Numerical Modeling of the 13 December 1990 M 5.8 East Sicily Earthquake at the Catania Accelerometric Station

The record of Catania ENEA–ENEL accelerometric station, with its anomalously high amplitude, represents an outlier for the dataset of seismograms recorded by the Italian accelerometric network during the 13 December 1990 M 5.8 East Sicily earthquake. Using numerical modeling tools, we corroborate the interpretation of this anomaly as an effect of the crustal structure or site response, rather than as a finite source effect. In the first part of this study, we compare the recorded seismograms to those computed by two numerical methods—the 2D Spectral Element Method (SPEM), a technique which solves the 2D full-wave propagation through a complex geological structure; and the Wavenumber Integration Method (WIM), which solves the 3D full-wave propagation in a horizontally-layered structure. The comparison shows that the recorded waveforms are reproduced accurately using a laterally heterogeneous structure, whereas none of the representations in terms of horizontal plane layers, which simplify the same structure, provides satisfactory results. Furthermore, we compare the horizontal-to-vertical spectral ratios (HVSR) obtained from (1) the earthquake record, (2) the seismogram simulated by the SPEM, and (3) the seismic noise recorded at the same site. Again, the overall agreement is very good. This study has several outcomes. Firstly, that the numerical modeling approach based on the SPEM, and used in a previous study to simulate ground shaking for a destructive scenario earthquake, provides reliable results. Secondly, that the 2D model used to represent the crustal structure beneath this area is realistic. Indeed, simplified 1D models may not be an adequate means to reproduce realistic seismograms and predict the ground motion in the frequency band of interest for seismic hazard (i.e., 0.5–10 Hz). Finally, the high amplitude displayed by the Catania station during the 1990 earthquake is explained as a combined effect of site and structure-path, while finite source models appear unnecessary. In general, this study emphasizes the importance of methods that accurately model the wavefield propagation through realistic geologic structures for predicting ground motion.