ALBERT

Description: In stable continental regions (SCRs), the process of probabilistic seismic-hazard assessment (PSHA) remains a scientific and technical challenge. In producing a new national hazard model for Australia, we developed several innovative techniques to address these challenges. The Australian seismic catalog is heterogeneous due to the variability between magnitude types and the sparse networks. To reduce the resulting high epistemic uncertainty in the recurrence parameters, a and b , the magnitudes of pre-1990 earthquakes have been empirically corrected to account for changes in magnitude formulas around 1990. In addition, existing methods for estimating recurrence parameters (e.g., maximum likelihood estimation) were found to be unstable. To overcome this problem, a new method was developed that removes outlier earthquakes before applying a regression. The incorporation of a model of episodic seismicity into the new hazard model required deviation from the more conventional method of PSHA. The selection of the maximum earthquake magnitude M max is based on the analysis of surface ruptures from paleoearthquakes, with M max thought to vary between geological domains (e.g., 7.2–7.6 in nonextended SCR and 7.4–7.8 in extended SCR). The sensitivity of PSHA to M max , source zone boundary location, recurrence parameters, and ground-motion prediction equations (GMPEs) was examined in this study. The hazard was found to be generally insensitive to M max in the estimated preferred magnitude range. The uncertainty in recurrence parameters was found to contribute a variation in hazard comparable to the epistemic uncertainty associated with the different GMPEs used in this study. For sites near source zone boundaries, a similar variation in hazard was observed by reasonable changes in the position of the boundaries. Aleatory variability and epistemic uncertainty in GMPEs are routinely incorporated in PSHAs, as is variation in M max . However, the uncertainties in recurrence parameters and source zone boundaries are generally given less attention.

Description: On 23 March 2012, at 09:25 UTC, an M w  5.4 earthquake occurred in the eastern Musgrave Ranges of north-central South Australia, near the community of Ernabella (Pukatja). Several small communities in this remote part of central Australia reported the tremor, but there were no reports of injury or significant damage. This was the largest earthquake recorded on mainland Australia in the past 15 years and resulted in the formation of a 1.6 km long surface deformation zone that included reverse-fault scarps with a maximum vertical displacement of more than 0.5 m, extensive ground cracking, and numerous rock falls. The earthquake occurred in nonextended stable continental region (SCR) cratonic crust, more than 1900 km from the nearest plate boundary. Surface deformation from the Ernabella earthquake provides additional constraint on relations of surface-rupture length to earthquake magnitude. Such relations aid in interpreting Australia’s rich record of prehistoric seismicity and contribute to improved estimates of SCR seismic hazard worldwide. Based upon an analysis of new and reinterpretation of existing surface-rupture length data, faults in nonextended stable cratonic Australia appear to produce longer surface ruptures (for earthquakes larger than M w ~6.5) than rupture lengths estimated using existing moment-to-rupture length scaling relations. The implication is that the estimated maximum, or characteristic, magnitude of paleoearthquakes in such settings may be overestimated where the estimate is based only on the length of the prehistoric fault scarp.