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

The spectral decay parameter k (Anderson & Hough, 1984) has been estimated to assess the high frequency attenuation of thick alluvial deposits which characterize the shallow geological structure of the Po Plain (Northern Italy). Data comes from a microseismic network that samples a uniform lithological condition in the Central Po Plain near Cortemaggiore (Piacenza). The network is composed of three surface and four 100 m deep borehole stations, all equipped with 1 Hz seismometers, with an inter-station distance of about 2 km. Starting from a recordings dataset that comprises earthquakes with ML magnitude between 2.0 and 5.2 and epicentral distance (Re) between 20 and 200 km, seismograms showing a signal to noise ratio greater than 3 were selected for the analysis. A standard procedure (Gentili & Franceschina, 2011; Ktenidou et al., 2014) has been applied to assess k = k0 + kR · Re, giving k0 values of about 70 ms at surface and 55 ms at 100 m depth. Taking into account shear-wave velocity (Vs) measures obtained by cross-hole tests performed in similar lithological conditions not far from our network, we can consider an average Vs of 300 m/s in the shallower 100 m. This value, which can be also inferred from the analysis of spectral holes found in down-hole recordings, allows to estimate an average value of about 20 for the shear-wave quality factor (Qs) of the shallower 100 m alluvial deposits.