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

In this short article, the possible reduction in the standard deviation of empirical ground motion estimation equations through the modelling of the effect 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 crustalmodels of the Pyrenees, the western Alps and the upperRhine Graben. Through the method of equivalent hypocentral distance introduced by Douglas et al. [(2004) Bull Earthquake Eng 2(1): 75–99] to model the effect of crustal structure in empirical equations, the scatter associated with such 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, impractical. The relative importance of each parameter in affecting the decay of ground motions is ass