzerland

The Devil’s Mountain fault is an active fault included in the 2014 USGS hazard model for Washington State. Recent neotectonic investigations have suggested that a west-northwestwards extension of the fault (the Leech River fault) has sea-bottom and onshore evidence pointing to recurrent young offsets. Accordingly, a logic tree model for the Leech River – Devil’s Mountain fault system (LRF-DMF) incorporating various fault lengths, slip rates of 0.25 mm/yr with upper and lower alternatives of 0.15 and 0.35 mm/yr, and interactions between the faults was developed and added to Canada’s 6th Generation seismic hazard model. The LRF was given a 50% chance of being active. Although the slip rate is low for an active tectonic region, the fault system passes through greater Victoria, British Columbia, and contributes to the overall seismic hazard for southernmost Vancouver Island. We calculate the hazard in greater Victoria with and without the LRF-DMF in order to estimate its effect. The hazard in downtown Victoria is already high (coming mainly from in-slab sources at short periods and the Cascadia subduction zone at long periods) and decreases slowly northwards. The hazard increment due to the LRF-DMF is quite small, even very close to the fault, and as expected its contribution to the hazard decreases away from the fault so that in Sidney at ~25 km distance it is insignificant. The importance would have been very different in a lower hazard region, or if the slip rate on the LRF-DMF had been considerably higher.