GNGTS 2013 - Atti del 32° Convegno Nazionale

Geology and tectonics of the study area. The study area is formed by a thick crustal plateau belonging to the Africa foreland domain. It flexures northward to form the Gela–Catania foredeep, a subsiding area filled by Quaternary sediments, that separates the Hyblean Plateau from the overthrusting Apenninic–Maghrebian orogenic sectors (see Fig. 1b). Eastward, the Hyblean Plateau is separated from the oceanic-type domain of the Ionian Basin by the Malta– Hyblean Escarpment. At regional scale, the tectonics of south eastern Sicily is characterized by a NE-SW striking normal fault system, located inland (Scicli line), and the Malta-Hyblean escarpment, trending NW-SE, located offshore in the Ionian sea (Fig. 1b). Seismotectonic information and interpretations available suggest that both fault systems can be identified as possible sources for the seismic activity that affected in historical time the town of Siracusa. A significant improvement to the knowledge of the seismotectonic setting in south-eastern Sicily was achieved by the analysis of the December 13 th 1990 earthquake and its aftershocks. This event appear to be located along a 5 km long, transverse segment striking EW separating two sub-parallel segments of the Malta-Hyblean fault scarp, whose presence was suggested by Finetti (1982) using bathymetric data. The focal mechanism of the 1990 event shows a dominant right-lateral strike slip solution (Amato et al ., 1995). In any case, relations between the main offshore fault segments and the historical earthquakes coupled with tsunamis, occurring in this region and high intensity earthquakes, up to XII MCS (e.g. 1169 AD, 1693 AD events), suggests that this tectonic lineament represents one of the major seismogenic sources of the whole southern Italy (Bianca et al ., 1999; Monaco and Tortorici, 2007). In the study area the substratum outline a horst structure formed by a Meso-Cenozoic carbonate sequence with interbedded volcanics (Grasso and Lentini, 1982) cropping out in the northern part of Siracusa (Fig. 1). The Cretaceous volcanics, having thickness up to 500 m, locally represent the deepest term (Tortorici, 2000) which is unconformably covered by sub-horizontal carbonate sequences that stand for the lithotypes more frequently cropping out in the Siracusa town. The above mentioned carbonate sequence is distinguished in two main units, having similar geotechnical features, known in the literature as Mt. Climiti and Mt. Carruba formations. The former, having thickness ranging between 20 and 80 m, lays on the Cretaceous volcanics and consists of compact and well cemented calcarenites, the latter, with an average thickness of about 20 m, is characterized by alternating calcarenites and marlstones. In some sites the carbonate sequence is directly overlaid by sub-horizontal poorly consolidated calcarenites up to 20 m thick (Tortorici, 2000), whereas, in the southern part of the study area, sands and sandy clays, up to 20 m thick, overlay the Meso-Cenozoic carbonate bedrock. Finally, alluvial deposits fill out the graben of the “Pantanelli” plain (see Fig. 1) whilst detritus, having thickness of about 6-8 m due to anthropic activity and historical ruins, is mainly outcropping in the downtown Ortigia area. Methodology. The dynamic properties of an edifice are usually described through its natural frequency and the damping ratio (ζ). The latter parameter represents the energy loss of an oscillating system that can be either internal (material damping) or due to another system (radiated damping). The damping ratio is important in seismic design since it allows to evaluate the ability of a structure to dissipate the vibration energy during an earthquake. Such energy causes a structure to have the highest amplitude of response at its fundamental frequency, which depends on the structure’s mass and stiffness. Therefore, the damping level, as well as the knowledge of the fundamental period ( T ) of the building are particularly important for estimating the seismic base shear force F in designing earthquake resistant structures. Following the Eurocode8 (2003), F can be expressed as: F = Sa ( T ; ζ) · m · λ where Sa ( T ; ζ) is the ordinate of the target spectrum at period T and damping ζ, m is the total mass of the building above the foundation or above the top of a rigid basement and λ is a correction factor. The Sa ( T ; ζ) is evaluated considering the local geological features, which influence the site response in term of amplification of the ground motion, as the seismic input travels from the bedrock to the overlying soil deposits. 105 GNGTS 2013 S essione 2.1