Capturing the center, the body and the range of ground-motion predictions is an indispensable
element of site-specific probabilistic seismic hazard analyses (PSHA), for which the logic tree is
the ubiquitous tool in current practice. The criteria for selecting the ground-motion models (GMMs)
used in such studies have generally been focused on their potential applicability to the region and
site for which the PSHA is being conducted. However, except for applications within the few
regions with abundant ground-motion databases, it will rarely be the case that GMMs can be
identified which are perfectly calibrated to the characteristics of the target study region in terms
of source and path properties. A good match between the generic site amplification model within
the GMM and the site-specific dynamic response characteristics is equally, if not more, unlikely.
Consequently, adjustments are likely to be made to the selected GMMs to render them more
applicable to the target region and site. Empirical adjustments for host-to-target region source
differences using local recordings are unlikely to be robust unless these have been generated by
earthquakes from a wide range of magnitudes. Empirical adjustments for site characteristics are
impossible unless there are recordings from the target site. Therefore, the preferred approach
makes parametric adjustments to empirical GMMs, isolating each host-to-target difference to map
the individual contributions to the epistemic uncertainty. For such an approach to be applied, the
emphasis moves from selecting GMMs on the basis of their applicability to focusing on their
amenability to being adjusted to the target region and site. An adaptable equation is characterized
by well constrained host-region source, path and site characteristics and a functional form in
which response spectral accelerations scale with source, path and site characteristics in a manner
similar to the scaling implicit in stochastic simulations based on Fourier amplitude spectra.