Geoscience Reference
In-Depth Information
depth are the same everywhere. Amplification is caused by impedance contrast, where
the properties of two media play an important role: those of the sediments and those of
the bedrock. If we take into account only the properties of one medium disregarding the
other, inevitably there will be sites where the actual amplification will be very different
from that predicted.
Whenweareabletoindicateinamapthosezoneswhereweexpectratherhomogeneous
site effects, and characterise site effects for each one of them in simple terms, we can
produce a microzonation map. Microzonation has also been the subject of specialised
international conferences, and once again this paper cannot do justice to this subject.
A microzonation map is a tool that can have an immediate impact in risk reduction and
can be directly incorporated into building codes. Ideally, this map will evolve as more
information becomes available. In the case of Mexico City, the first microzonation map
was included in the building code of this city published in 1959. Since then, it has been
revised several times. Currently, the code includes a “nanozonation” (a term introduced
by M. Ordaz, per. comm.) of Mexico City, where the dominant period at any site within
the city is given. The design spectrum imposed by the code at each site is constructed
using five parameters, all of which are derived from the dominant period value. This
has been possible thanks to two factors. The first is that local amplification is strongly
governed by the very large impedance contrast between a soft surficial layer and its sub-
stratumwehavealreadymentioned.Agoodestimateofthisamplificationcanbeobtained
assumingahomogeneousflatlayer(whosethicknessisassumedtochangewiththeloca-
tion of the site of interest) overlying a rock basement. The second is the large number
of strong motion stations that were installed after the disastrous 1985 earthquake. When
the records are analysed using spectral ratios, and the resulting transfer functions inter-
preted in terms of a 1D model, it becomes possible to interpolate site effects and predict
the response everywhere. This imposes, however, that we ignore the very large scatter
observed in spectral ratios. The stability of the average that is obtained is very good, as
long as we are interested in response spectra. The reason is the extremely large amplifi-
cation due to a single subsoil interface, so large that it thwarts the significance of other
factors.However,the1Dmodelhasstronglimitations,evenatMexicoCity,asshownby
Ch´avez-Garc´ıaandBard(1994).Wearefacedthenbytheparadoxthatamodelthatisnot
physicallycorrectisveryuseful,whilethemorephysicallycorrectmodel(Ch
´
avez-Garc
´
ıa
and Salazar, 2002) isnot yet useful inpractical applications.
Appealingasthe“nanozonation”is,thisapproachiscurrentlynotfeasibleatmostcities.
Inaddition,thisapproachcannotbeenvisagedwhenmorethanonefactorcontributessig-
nificantly to site amplification. An extreme example is provided, once again, by the city
of Colima. We mentioned this city with regard to the failure of ambient noise measure-
ments to estimate site effects. In contrast, the analysis of dispersion using microtremor
data allowed to build 1D models from which site amplification was computed, which
was in good agreement with previous observations. The results, however, could not dif-
ferentiate zones within the city with different site response characteristics. In the case of
Colima,theconclusionwasthatitwasnotpossibletodrawamicrozonationmapforthis
city and that the better solution was to consider homogeneous amplification throughout
