GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 15 for the most of the stations indicating that the NE-SW set represents potential failure surfaces for future rockfalls. Moreover, the weakening of the rocky slope, along with the enlargement of the fractures, is likely enhanced by water circulation dissolving the limestone and by the presence of the caves. Therefore, seismic shaking, which in turn can drive slope instability, can be considered a cause for the rock fracturing superimposed on tectonic stress. Seismically induced brittle deformation on rock, usually found close to active fault zones worldwide (Montenat et al. , 2008), is documented locally at Vendicari (10 km south from the study site) (Pirrotta and Barbano, 2011). The Mt. Gisini landslide (Figs. 1 and 3) occurred to the west of Avola Vecchia. It was a planar translational rockslide, with a steep slip surface, involving a volume of 2 x 106 m 3 of sub-horizontally bedded Tortonian and Messinian calcarenite and marly limestone (Gringeri et al. , 2002). The accumulation, about 250 m wide and 370 m long, is around 60 m high above the valley bottom. Despite the high degree of fracturing, the rock stratification remains sub- horizontal. The landslide produced the obstruction of the Miranda river causing a dam belongs to type II (spanning the entire valley) according to the classification of landslide dams by Costa and Schuster (1988) (Nicoletti and Parise, 2002). Nowadays the landslide body is re-incised by the Piscitello stream that creates a V valley on it. The erosion processes on the downstream face have triggered a secondary landslide on the downstream side of the same landslide body. The Mt. Gisini landslide seems to have reached the position of minimum potential energy and, therefore, the probability of a further failure seems low (Nicoletti and Parise, 2002). Discussion . The Avola Vecchia area shows several effects of active tectonics and earthquake shaking. We have observed brittle deformational structures and seismically induced features such as the Mt. Gisini landslide. Fig. 3 – The Mt. Gisini landslide that dammed the River Miranda on 11 January 1693, and destroyed three mills killing people.