Velocity gradients choice affecting seismic site response in deep alluvial basins: Application to the Venetian Plain (Northern Italy)

The average shear-wave velocity of the first 30 metres of subsoil and the depth of the engineering bedrock are considered the key parameters for simplified seismic site response modelling. However, a reliable estimate of the site amplification should consider the entire shear-wave velocity profile from the ground surface down to the engineering bedrock. In deep alluvial basins, a typical geological context where the soil-bedrock interface may lie below the penetration depth of most common prospecting methods, only the shallow velocity profile can be defined in detail, while the deeper structures are commonly extrapolated with linear equations. The choice of a realistic interpolation between the shallow and deep soil still remains an open issue. We compute the 1D seismic site response of two sectors of the Venetian Plain (Northern Italy) characterised by gravelly and sandy deep formations. We model the 1D soil columns using theoretical non-linear gradients proposed in literature for deep alluvial basins. The numerical modelling results, in terms of strong motion parameters, show variations in the seismic site response up to 20%. The effect of the velocity gradients is also evaluated comparing the numerical simulations with real accelerometers recorded by a deep borehole seismometer and a seismic station located at the top of the borehole. These results demonstrate that the selection of the velocity gradient is crucial for seismic site characterisation of deep alluvial basins. In particular, the study suggests which is the most conservative gradient among the ones tested in terms of ground motion hazard estimation.