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/)

<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>

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”

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