The influence of velocity gradients choice in deep alluvial basin seismic site response

The characterization of seismic site response represents one of the most important issues of seismic hazard assessment and risk mitigation planning. Characterizing the site conditions involves the measurement of several soil properties such as the shear-wave velocity (Vs), density and damping properties as a function of depth. Therefore, most of the site-effect studies in earthquake ground motions are based on the properties of the upper 30 meters and the anti-seismic building codes propose in most cases a simplified analysis based on shear wave velocity of the shallow subsoil. From a seismological perspective, the upper 30 meters would almost never represent more than 1% of the distance from the source. This should be taken into account especially for large and deep alluvial basins, representing the most inhabited geological environment of the world, where could be difficult to estimate the thickness and the velocity profile of the soft sediment overlying the rigid seismic bedrock.

The common approach adopted to characterize greater depths is then an extrapolation of shear wave velocity in depth, considering a selected linear or non-linear velocity gradients till the depth of the considered seismic bedrock (usually set to Vs ≥ 800 m/s). These gradients are generally derived from geological information or from literature, but how much the gradients choice affects the final site response analyses is often a neglected aspect.

In this work we try to investigate the generic case of deep alluvial basins. We consider the shallow subsoil as characterized by several in-situ tests in northern Italy. We extrapolate the deeper soil structure considering different literature velocity gradients obtained for deep basins in different geological contests: tectonic basins (Lower Rhine Basin and Po Plain) and Alpine basins (Grenoble and Lucerna Basins). We perform one-dimensional analysis of shear waves with the Linear Equivalent Method. The study demonstrates how relevant can be the role of velocity gradient choice for the ground response scenario. Starting from the same shallower Vs structures, the computed seismic motion at surface can present variation in the order of 50% varying the velocity gradients in depth. The results are of relevant interest for the analysis of seismic hazard in the deep alluvial basins environments, which host the main urban areas around the world.