Geoscience Reference
In-Depth Information
The second objective is not so focused on an immediate application and could be stated
astheunderstandingofthecharacteristicsofgroundmotion.Itisclearthatthisobjective
may be out of the question for many engineering projects, yet this is the more satisfying
objective, at least from the scientific point of view. One could translate understanding a
particularsituationofsiteamplificationastheorderingofthefactorsthatcontributetothe
site response. Take for example Mexico City, a paradigmatic case for site amplification.
Our current understanding of site effects there is that the first and most important factor
intheobservedresponseistheimpedancecontrastbetweenathinlayerofextremelysoft
clay and its volcanic substratum. This factor alone allows reproducing the amplification
of ground motion and its geographical distribution. However, this factor is insufficient
to explain site response because it is incomplete. Models based on 1D approximation
fail to reproduce the observed long duration of strong ground motion in this city. The
reason for this failure is that the long duration is the result of a very long excitation
wavefield, produced by the distance between Mexico City and the epicentral zone and
the diffraction of the incident wavefield by the geological heterogeneities that plague the
crustal structure in central Mexico. We note that, in the case of Mexico City, site effects
are not to be understood if we do not consider the whole path between the subduction
zone and the city (more than 300 km, see, e.g., Furumura and Kennett, 1998; Ch´avez-
Garc´ıaandSalazar,2002).Thisisasoberingreminderthatsiteeffectsarenotnecessarily
related exclusively to the site and that it is not always valid to separate source and path
effects from them. The classification of the contributing factors in order of importance is
fundamentalherebecauseatpresentwearenotabletomodeleverypossiblecontributing
factor. We must choose what to model and where to direct the available resources. Site
effects continue tobe avery exciting and useful field of research.
Acknowledgements
The comments by D. Raptakis greatly improved the manuscript. This research
was supported by Consejo Nacional de Cienciay Tecnologia, Mexico, through the con-
tract SEP-2003-C02-43880/A. Additional support was received from Coordinacion de la
Investigacion Cientifica and Instituto de Ingenierva, Universidad Nacional Autonoma de
M´exico.
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