Vs measurements in volcanic urban areas from ambient noise Rayleigh waves.

An original approach for VS measurements in highly urbanized areas is proposed that is based on FTAN and Hedgehog methods applied to cross-correlation of synchronous ambient seismic noise recordings at two sites. In fact, theoretical research has shown and experimental studies have confirmed that the cross- correlation of noise recordings gives a reliable estimate of the Green’s function (surface wave packet). We show examples of VS measurements performed in the urban areas of Napoli and Ischia island, characterized by the presence of pyroclastic soils and rocks, chaotically distributed. As the investigated thickness depends on the receiver distance, we deal with VS models extending to depths from some hundred of metres to some kilometres. The obtained VS models reach depths that are prohibitive for controlled source experiments and allow the definition of the seismic basement, a key information for reliable microzonation studies.