SITE RESPONSE FROM AMBIENT NOISE MEASUREMENTS - NEW PERSPECTIVES FROMAN ARRAY STUDY IN CENTRAL ITALY

In this study, we used both spectral and array techniques to investiga te microtremor characteristics in an urban area in Central Italy and t o relate them to the properties of shallow geological structures. We m ade noise measurements for sites located on the various surface geolog ical conditions that can be found in the town of Rieti. Both sediments and bedrock are present at the surface, and the transition zone betwe en them is very narrow and well localized. Using a small area reduced the effects related to the poor knowledge about the distribution of no ise sources; using spectral ratio techniques, a good image of resonanc e frequencies on the alluvial sediments was obtained. Moreover, the us e of an array allowed to describe better the morphology of the bedrock and to study Rayleigh-wave dispersion. Starting from the obtained sur face-wave dispersion curve, we calculated a velocity model producing b oth a theoretical dispersion relationship and resonance frequencies fo r sedimentary layers in good agreement with experimental data. Our wor k suggests that the use of array techniques can be very useful to incr ease the quality of microtremor data, giving new answers to microzonin g problems.