This paper examines the question of which sources of uncertainty in ground motion model most strongly affect the seismic performance of structures in a future earthquake. We measure the sensitivity of seismic demand to ground motion parameters using an information- theoretic approach. We perform this study using a high-rise steel moment-frame building structure. We find that the top two contributors to uncertainty are the Arias intensity and the central frequency of earthquake ground motion. The results of analysis can be useful in the modelling of ground motion in some countries, such as Iran, with lack of recorded strong motion.