On the regional dependence of earthquake response spectra

It is common practice to use ground-motion models, often developed by regression on recorded accelerograms, to predict the expected earthquake response spectra at sites of interest. An important consideration when selecting these models is the possible dependence of ground motions on geographical region, i.e., are median ground motions in the (target) region of interest for a given magnitude and distance the same as those in the (host) region where a ground-motion model is from, and are the aleatoric variabilities of ground motions also similar? These questions can be particularly difficult to tackle in many regions of the world where little observed strong-motion data is available since there are few records to validate the choice of model. Reasons for regionally dependent ground motions are discussed and possible regional dependence of earthquake response spectra is examined using published ground-motion models, observed accelerograms and also by using ground motions predicted by published stochastic models. It is concluded that although some regions seem to show considerable differences in spectra it is currently more defensible to use well-constrained models, possibly based on data from other regions, rather than use predicted motions from local, often poorly-constrained, models.