GNGTS 2013 - Atti del 32° Convegno Nazionale

CHARACTERIZATION OF THE SUBSOIL IN THE AREA OF THE FRIARS MINOR CONVENT OF ISPICA (SOUTH-EASTERN SICILY) S. Grassi 1 , S. Imposa 1 , G. Coco 2 , M. Corrao 2 1 Dip. di Scienze Biologiche, Geologiche e Ambientali, Sezione di Scienze della Terra, Università di Catania, Italy 2 Geocheck s.r.l., Aci Sant’Antonio, Catania, Italy Introduction. The main goal of this work has been the characterization of the subsoil beneath the convent of the Friars Minor of Ispica, a village located in south-eastern Sicily. This building, located on top of a calcarenitic cliff, shows evidence of failure and instability. To achieve this target, we have estimated the Vs 30 in three mechanical drillings using the down-hole technique (DHT) and carried out a seismic tomography survey. The present study complements previous geognostic and structural surveys published by Imposa et al. (2010). Considerable advancement has had, recently, the analysis of refraction data for the characterization of rocky terrains in terms of seismic waves velocity and amplitude, for the identification of deep interfaces and the determination of the degree of fracturing. The degree of fracturing in a medium affects the propagation of seismic waves: fractured volumes of rocks show lower P and S waves velocity, dispersion and attenuation, anisotropy of velocity (e.g., Abu-Zeid and Vuillermin, 1996); mapping Vs variations allows to infer variations of mechanical properties throughout the medium. Above the calcareous cliff where the Franciscan convent stands, the morphology is rather flat and of karstic type, interrupted by long and deep stream incisions locally called caves (“cave”). Ispica is in fact located at the farest end of a ~ 12-km long, ~ 100 m deep, stream incision called Cava d’Ispica (Ispica Cave), a renowned archaeological site with caves inhabited some 2,000 years BC and hundreds of graves of Christian age (IV–V century). The study area is characterized by outcrops of alternating calcarenites and marls of the Ragusa Formation, with levels of calcarenites belonging to the Irminio Member (Carbone et al. , 1982; Grasso et al. , 2000). It is located within the Hyblaean calcareous highland that is part of the foreland domain outcropping in southeastern Sicily (Fig. 1). The foreland units deepen northward beneath the Gela nappe that represents the most external front of the chain. Fig. 1 – Geological map of south-eastern Sicily (Lentini et al. , 1987). 46 GNGTS 2013 S essione 3.1