Bayesian analysis of ground motion models using chimney fragility curves: 2021, 5.9-Mw Woods Point intraplate earthquake, Victoria, Australia

Abstract

The 22 September 2021 (AEST) MW 5.9 Woods Point earthquake occurred in an intraplate setting (Victoria, southeastern Australia) approximately 130 km ENE of the central business district of Melbourne (pop. ~5.15 million). A lack of seismic instrumentation and low population density in the epicentral region resulted in a dearth of near-source instrumental and “felt” report intensity data. To evaluate the relative performance of ground motion models (GMMs) used in seismic hazard analysis for the region, we first surveyed unreinforced masonry chimneys following the earthquake to establish damage states and develop fragility functions. Using Bayesian inference and including pre-earthquake GMM rankings as Bayesian priors, we evaluate the relative performance of GMMs in predicting chimney observations for different fragility functions and seismic velocity profiles. GMM relative performance in the near-field of the Woods Point earthquake is generally consistent with pre-earthquake expert elicitation derived GMM rankings, although individual GMM weightings vary significantly. Consideration could be given to refining the weightings of GMMs in future national seismic hazard models for Australia. GMMs used within the NSHA18 for southeast Australia outperform non-NSHA18 GMMs with Allen (2012), Atkinson and Boore (2006), and Chiou and Youngs (2008) the highest ranking NSHA18 GMMs at a Vs30 of 1100 m/s.