Use of different approaches to estimate seismic hazard: the study cases of Catania and Siracusa, Italy

The identification of the causative faults in south-eastern Sicily (Italy) is a complicated task although the seismicity of the area has been well studied for years. For this reason a seismic hazard assessment was carried out for the Catania and Siracusa towns providing a comprehensive re-examination and re-processing of all the available seismic data. The site approach and the seismotectonic one were used and compared. The hazard assessment, using both methods, was performed following a logic-tree approach in order to consider and reduce the epistemic uncertainties. Data available in the literature, coming out from recent studies oncerning the parametric earthquake catalogue and a complete database of site intensity in Italy were used. For the seismotectonic approach, two source zone models were considered for southeastern Sicily. In the first model, the same source zone defined for the current Italian reference seismic hazard map is used; in the second model, two separate zones are considered, one includes the normal-fault structures located in the eastern part of the Hyblean area, and the other roughly corresponding to the western portion of the Hyblean front. The combined use of these approaches allowed us to obtain useful elements to define the seismic hazard in Catania and Siracusa. When the seismic history site is used, the town of Catania shows hazard values higher than the ones found for Siracusa, for each considered time interval. On the contrary, when the seismotectonic method is used, the hazard curves show a different behaviour according to the different geometry and size of the adopted source zones. The comparison between the results obtained through the two approaches is recommended since it allows us to verify the robustness of the hazard estimates performed.