New Insights into Long-Period (>1 S) Seismic Amplification Effects in Deep Sedimentary Basins: A Case of the Po Plain Basin of Northern Italy

This study investigates and quantifies the influence of the shallower deposits (down to few hundreds of meters) of the Po Plain sedimentary basin (northern Italy) on the long-period component (i.e., 1 s < T < 3 s) of seismic ground motion, in which amplification effects due to the soft sediments above seismic bedrock were observed. A new seismostratigraphic model of the shallow deposits of the entire basin is provided with an unprecedented detail by taking advantage of recently acquired geophysical data. The seismostratigraphic model is used to simulate the ground motion amplification in the Po Plain by means of extensive 1D ground response analysis. Results are compared with seismic observations available at a number of sites equipped with borehole seismic stations, where earthquakes have been recorded both at the surface and at the seismic bedrock depth. Despite the general agreement with observations concerning the seismic resonance frequencies, our model may fail in capturing the amplitude of the actual seismic amplification of the basin in the long-period range. We observe that 3D basin effects related to surface waves generated at the edge of the basin may play a significant role in those zones where seismic hazard is controlled by distant sources. In these cases, 1D modeling leads to average underestimations of 30%, up to a maximum of 60%. The amplification functions need to be corrected for a basin-effects correction term, which in this case is provided by the ground-motion prediction equation of the study area. The corrected amplification functions agree with the empirical observations, overcoming the uneven distribution of the recording stations in strong-motion datasets. These results should be taken into account in future seismic microzonation studies in the Po Plain area, where the 1D approach is commonly adopted in ground response analyses, and in site-specific seismic hazard assessments aimed at the design of structures that are sensitive to the long-period component of seismic ground motion (e.g., long-span bridges and tall buildings).