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We present a methodology for the selection of accelerometric time histories as input for dynamic response analyses over vast areas. The method is primarily intended for seismic microzonation studies and regional probabilistic seismic hazard assessments that account for site effects. It is also suitable for structural response analyses if one would like to use a fixed set of ground motion records for analyzing multiple structures with different (or unknown) periods. The proposed procedure takes advantage of unsupervised machine learning techniques to identify zones (i.e., groups of sites) with homogeneous seismic hazard, for which the same set of earthquake recordings can be reasonably used in the numerical simulations. The procedure consists of three steps: (1) data-driven cluster analysis to identify groups of sites with comparable seismic hazard levels for a specified mean return period (MRP); (2) for each zone, definition of a single, reference uniform hazard spectrum (UHS) corresponding to the MRP of interest; (3) selection of a set of accelerometric recordings that are consistent with the magnitude-distance scenarios contributing to the hazard of each zone, and meet the spectrum-compatibility requirement with respect to the reference UHS. An application of the procedure in the Po Plain (Northern Italy) is described in detail. [ABSTRACT FROM AUTHOR]

In the framework of the S2 DPC-INGV 2014-2015 project (), several field surveys were designed with the aim to investigate the local seismic site response of selected Italian Accelerometric Network strong motion stations. In this study the results obtained at Sorbolo, Lesignano de' Bagni, and Castelfranco Veneto, three sites located in the Po alluvial basin (northern Italy), were presented. In particular, at each site, ambient vibration surveys were performed both in single stations and two dimensional arrays configurations. The ambient vibration arrays were analyzed to obtain at each site the shear-wave velocity (Vs) profile, until depths in correspondence of which the Vs show values higher than 800 m/s (i.e. lower bound for the soil category A, as now defined in the Italian seismic code for building). In this way, data were processed through a joint inversion computed using both the Rayleigh waves dispersion curve and the ellipticity curve deduced from the horizontal-to-vertical spectral ratios calculated from the microtremor single station measurements. In general, the obtained profiles show an increasing depth of the main Vs discontinuity (i.e. the seismic bedrock) from 60 to 320 m, moving from the edge to the central part of the Po alluvial basin. To support the outcomes of the field experiments, the available earthquakes recorded at each station in the last decades were processed in terms of the horizontal-to-vertical spectral ratios. Although in many cases the resonance frequencies are comparable with those obtained from microtremor, a high variability is observable depending on the selected portions of each single earthquake. Finally, residual analysis were carried out comparing the recorded ground motion to predicted data as provided by the actual Italian ground motion prediction equations, calibrated both at national and at regional scale. [ABSTRACT FROM AUTHOR]