The importance of crustal structure in explaining the observed uncertainties in ground motion estimation

Abstract : In this paper, the possible reduction in standard deviation of empirical ground motion estimation equations through the incorporation of crustal structure is assessed through the use of ground-motion simulations. Simulations are computed for different source-to-site distances, focal depths, focal mechanisms and for crustal models of the Pyrenees, the western Alps and the upper Rhine Graben. Through the use of the method of equivalent hypocentral distance introduced by Douglas et al. (2004) to model the effect of crustal structure in empirical equations the scatter associated with ground motion estimation equations derived using these simulated data could be reduced to zero if real-to-equivalent hypocentral distance mapping functions were derived for every combination of mechanism, depth and crustal structure present in the simulated dataset. This is, obviously, unrealistic for a practical use of the method. The relative importance of each parameter in affecting the decay of ground motions is assessed here. It is found that variation in focal depth is generally more important than the effect of crustal structure when deriving the real-to-equivalent hypocentral distance mapping functions.