High frequency attenuation of S waves in alluvial deposits of the central Po Plain (northern Italy)

Estimation of local seismic response plays a key role in assessing local seismic hazard and particularly in the design of shaking scenarios. Modelling local seismic response involves knowing of the shear wave velocity (V-S) and quality factor (Q(S)) profiles for the site in question. The many techniques that have been developed to assess V-S in surface deposits produce reliable measurements of V-S, but these rarely correspond to direct measurements of Q(S). The latter is often considered through damping measures from laboratory tests on small-scale soil samples, which can provide information primarily on intrinsic attenuation, neglecting the contribution of scattering effects. In this paper, using seismic recordings obtained at the surface and in boreholes at 100 m depth, we estimate an average value of Q(S) of some characteristic alluvial deposits of the Po Plain (northern Italy). Data come from a microseismic network which sampled an almost uniform lithology in the central Po Plain and consisted of three surface and four borehole stations with an interstation distance of about 2 km. The average value of Q(S) of the shallowest 100 m of the sedimentary strata, Q(S,100), is estimated by considering: (1) the high-frequency attenuation of seismic waves due to propagation through the corresponding stratigraphy and (2) the interference between incident and surface-reflected waves observed at borehole stations. We parametrize the first through Delta k(0100), the difference between the values of the spectral decay parameter kappa (k) estimated at the surface and at the boreholes depth, respectively. We use the second in order to compute V-S,V-100, the time-averaged V-S referred to the uppermost 100 m stratigraphy. We obtain: Delta k(0100) = (11 +/- 3) ms, V-S,V-100 = (309 +/- 11) m s(-1) and Q(S,100) = 31 +/- 10. At the surface, the estimated values of the site-specific kappa, k 0, are found to range from 75 to 79 ms. As expected, these results are in good agreement with studies performed in other sites characterized by sandy or clayey lithologies, and can be usefully used in site response analysis at sites where the rigidity is mainly controlled by lithostatic pressure.