FREQUENCY-DEPENDENCE OF SITE AMPLIFICATION FACTOR IN THE MESSINA STRAIT AREA

The spectral ratio method is used here for evaluating the frequency de pendence of a site amplification factor in the Messina Strait area. Al l stations exhibit the same features of spectral ratios evidencing two peaks at about 10 Hz and 16 Hz. We relate this observation to the sam e geological structures. In fact, all the stations (except the referen ced one) are situated over pleistocenic sediments with a similar grain size. This causes the disappearing of any site effect when the averag e spectrum is used as a reference. MES station, situated in the city o f Messina, presents a clear site effect at a different frequency (6 Hz ) which cannot be related to any geological structure because no subst antial difference is observed between the sediments on which this stat ion is situated and the sediments on which the other stations are situ ated. MT1 station exhibits a spectral peak at about 2 Hz which can be explained with oscillations of a little sedimentary basin.