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

<div>An earthquake catalogue based on the moment magnitude scale (<em>M</em>W) is a prerequisite for global best practice seismic hazard analyses. The 2018 National Seismic Hazard Assessment (NSHA18) was the first national-scale seismic hazard assessment for Australia to apply magnitude conversions to express earthquake magnitudes uniformly in terms of <em>M</em>W. This approach led to the single-biggest change in seismic hazard estimates between Geoscience Australia-led national seismic-hazard models. Between the 2012 and 2018 assessments, the hazard reduced because of: 1) the general reduction in the number of earthquakes of magnitude 4.0 and larger due to the correction of local magnitudes (<em>M</em>L) and subsequent conversion to <em>M</em>W, and; 2) the increase in the Gutenberg-Richter <em>b</em>-value due to the non-linear conversion of local magnitudes <em>M</em>L to <em>M</em>W.</div><div>Using a new continental-scale attenuation model, independent assessment of <em>M</em>W has been performed for over 300 earthquakes recorded between 1990 and September 2024. After recalculating <em>M</em>L for the same earthquakes using improved filtering and time-domain windowing criteria, the <em>M</em>W catalogue is used to test and validate the <em>M</em>L to <em>M</em>W conversion equations used in the 2023 National Seismic Hazard Assessment (NSHA23). The earthquakes are partitioned into their regional magnitude polygons as applied by Geoscience Australia in its real-time operations; notionally central and western Australia, South Australia (Mt Lofty and Flinders Ranges) and eastern Australia. The performance NSHA23 <em>M</em>L to <em>M</em>W conversion equation is then assessed for each of these magnitude regions. Overall, the NSHA23 <em>M</em>L to <em>M</em>W conversion performs very well relative to continental-scale earthquake dataset. The sensitivity of this conversion to an earthquake’s static stress drop is also assessed. There is evidence that minor adjustments could be applied to the NSHA23 <em>M</em>L–<em>M</em>W conversion equation for larger-magnitude events with high stress drops.</div><div><br></div> Presented at the Australian Earthquake Engineering Society (AEES) National Conference 2024