GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 13 coast (Fig. 1), following the geometrical and regular plan designed by the architect friar Angelo Italia (Gringeri Pantano, 1996). Among the numerous historical sources, reporting the effects of the 1693 earthquakes in Avola Vecchia, we selected the two witness contemporary and reliable accounts by Dell’Arte (1699) and Di Maria (1745), who described in detail the effects of earthquakes on the city and on the territory. “In Avola Vecchia, the January 9 at four-thirty (Italian time, used in the XVII century in Sicily) in the night (~21 GMT), a strong earthquake destroyed almost the whole quarters known as di Sopra and Marchi, ruining houses since foundations with the loss of 500 citizens. 40 hours after the first shock, on January 11 at 20 hours and a quarter (~13 GMT), the earthquake was so proud and terrible that destroyed the entire city. No stone remained upon stone, including caves, and people was not able to distinguish one house from the other houses” (Dell’Arte, 1699). The earthquake “has unhinged stones above which Avola was built; then destroyed throughout the whole city” (Di Maria, 1745). Five hundred people died for this shock. In Avola, the whole fatalities (9 and 11 January shocks) were 1,000 out of 6,225, in minor percentage than other Sicilian cities, because most of the inhabitants, were outdoors, having felt another slight shock at 16 hours (~9 GMT). The fortified castle, located on the acropolis of Mt Aquilone, was destroyed, although it had been rebuilt a first time after the December 10, 1542 earthquake, “which had ruined the castle and many houses” (Gallo, 1966). “The Mount called Gisini split, and almost half Mount, breaking away with fury, sank in the bed of the Valley called Carnevale, remaining under the portentous mass three mills with many people inside them” (Di Maria, 1745). The Mt Gisini landslide is further documented by an archive plan concerning the construction of a canal system to bypass the occlusion and provide water to the plantations and factories (Gringeri Pantano, 1996). Data analysis . Aerial-photos interpretation, field survey and mesostructural analysis near Avola Vecchia allowed us to observe some devastating effects of the 1693 earthquakes still evident on the surface. Mechanical discontinuities with decimetric spacing affect the calcarenites, marly limestones and limestones of Miocene age, outcropping at Mt Aquilone. Numerous of these fractures affect both the cave-tombs of the Sicels’ necropolis and the Byzantine-medieval cave houses (Figs. 2A and 2B). Since they can give an evidence of active tectonics, we performed systematic and mesostructural analyses (stereographic projections and rose diagrams) of 137 fractures disturbing 19 caves of Mt Aquilone (Fig. 1B). These fractures are up to several metres long, often opened from few millimetres to several centimetres (Figs. 2B and 2C) and sometimes filled by re-crystallized calcite deposit. They show no, or few, evidence of shear motion, being originated as purely extensional fractures. This type of brittle features is known as “extensional joints” and they have been recognized as one of the most common deformational structures in every tectonic environment (Caputo, 2010 and references therein). The joints are grouped in two orthogonal sets with main directions N45 and N140 (Fig. 2D). According to Caputo (2005), two orthogonal joint sets are due to stress swapmechanisms between σ 2 and σ 3 stress axes that occur locally causing stress field deformation. However, the prevalence of fractures with direction NE-SW indicates that the local tectonic maximum extension is almost NW-SE oriented (Fig. 2E), which is compatible with the regional stress field. This stress field is also responsible of the Avola fault (AF in Fig. 1A). It is worth noting that the study site is located on the footwall of the Avola Fault, which according to some authors may be active and could be the source of the 9 January 1693 earthquake (Monaco and Tortorici, 2000). Nevertheless, other mechanisms such as seismic shacking can have triggered the fractures. Indeed historical accounts describe damage in the cave houses and “unhinged stones” during the 1693 earthquakes. We have also compared the trend of the NE-SW fractures, with that of the slope at the location of each mesostructural station. A similitude between the two directions was observed