Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 218

main characteristics of these ground motion types, taking into account the source
typologies. This target can be obtained by processing the recorded earthquakes, or,
as a new challenge in Earthquake Engineering, by studying the rupture processes
and propagation of seismic waves by numerical modeling.
7.1.2 Uncertainties in Selection of Proper Design Ground Motions
Historically, the codes were developed based on the experience of few recorded
ground motions, not sufficiently close to the causative faults, due to the absence of
a dense network of recording stations. Recently this situation has changed in some
very urbanized seismic zones, with more near-fault records. But, unfortunately,
there is still a large number of seismic zones without adequate information. In
addition, due to the fault network or to the presence of some little knowledge about
active faults different from the known ones, it is practically impossible to design a
recording network station in such a way to get records on the sites with the
maximum ground motions. So, the recorded values have a great incertitude, and
only by chance, some of them are situated near the fault. But these records can be
influenced by some local site conditions, so they are available only for the
recording station. As a consequence, the design spectra are not, generally,
applicable for other seismic sites. In these conditions, it is very difficult to detect
the differences between the ground motions produced by different source types.
Therefore, the design of earthquake-resistant structures is affected by many
uncertainties. One of the most important is related to the selection of the proper
design ground motion. Conceptually, it should be done by considering all possible
ground motions which can occur in a given site and can drive the structure to a
critical response during its service life (Anderson and Bertero, 1987). The
quantification of this concept creates serious difficulties due to the differences
among the characteristics of different earthquake types.
During the past decades, the increasing databases of recorded earthquakes has
indicated that the dynamic characteristics of the ground motion can vary
significantly among recording stations which are located in the same area or in
different areas. For one recorded earthquake at the different stations situated in the
same area, the differences are significant especially for the stations located near the
epicentral region, due to the effective distances from the epicenter and the direction
of rupture. Examining the records at different sites, one can observe that they vary
significantly due to several factors (fault mechanism type, path effects, local site
conditions, etc.). The main differences are observed in recorded accelerations,
velocities and displacements, vertical components, directivity duration, number of
significant pulses and natural period of ground movements. In function of some
conditions, these differences can be determinant or not in the selection of the
proper design ground motions for a structure.
The recorded ground motion is influenced by the effects of some important
factors (for instance the source type and local soil conditions), over which the
effects of some secondary factors are superposed, changing in a certain extent the
original characteristics. This is the reason why the recorded values differ so much