Seismic hazard assessment in the Catania and Siracusa urban areas (Italy) through different approaches

Catania and Siracusa are located in the eastern coast of Sicily (southern Italy). The high level of seismicity that affects the area, together with the considerably high density of inhabitants, contribute to classify this towns among the ones having the highest hazard in Italy. The aim of this study is to carry out a detailed probabilistic seismic hazard analysis (SHA) for the two towns using two probabilistic approaches. The “site” method (Albarello and Mucciarelli, 2002) and the “seismotectonic” methodology (Cornell, 1968; Esteva, 1967) were compared taking also into account different seismogenic sources. Response spectra in acceleration, assuming rock-site conditions, were computed at both towns and a two dimensional disaggregation in magnitude and distance was performed as well in order to identify the design earthquake that characterize the local seismic hazard. The outcomes obtained by either the site or the seismotectonic method are only partially comparable. The results set indeed into evidence that both the choice of the approach and the assumption of a suitable source model play an important role. It can also be observed that the site approach assign to Catania higher hazard than in Siracusa for all exceedance probabilities. Such finding can be interpreted as the consequence of the higher damage historically suffered by this town and its average shorter distance from seismogenic sources of major historical earthquakes. Furthermore, PGA values obtained through the site method are always greater than those computed by the Esteva-Cornell one. Such difference could be a consequence of the PGA-Intensity converting law which correlate the macroseismic intensity, evaluated into a relatively wide area, with acceleration values that are assessed at a more local scale.