Local site amplification maps for the volcanic area of Trecastagni, south-eastern Sicily (Italy)

On 26th December 2018, an earthquake (Mw = 4.9) hits the eastern flank of the Etna volcano (Italy). It was the strongest seismic event among seventy with Mw > 2.5 occurring from 23rd December 2018. After the 2018 seismic sequence, seismic microzonation activities have been conducted for several municipalities located in the Etna area. The paper presents the results of seismic microzonation studies for the municipality of Trecastagni damaged by the 2018 seismic sequence. One important aspect in seismic microzonation studies is the definition of subsoil models derived from geological and geotechnical investigations. In this work, an intense investigation campaign has been carried out to define the subsoil models to be used in local seismic response analyses. A peculiarity of the area is constituted by the presence of shear wave velocity inversions typical of volcanic contexts. Another key aspect in seismic microzonation studies is the selection of the most suitable input motions matching on average a reference spectrum in a defined range of periods. In this study, the acceleration response spectrum prescribed by the Italian seismic code for outcropping rock condition and 475 years return period has been assumed as reference. The results of the site response numerical analyses are presented in terms of elastic acceleration response spectra, amplification functions and amplification factors of spectral acceleration defined according to national guidelines and standards for microzonation studies. Findings have been synthetized into three maps representing the amplification factors computed within three ranges of periods. In order to establish an absolute ranking of seismic hazard, a new methodology has been applied by a synthetic damage-constrained parameter (called H) whose classification is based on the correlation with the European Macroseismic intensity Scale. Finally, a damage grade map has been developed for a seismically homogeneous microzone based on the macroseismic intensities and the vulnerability classes of the buildings encountered in the area. This study highlights the importance of the H parameter in developing emergency planning and mitigation policies.