Seismo-stratigraphic model of the Po Plain (Italy)

Abstract

The aim of this study is to provide a seismo-stratigraphic model of the Po Plain sedimentary basin (Northern Italy), to be implemented in soil hazard studies at regional scale. The proposed model characterizes the subsoil up to the seismic bedrock depth. Mascandola et al. (2018) identifies the seismic bedrock of the Po Plain in correspondence with a marked increase in the mechanical properties of the subsoil materials, which produces a measurable resonance effect at the surface in the medium-to-long-period range. To map the seismic bedrock depth we relies on an extensive collection of both existing and newly acquired ambient vibration measurements, with the aim of defining the soil resonance frequencies and the shear-wave velocity gradients within the soft sediments above seismic bedrock. Based on the collected data, an empirical regression model that relates the thickness of the soil deposits above the seismic bedrock to their resonant frequency is defined and applied to map the seismic bedrock depth in the Po Plain area. The resultant seismic bedrock map is correlated with depth of the main unconformities recognized inside the Quaternary succession (Regione Emilia-Romagna,ENI–AGIP, 1998; Regione Lombardia, Eni Divisione Agip,2002). The shear-wave velocity model above seismic bedrock is derived through the interpolation of 51 S-wave velocity profiles selected after a quality check on the available data. The velocity gradients highlights two different zones inside the study area: one at Northwest and another at East-Southeast with higher and lower velocity gradients respectively. To compute the soil amplification functions, the velocity model is discretized into a grid. For each grid node, a 1D soil model is defined and a numerical ground response analysis is carried out. The gridded soil amplification model is checked at those sites with both borehole and surface seismic sensors by comparing the theoretical and empirical soil amplification functions. These results will be included in regional seismic hazard studies, to account for soil amplification in seismic hazard estimates.