Epistemic Uncertainty for NGA‐West2 Models

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. The NGA‐West2 developers indicate that additional epistemic uncertainty needs to be incorporated into the median ground motion estimation from each of the five NGA‐West2 GMPEs in order to more fully represent an appropriate level of epistemic uncertainty. A proposed minimum additional epistemic uncertainty is evaluated based on statistical estimates of the uncertainty in the median predictions of each GMPE. The proposed additional epistemic uncertainty model is distance‐independent but depends on magnitude, style of faulting (SOF), and spectral period. The epistemic uncertainty in the median predictions from each GMPE is then modeled using a three‐point discrete approximation to a normal distribution.