Probabilistic seismic hazard studies typically address the epistemic uncertainty in ground motion estimation by using a weighted set of alternative ground motion models. This approach relies on the assumption that the alternative models are developed independently, and the resulting range of model predictions adequately captures the epistemic uncertainty in ground motion estimation. The development of the NGA-West2 ground motion prediction equations (GMPEs) is a collaborative effort with many interactions and exchange of ideas among the developers. Despite the fact that the NGA-West2 models have different functional forms and use different subsets of the available empirical data, the high degree of interaction indicates that the models are not independent. The NGA developers all agree that an additional epistemic uncertainty needs to be incorporated into the median ground motion estimation from these models.
In this report, we present an approach for estimating the minimum epistemic uncertainty in the median NGA-West2 GMPEs based on model-to-model differences and the uncertainty in the median predictions of each GMPE. Results of the model-to-model and within-model uncertainty for the 5 NGA-West2 GMPEs are presented. A model is proposed to incorporate a minimum epistemic uncertainty in the median of individual NGA-West2 GMPEs.
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