Toward physics-based PSHA study in northern Italy: 3D velocity model validation and broadband seismic signals synthesis.

Physics-based approaches are increasingly recognized as essential for improving seismic hazard assessment, however, no fully physics-based probabilistic seismic hazard analysis (PSHA) exists for the Italian territory. This gap is particularly relevant in the Po Plain area in northern italy, where deep sedimentary deposits strongly amplify seismic waves and prolong shaking, even for moderate-magnitude events. In this context, broadband ground-motion simulations represent a key requirement for capturing both long-period basin effects and high-frequency scattering. In this study, we generate synthetic seismograms spanning the engineering-relevant 0.1–10 Hz bandwidth using a hybrid approach that combines deterministic low-frequency (<1 Hz) simulations with stochastically generated high-frequency (1–10 Hz) ground motion. The low-frequency component (<1 Hz) is computed using the SPECFEM3D Cartesian code, which implements the spectral element method to solve the full seismic wave equation in complex 3D media. A central goal of this work is the validation of the 3D MAMBo velocity model (Molinari et al., 2015). We test the model using several earthquakes and compare its performance against alternative candidate 1D and 3D velocity models, highlighting the critical role of a detailed 3D representation of basin geometry and major velocity discontinuities. The synthetic seismograms are quantitatively evaluated using time–frequency misfit and goodness-of-fit metrics. Our results show that the 3D characterization significantly improves the agreement with observed waveform shapes and durations, and they provide a foundation for future refinement of the regional velocity model. The resulting broadband synthetics are suitable for seismic-hazard analysis and engineering applications in the densely populated and economically important Po Plain. Overall, this study outlines a pathway toward fully physics-based probabilistic seismic hazard analysis (PSHA) in northern Italy, grounded on validated 3D structure and physics-based broadband ground-motion simulations.