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the city within a specified frequency band. Thus, the microzonation efforts for Colima
resulted inthe absence of any zonation at all.
Finally, with a clear bias, we will mention the tentative proposal by Ch´avez-Garc´ıa and
Faccioli (2000) to formulate a way to include 2D site effects in the framework of build-
ingcodes.Thenumberofparametersinvolvedwiththeresponseateachsiteinverylong
alluvial valleys makes this a daunting task. Indeed there does not seem to be an easy
way to correctly include all the effects on seismic motion due to lateral heterogeneities
in simple expressions. However, if one restricts oneself to response spectra, site to site
variations become smaller. This made possible for Ch ´ avez-Garc ´ ıa and Faccioli (2000)
to propose an “aggravation factor”. The hypothesis is that amplification is controlled
by impedance contrast. Lateral irregularities increase this amplification by a variable
amount. If impedance contrast is large, the lateral variations of that additional amplifi-
cation can beneglected and a singlefactor can beused throughout the valley. Thisfactor
would affect (one would hope it would correct) 1D amplification by the effects of lateral
irregularities. Those authors found this factor to be within 1.5 to 2 in terms of 5% damp-
ing response spectra. This factor was found to apply also to the case of Parkway valley
(Ch´avez-Garc´ıa, 2003), notwithstanding the 3D character of its seismic response. This
seems to be a viable alternative that could at least be investigated, while we wait for the
developmentofbetteroptionstotake2Dor3Dsiteeffectsintoaccountinbuildingcodes.
Of course, ifground motion duration is an issue, the “aggravation factor” isnot useful.
5. Concluding remarks
Wehavediscussedtwopossiblepathstoestimatesiteeffects.Thefirstoneisdirectmea-
surement of the local amplification. This path is more direct but offers less flexibility. In
addition, there may be cases where we run the risk of accepting an erroneous estimate
of local amplification because of the difficulties involved in its verification. This risk is
reducedwhenweareabletousemorethanoneindependentestimate.Thesecondpathis
a very indirect one; determine subsoil structure and from there compute expected ampli-
fication.Thissecondpathcompensatesitsshortcomingsandtortuousnesswithitsgreater
flexibility. Of course no single approach is appropriate for all circumstances. An impor-
tant consideration is that, in many cases, developing countries face a large seismic risk.
The scientific challenges related to site effect estimation are the same as anywhere else.
However,theresourcesavailablewithwhichtocopewiththemaremuchsmaller.Thisis
an important consideration tomake when developing techniques.
Indealing withsiteeffects,one must separate twodifferentobjectives. Although thetwo
arerelated,eachprojectmayrequiretheemphasistobeplacedononeofthemasbothare
necessaryandonecannotsubstitutetheother.Thefirstobjectiveisthepredictionoflocal
amplification at a given site, maybe to modify a regional estimate of expected ground
motion. A microzonation map is a clear example. This objective is better served by an
approach relying on direct estimates of the local transfer function or some proxy for it,
suchasregressionsofintensityestimatesortheuseoftheH/Vspectralratiousingeither
earthquake or noise data.
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