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Fig. 3.1. Location of Colimacity, Mexico, and itsregional geology (After
Ch´avez-Garc´ıa et al., 2007.)
effects. Whenever possible, the more adequate results are obtained through the compari-
sonandcomplementationofdifferenttechniques(e.g.,LermoandCh´avez-Garc´ıa,1994;
Ch´avez-Garc´ıa et al., 1999; Horikeet al., 2001).
It is to be expected that this conference will show a few cases where H/V provided good
results. For this reason, this paper would like to illustrate the opposite, the case where
these measurements were not very useful, exemplified by the city of Colima in Western
Mexico (Ch´avez-Garc´ıa et al., 2007). Large earthquakes occur in this region, the north-
ernsectionofthePacificcoastsubductionzoneinMexico (Figure3.1)buttheseismicity
level is much smaller than further South along the subduction zone making it difficult to
recordearthquakes.Previousstudies(Guti´errezetal.,1996)hadshownthatlocalamplifi-
cation(measuredusingearthquakespectralratios)issignificantinthiscity;aboutafactor
6distributedinawidefrequencyband.Colimaisunderlainbyacolumnofvolcanicsedi-
mentsthatattains800mthickness,fillingaNorth-Southvalleysurroundedbylimestone.
These volcanic sediments are characterised by their large irregularity, with a mixture of
differentkindsofavalanche,debrisandlahardeposits.Figure3.2showsthesurfacegeol-
ogy in the city. Different volcanic formations have been identified by geologists, but it
is far from evident that those differences are related to changes in the mechanical prop-
erties that condition seismic response. As mentioned already, previous studies indicated
significant amplification, but were unable toanalyse itsdistributionwithinthe city.
