Site response from ambient noise measurements: New perspectives from an array study in Central Italy

In this study, we used both spectral and array techniques to investigate microtremor characteristics in an urban area in Central Italy and to relate them to the properties of shallow geological structures. We made noise measurements for sites located on the various surface geological conditions that can be found in the town of Rieti. Both sediments and bedrock are present at the surface, and the transition zone between them is very narrow and well localized. Using a small area reduced the effects related to the poor knowledge about the distribution of noise sources; using spectral ratio techniques, a good image of resonance frequencies on the alluvial sediments was obtained. Moreover, the use of an array allowed to describe better the morphology of the bedrock and to study Rayleigh-wave dispersion. Starting from the obtained surface-wave dispersion curve, we calculated a velocity model producing both a theoretical dispersion relationship and resonance frequencies for sedimentary layers in good agreement with experimental data. Our work suggests that the use of array techniques can be very useful to increase the quality of microtremor data, giving new answers to microzoning problems.