The uppermost crust structure of Ischia (southern Italy) from ambient noise Rayleigh waves

Ambient noise measurements were performed at the island of Ischia (southern Italy) along alignments of 2.4– 7 km by using two three-component seismic stations. Synchronous noise recordings of 2–20 h were crosscorrelated over 20–30 s windows, stacked and iteratively band-pass filtered to enhance the dispersive wave trains. Frequency time analysis was performed on the vertical and radial components of cross-correlations and the fundamental-mode Rayleigh wave group velocity was obtained. Validation of the dispersion data was possible with those obtained from an earthquake recording along a close path. The non-linear inversion of average Rayleigh wave group velocity dispersion curves along 13 paths (receiver inter-distances) allowed the definition of shear wave velocity models in the uppermost 1–2 km of the crust. The correlation of VS profiles vs. depth and drilling stratigraphy allowed to attribute VS lower than 1 km/s to tuffs and VS of 1.41km/s to very fractured lavas. Higher VS are found in the central area of the island, in correspondence of the resurgent area. The top of the trachytic lava basement, with VS of 2.2–2.4 km/s and density of 2.3 g/cm3 is about 0.6–0.7 km deep b.s.l. in the centre of Ischia, below altered, very fractured lava or thermally altered tuff.