Civil Engineering Reference
In-Depth Information
The main challenge in the fi eld of engineering seismology is to fi nd an
appropriate balance between scientifi c rigour and engineering pragmatism
and this necessarily means that simplifying assumptions and approxima-
tions must be made. The primary purpose of the present chapter is to outline
the key features of uncertainty that are associated with ground-motion
models as used in seismic hazard and seismic risk analyses today. A second-
ary objective is to explore some of the shortcomings that are currently
associated with these models, as well as to consider some of the recent
advancements that have been made in the fi eld of ground-motion modelling
that look to try to overcome some of these issues.
In order to achieve these objectives, the chapter starts by explaining
ground-motion equations in terms of their role in seismic hazard and risk
applications, the different types of ground-motion models that are encoun-
tered in practice, and how these models are derived. With this context
having been set, the latter parts of the chapter focus upon the consideration
of uncertainty in ground-motion models, with a particular emphasis on how
epistemic uncertainty infl uences empirical ground-motion models.
2.2
Explanation of ground-motion prediction
equations (GMPEs)
Ground-motion prediction equations (GMPEs) are frequently also referred
to as attenuation relationships, empirical ground-motion models, or just
ground-motion models. Although various parties argue that the use of one
of these terms is more appropriate than others, for practical purposes all of
these terms can be taken to mean the same thing. While the use of the term
ground-motion prediction equations
has become more common in recent
years, the use of the word prediction has implications that are not helpful
for enabling people to understand their true purpose. This point will be
discussed in the following section.
Regardless of how we refer to it, a GMPE is essentially a function that
takes a number of input parameters that relate to the properties of an
earthquake and its spatial relationship with a site, and provides two outputs
that together defi ne the distribution of ground-motion values that is to be
expected for the considered scenario.
Before discussing this further it is instructive to fi rst consider what we
mean by 'ground-motion'. An earthquake manifests via the radiation of
waves that are related to the release of elastic strain energy previously
stored in the vicinity of the earthquake source. These waves result in time-
dependent variations in the position of points on the Earth's surface and
earthquake-induced ground-motions can therefore be described in terms
of the current position of a given point at a given time, or equivalently by
the motion of such a point in terms of its velocity or acceleration. In order
to understand the characteristics of these motions, we deploy accelero-
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