GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 11 ANALYZING SEISMOINDUCED EFFECTS AND FRAGILE DEFORMATION IN THE AVOLA VECCHIA AREA (SOUTHERN SICILY): IMPLICATION FOR ACTIVE TECTONICS AND SEISMICITY M.S. Barbano, C. Pirrotta, G. De Guidi, C. Farina Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Catania, Italy Introduction. Although in active areas tectonics is considered the most probable trigger mechanism of fragile deformation of the rocky masses, seismic shock is another highly probable cause (Caputo, 2005; Montenat et al. , 2008). Indeed, strong earthquakes can trigger several soil deformation phenomena, such as liquefaction, ground fracturing and landslides, which can often cause more damage than the seismic shaking itself. Ground fracturing, which is among the most diffuse seismo-induced effects, can also contribute to increase terrain instability. Rocks and sediments can record such effects as evidence of paleoearthquakes. The study of these markers of seismicity can be a useful tool to obtain data on ancient earthquakes occurring in a region. Indeed, although the recognition of off-fault seismo-induced structures does not provide precise and direct information on seismogenic fault and earthquake parameters (magnitude, intensity, fault length and elapsed time), it can give important information on the epicentral distance of the site, earthquake magnitude threshold and intensity reached at the site. Moreover, the finding of features dated before historical records, can be useful to extend the seismic catalogues back in time, to assess recurrence time of earthquakes and so to better characterize the seismicity of an area. Eastern Sicily is considered among the most seismically active area in Italy. Several strong earthquakes with Maw 6.0–7.4 (Rovida et al. , 2011) have occurred in the last millennium and numerous Quaternary faults modified the landform. However, the lack of instrumental data for strong historical earthquakes, and poor exposures of faulted Quaternary sediments do not allow the unambiguous identification of active, seismogenic faults. An extensional belt running, for ≈ 370 km, from eastern Sicily to South Calabria, the Siculo Calabrian Rift Zone (SCRZ), is considered responsible of the crustal seismicity of these areas (Catalano et al. , 2008). This belt is composed by faults up to 50 km long and some of these are considered seismogenic only on few evidence of geological and structural studies and macroseismic data (Bianca et al. , 1999; Catalano et al. , 2008). Seismicity of south-eastern Sicily is tentatively related to some tectonic structures of the northern and western Hyblean Plateau. The 1169 and the 1693 earthquakes were located off- shore and associated with Malta Escarpment fault system since the strong tsunamis triggered by these events and the lack of surface faulting evidence on-shore (e.g. Azzaro and Barbano, 2000; Monaco and Tortorici, 2000; Argnani et al. , 2012). The Avola Fault segment has been considered responsible of the 9th January 1693 foreshock only on the basis of the damage area that extend along a narrow belt, on land, along the eastern edge of the Hyblean Plateau (Bianca et al. , 1999). The 1169, 1542, 1693, 1848 and 1990 south-eastern Sicily earthquakes caused damage, numerous fatalities and triggered several ground failures, as reported by historical sources. Geological evidence of liquefactions, correlated to some of the strongest earthquakes, were found in the Holocene deposits of the Mascali area and in the Catania Plain, both characterized by a continental fluviatile sedimentation environment (Guarnieri et al. , 2009). Moreover, Pirrotta and Barbano (2011) reported seismically induced deformation structures along the rocky coast of Vendicari (southeastern Sicily). Traces of seismically induced features can be particularly significant to assess earthquake recurrence time in areas that, like eastern Sicily, have poorly defined seismogenic sources. In this study we analyse a seismically induced landslide along with some fractures affecting archaeological structures in the ancient site of Avola Vecchia, southwest of Syracuse (Fig. 1), and correlate them with historical and prehistorical events. Avola Vecchia is located in the eastern coastal sector of the Hyblean Plateau, which is the emerged part of a gently deformed segment of the African continental margin representing