## 1 Introduction  
Recent strong earthquakes worldwide (e.g., Michoac \(\mathfrak{a}\), Loma Prieta, Kobe, Izmit) have provided clear evidence that the damage that results at a site is not merely a function of the energy released from the earthquake source. In fact, the level of damage and devastation in urbanized area might follow a very complex pattern also related to a phenomenon called'site effect' that is due to those variations of geological and geotechnical conditions at shallow depth (_i.e._, essentially the shear-wave velocities of soft-sediments and of the bedrock) that significantly affect the seismic shaking at the surface. For this reason, knowledge of the local near-surface shear wave (S-wave) velocity profile is critical for estimating the damage and loss potential patterns from future earthquakes, as it plays the main role in effects such as ground-motion amplification, landslides or liquefaction. The evaluation of site-effects is therefore one of the key components for mitigating the effects of earthquake disasters. Hence, to mitigate the risk associated with the recurrence of earthquakes, a number of procedures suitable for mapping the mechanical properties of the ground (_i.e._, the S-wave velocity profile) near the surface (_i.e._, typically between the first thirty and one hundred meters) and to account for site effects in the levels of ground shaking expected have been provided. The application of these procedures is defined seismic microzonation. However, the mitigation of seismic risk in urban area requires the estimation of the S-wave velocity profile, and thus of the earthquake ground motion amplification, over large areas. This can be accomplished only if methods suitable for the particular urban environment are developed and applied. Conventional seismic methods (reflection, refraction, cross-hole, down-hole, _etc._) require artificial sources or the drilling of boreholes, which are both expensive, effective for restricted investigation depth only (a few tens of meters), and difficult or impossible to implement in urban or environmentally sensitive areas. For this reasons, in the last decades the analysis of the very small amplitudes Earth's surface vibrations (defined'seismic-noise' or'microtremors', and having displacement generally included in the range 10\({}^{-4}\) 10\({}^{-2}\) mm) produced by natural or anthropic sources, and that can be recorded with good lateral coverage and at reasonable costs, captured the interest of the geophysicist community. In particular, since the pioneering work of [1], two-dimensional (2D) seismic arrays have been used at small scales (_i.e._, maximum aperture of the array is of the order of tens to hundreds of meters) for the characterization of surface-wave propagation, and the extraction of information about the shallow subsoil structure (_i.e._, the estimation of the local S-wave velocity profile). Over the last few years, due to the focus of seismologists and engineers on estimating the amplification of earthquake ground motion as a function of local geology, and the improvements in the quality and computing power of instrumentation, the analysis of seismic noise recorded by 2D arrays has been confirmed to be particularly successful in deriving the subsoil S-wave structure (e.g., [2-6]). Using just a few minutes of seismic noise recordings and combining this with the well-know horizontal-to-vertical spectral ratio (H/V) method, it has also been shown that it is possible to investigate the average one-dimensional (1D) velocity structure below an array of stations in urban areas with a sufficient resolution to depths of also few hundreds of meters that would be prohibitive with active source array surveys, and while also reducing the number of boreholes required to be drilled for site-effect analysis.