In this article, a novel strategy to generate broadband earthquake ground motions from the results of 3D physics‐based numerical simulations (PBSs) is presented. Physics‐based simulated ground motions embody a rigorous seismic‐wave propagation model (i.e., including source, path, and site effects), which is however reliable only in the long‐period range (typically above 0.75–1 s), owing to the limitations posed both by computational constraints and by insufficient knowledge of the medium at short wavelengths. To cope with these limitations, the proposed approach makes use of Artificial Neural Networks (ANNs), trained on a set of strong‐motion records, to predict the response spectral ordinates at short periods. The essence of the procedure is, first, to use the trained ANN to estimate the short‐period response spectral ordinates using as input the long‐period ones obtained by the PBS, and, then, to enrich the PBS time histories at short periods by scaling iteratively their Fourier spectrum, with no phase change, until their response spectrum matches the ANN target spectrum. After several validation checks of the accuracy of the ANN predictions, the case study of the 29 May 2012 $Mw$ 6.0 Po Plain earthquake is illustrated as a comprehensive example of application of the proposed procedure. The capability of the proposed approach to reproduce in a realistic way the engineering features of earthquake ground motion, including the peak values and their spatial correlation structure, is successfully proven.

In this paper, we present SEM3D: a 3D high-fidelity numerical earthquake simulator that is tailored to predict the seismic wave field of complex earthquake scenarios from the fault to the epicenter site. SEM3D solves the wave-propagation problem by means of the spectral element method (SEM). The presented demonstrative test case was a blind $M_W$6.0 earthquake scenario at the European experimental site located in the sedimentary basin of Argostoli on the island of Kefalonia (Western Greece). A well-constrained geological model, obtained via geophysical inversion studies, and seismological model, given the large database of seismic traces recorded by the newly installed ARGONET network, of the site were considered. The domain of interest covered a region of 44 km × 44 km × 63 km, with the smallest grid size of 130 m × 130 m × 35 m. This allowed us to simulate the ground shaking in its entirety, from the seismic source to the epicenter site within a 0–10 Hz frequency band. Owing to the pseudo-spectral nature of the numerical method and given the high polynomial order (i.e., degree nine), the model featured 1.35·10${}^{10}$ DOFs (degrees of freedom). The variability of the synthetic wave field generated within the basin is assessed herein, exploring different random realizations of the mean velocity structure and heterogeneous rupture path.

Realistic physics-based 3D earthquake simulation for source-to-structure wave propagation consists of a powerful numerical tool for seismic response prediction of critical structures submitted to high safety standards. Structural response considering soil–structure interaction (SSI) is usually estimated by Finite Element Method (FEM) approach, as it is considered as the most flexible numerical approach for nonlinear structural dynamics. However, current engineering practice considers seismic input motion as vertically incident plane waves, despite the fact that this assumption excludes wave passage effects for large infrastructures and surface waves appearing from possible local basin effects. In this framework, a realistic input excitation needs to be defined as an input excitation of the FEM model, accounting for (i) a realistic dynamic excitation, (ii) wave propagation path in the regional scale, and (iii) local site-effects. The Domain Reduction Method (DRM), which allows for the imposition of a 3D complex incident wave field as an input to the SSI model, is adapted and examined here in a Spectral Element Method (SEM) – FEM weak coupling approach. The weak coupling is verified at first for a canonical case-study and for an increasing complexity of the dynamic excitation: (i) double-couple point-source and (ii) extended fault. An optimization approach, based on the decimation of SEM output signal, is then examined in order to decrease the computational burden by maintaining the same accuracy of the final solution. The SEM-FEM weak coupling is then used to study the SSI problem, where the impact of the reduced domain size on structural response is examined at first. The current study shows that a reduced domain of dimension greater than $4\times {\lambda }_s$, where ${\lambda }_s$ is the maximum wavelength, is sufficient for a proper representation of structural response. For a fixed size model, structural and soil responses are then examined for a hypothetical case-study.