Interpretation of microtremor 2D array data using Rayleigh and Love waves : the case study of Bevagna (central Italy)

In the last decades, geophysicists and seismologists have focused their attention on the inversion of empirical surface-waves' dispersion curves from microtremor measurements for estimating the S-wave velocity structure at a site. This procedure allows a fast and convenient investigation without strong active sources, which are difficult to deploy especially in urban areas. In this study we report on a 2D seismic noise array experiment carried out at Bevagna (central Italy) near the station BVG of the Italian Accelerometric Network (RAN). The site was investigated within the DPC-INGV S4 Project (2007-2009). The Rayleigh- and Love-waves dispersion characteristics were estimated using different methods. The inversion of the dispersion curves was then performed independently, obtaining two estimations for the S-wave velocity profiles. The results of cross-hole logging near the seismic station are used for a comparison. The shear-wave velocity profiles estimated by microtremor analyses range up to 150 m depth. The two independent procedures provide consistent shear-wave velocity profiles for the shallow part of the model (20-30 m in depth) in agreement with the results of the cross-hole logging. Some problems arise between 30-40 m in depth in the profile estimated by surface waves. In this range cross-hole logging evidences an inversion of the S-wave velocity. Although the cross-hole logging stops at 40 m of depth, we are confident about the results provided by the Rayleigh-wave analysis below 40-50 m. This case study suggests that greater efforts should be devoted to exploit the potential of a coupled analysis of Rayleigh and Love waves from microtremor array measurements.