Deriving Wavefield Characteristics and Shear-Velocity Profiles from Two-Dimensional Small-Aperture Arrays Analysis of Ambient Vibrations in a Small-Size Alluvial Basin, Colfiorito, Itaky

We analyze the dispersion characteristics of ambient noise vibrations.
For this purpose, two-dimensional (2D) seismic array data were acquired in four
different sites in the Colfiorito plain, an alluvial intramountain basin that exhibits
strong site effects. Assuming seismic noise being mainly composed of surface waves,
we derive one-dimensional (1D) shallow shear-velocity profiles through the inversion
of dispersion curves measured by frequency–wavenumber (f-k) methods. The inverted
shear-wave velocity profiles are consistent with a priori information for those
sites that can be approximated by 1D simple models. In these cases, the use of passive
records of seismic vibrations can be a valuable tool for determining the shallow
velocity profile if a detailed depiction of velocity structure is not required. The theoretical
dispersion curves for Rayleigh and Love waves were compared with the measured
dispersion curves for vertical and horizontal components, respectively. This
allows us to discuss qualitatively the composition of ambient vibrations (outlining a large proportion of Love waves in the noise wave field) and the effects of higher
modes. We also use the single-station method for investigating the origin of the
horizontal-to-vertical (H/V) peak in the plain of Colfiorito in terms of ellipticity of the fundamental Rayleigh mode.