The increasing interest in performance-based earthquake engineering has promoted research on the improvement of hazard-consistent seismic input definition and on advanced criteria for strong motion record selection to perform nonlinear time history analyses. Within the ongoing research activities to improve the representation of seismic actions and to develop tools as a support for engineering practice, this study addresses the selection of displacement-spectrum-compatible real ground motions, with special reference to Italy. This involved (1) the definition of specific target displacement spectra for Italian sites, constrained—both at long and short periods—by results of probabilistic seismic hazard analyses; (2) the compilation of a high-quality strong ground motion database; and (3) the development of a software tool for computer-aided displacement-based record selection. Application examples show that sets of unscaled, or lightly scaled, accelerograms with limited record-to-record spectral variability can also easily be obtained when a broadband spectral compatibility is required.

This work presents a new high performance open-source numerical code, namely SPectral Elements in Elastodynamics with Discontinuous Galerkin, to approach seismic wave propagation analysis in viscoelastic heterogeneous three-dimensional media on both local and regional scale. Based on non-conforming high-order techniques, such as the discontinuous Galerkin spectral approximation, along with efficient and scalable algorithms, the code allows one to deal with a non-uniform polynomial degree distribution as well as a locally varying mesh size. Validation benchmarks are illustrated to check the accuracy, stability, and performance features of the parallel kernel, whereas illustrative examples are discussed to highlight the engineering applications of the method. The proposed method turns out to be particularly useful for a variety of earthquake engineering problems, such as modeling of dynamic soil structure and site-city interaction effects, where accounting for multiscale wave propagation phenomena as well as sharp discontinuities in mechanical properties of the media is crucial.whereas the matrix A associated to the bilinear form A. , / defined in (12) is such that for i, j D 1, : : : , D it holds A`k ij WD A.ˆj ,ˆk i / NI , for k,`D 1, .., 3.Now, we define V WD P U the vector of nodal velocities, we prescribe initial conditions U.0/ D u 0 and V.0/ D u 1 and we consider the system (13). Let us now subdivide the interval .0, T into N subintervals of amplitude t D T =N and set t n D nt , for n D 1, : : : , N .

Stimulated by the recent advances in computational tools for the simulation of seismic wave propagation problems in realistic geologic environments, this paper presents a 3D physics-based numerical study on the prediction of earthquake ground motion in the Po Plain, with reference to the M W 6.0 May 29 2012 earthquake. To respond to the validation objectives aimed at reproducing with a reasonable accuracy some of the most peculiar features of the near-source strong motion records and of the damage distribution, this study required a sequence of investigations, starting from the analysis of a nearly unprecedented set of near-source records, to the calibration of an improved kinematic seismic source model, up to the development of a 3D numerical model of the portion of the Po Plain interested by the earthquake, including the irregular buried morphology, with sediment thickness varying from few tens of m to some km. The spatial resolution of the numerical model is suitable to propagate up to about 1.5 Hz. Numerical simulations were performed using the open-source highperformance code SPEED, based on the Discontinuous Galerkin Spectral Elements (DGSE) method. The 3D numerical model coupled with the updated slip distribution along the rupturing fault proved successful to reproduce with reasonable accuracy, measured through quantitative goodness-offit criteria, the most relevant features of the observed ground motion both at the near-and far-field scales. These include: (i) the large fault normal velocity peaks at the near-source stations driven by up-dip directivity effects; (ii) the small-scale variability at short distance from the source, resulting in the out-of-phase motion at stations separated by only 3 km distance; (iii) the propagation of prominent trains of surface waves, especially in the Northern direction, induced by the irregular buried morphology in the near-source area; (iv) the map of earthquake-induced ground uplift with maximum values of about 10 cm, in substantial agreement with satellite measurements; and (v) the two-lobed pattern of the peak ground velocity map, well correlated with the distribution of macroseismic intensity.

In this paper, a novel strategy to generate broad-band earthquake ground motions from the results of 3D physics-based numerical simulations (PBS) 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 (ANN), 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 M6.0 Po Plain earthquake of May 29, 2012 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 proved.

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This study assesses the spatial correlation of broadband earthquake ground motions from 3D physics-based numerical simulations in near-source conditions. State-of-the-art models for predicting the spatial correlation are derived from wide datasets including densely recorded earthquakes in different areas worldwide and, therefore, they may be poorly representative of specific regions and near-source effects. A large set of broadband ground motions simulated by the SPEED code, and enriched in the high-frequency range with an Artificial Neural Network technique, is used to investigate the sensitivity of crucial parameters in geostatistical analysis (number of receivers), as well as of source, path, and site effects on spatial correlation, with a level of detail which could not be possible otherwise due to the paucity of recordings. First of all, the comparison of our results with those derived from earthquake recordings validates successfully the numerical approach in predicting the spatial correlation in a broad frequency range. Furthermore, the study points out that spatial correlation of response spectral accelerations is significantly affected by the magnitude, forward directivity effects, ground-motion directionality (fault normal versus fault parallel), and relative position from the causative fault. These features may make critical the use of isotropic and stationary models especially in near-fault conditions.

SUMMARYA theoretical approach is presented to study the antiplane seismic response of underground structures, subjected to the incidence of both plane and cylindrical waves. The structure is assumed to be a circular inclusion embedded in a homogenous, isotropic and linear visco-elastic halfspace. The inclusion may consist either of a cavity, with or without a ring-shaped boundary, or it may be filled in with a linear-elastic material, without loss of generality. The analytical solution is obtained using expansions of wave functions in terms of Bessel and Hankel functions, relying on the technique of images and the use of Graf's addition theorem to enforce the boundary conditions.The effects of underground cavities on surface earthquake ground motion are studied as a function of the size of the cavity, its embedment depth, the frequency content of the excitation, the incidence angle and the distance from the axis of symmetry of the cavity itself. A simple application of Rayleigh's method allows us to verify that the ground surface response is dominated by the fundamental vibration mode of the portion of soil between the cavity and ground surface itself, in the frequency range of interest for engineering purposes. A simple relationship to estimate the fundamental natural frequency as a function of the embedment depth of the cavity is given. Finally, amplification factors on response spectra are obtained, to provide a practical insight into the effect of an underground cavity on surface ground motion during real earthquakes.