Geoscience Reference
In-Depth Information
CHAPTER 3
SITE EFFECTS: FROM OBSERVATION AND MODELLING
TO ACCOUNTING FOR THEM IN BUILDING CODES
Francisco J.Ch´avez-Garc´ıa
Coordinaci on de Ingenierıa Sismol ogica, Instituto de Ingenierıa, Universidad Nacional
Aut onoma de Mexico
paco@pumas.ii.unam.mx
Abstract. Site effects are defined as the modifications that seismic motion undergoes when tra-
velling through the topmost layers before hitting the ground surface or a structure built on it. The
definition of topmost layers is frequency dependent. This paper compiles results regarding the
estimationofsiteeffectsandtheirusetodecreaseseismicrisk.Siteeffectsmaybeestimatedeither
by direct measurements or indirectly, meaning that subsoil mechanical properties and geometry
are evaluated and from them an estimate of local amplification is computed. The first approach
has the advantage of its directness. It has been shown that this approach is most effective when
local amplification is significant and brought about by a simple configuration (e.g., one soft layer
overlying a more rigid basement). However, this first approach lacks flexibility and when it fails
the invested effort is practically lost. The second approach, although more tortuous, offers larger
flexibility. It is very likely that efforts directed to the estimation of the subsoil structure, even if
not completely successful, will allow to build some model and to estimate an order of magnitude
of the amplification. Current methods used to estimate site effects based on earthquake or ambient
noise data are discussed. In addition, the main techniques that are used to estimate the subsoil
structurearereviewed,inparticular,thenewproposalsregardingtheuseofthecorrelationofnoise
measurementstodeterminethepropertiesofthesubsoillayers.Finally,somecommentsareoffered
on the current approaches available to take into account site effects in the framework of building
codes.
1. Introduction
Local amplification caused by surficial soft soils is a significant factor in destructive
earthquake motion. It is frequent that site effects condition the occurrence of damages
in moderate to large earthquakes. For this reason, site effects have received much atten-
tionduringthelastdecades.Moreover,specialthemeconferenceshavebeendedicatedto
thissubjectinitsmanyfacets(e.g.,duringAugust2006,theESGconferencespecialised
onsiteeffectswasheldatGrenoble,France).Obviously,asinglearticlecannotdojustice
tosuch an ample subject, and thispaper willdeal only withsome particular aspects.
The use of the term surficial soft soils is purposely vague. Very early it was recog-
nised that what we name local must be frequency dependent. Moreover, its definition
depends strongly on the medium. If the thickness of the soil layers is small, it is likely
that their effects will appear at higher frequencies. Conversely, we observe amplification
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