Development of new Australian magnitude conversion equations and its effects on seismic hazard analyses

All modern ground motion prediction equations (GMPEs) are now calibrated to the moment magnitude scale MW, it is therefore 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 Australia’s low-to-moderate level of seismicity, coupled with the relatively sparse seismic recording networks. 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. The reference catalogue served as the basis for the development of magnitude conversion equations between MW and ML. The conversions are developed using general orthogonal regression. Different functional forms for the conversion equations were considered and their impact on seismic hazard is explored. Synthetic earthquake catalogues with a “known” ­b-value are generated about an arbitrary location. These catalogues are subsequently perturbed according to different magnitude adjustment assumptions. It is found that the results of seismic hazard analyses at our site are sensitive to the implementation algorithm of such equations. For the considered scenario, the results show a 20-40% reduction in PGA hazard (at the 10% in 50-year probability of exceedance level), depending on the selection of the functional form as well as the method for applying the magnitude conversion equations. Presented at the 2018 Seismological Society of America (SSA) Annual Meeting