GNGTS 2015 - Atti del 34° Convegno Nazionale

16 GNGTS 2015 S essione 1.1 The occurrence of landslides and ground fracturing during strong earthquakes in eastern Sicily is also illustrated both by the historical accounts, reporting the observation of such phenomena in several places of this region, and by previous field works that studied similar seismically induced structures (Nicoletti and Parise, 2002; Guarnieri et al. , 2009; Pirrotta and Barbano, 2011). The large and inactive landslide occurred near Avola Vecchia is described by historical accounts as related to the 1693 earthquake. Other landslides were surveyed in the same area and in southeastern Sicily. Since this is a seismic region but commonly considered no prone to slide failure, these landslides are considered earthquake-triggered (Nicoletti and Parise, 2002). The landslides testify the occurrence of seismic events with magnitude greater than 5 and intensity greater than IX, that are the thresholds for which these seismo-induced features may develop in a site (e.g. Pirrotta and Barbano, 2011). According to empirical relationship between source parameters and epicentral distance of the site where landslides developed during historical earthquakes in eastern Sicily, the triggering earthquakes occurred at a distance lower than 20 km, considering the threshold parameters, or at a longer distance but with a higher magnitude than the threshold (Pirrotta et al. , 2007). The study of joint sets and grid-lock fracture systems, allows documenting the existence of an extensional tectonic regime, which is still active since fractures affect historical and archaeological man-made structures. The obtained stress orientations are in good agreement with the regional extensional tectonic stress field and support the recent activity of the Avola fault located close to Mt. Aquilone (Fig. 1A). Since the joints can be related and reopened either because of coseismic shaking, or for slope instability, additional surveys and analysis need to validate their connection to the recent stress field. Conclusion. Coeval sources describe with certainty a landslide triggered by the January 11, 1693 Sicilian earthquake near Avola Vecchia. The landslide occurred west of the town, at Mt. Ginisi, along the Miranda river, which was the economic and productive focus of the territory hosting five mills. The landslide destroyed three mills and dammed the river. Near Avola Vecchia, at Mt. Aquilone, we have recognized several fractures in rock masses as well as in anthropogenic caves (the so-called cave houses) and oven-shaped tombs, dating back to the Byzantine period and Sicels’ age, respectively. The mesostructural analysis of the rock masses showed that fractures are mainly classifiable as joints and that some of them are due to the tectonic stress field. The fact that fractures affect man-made structures confirms that in southern Sicily a WNW-ESE extensional regime is still active. Some fractures are due to seismic shaking and slope gravity effects, suggesting that several earthquakes have affected the area. Therefore, with the aim to provide new and useful information on ancient earthquakes, we plan to extend structural and morphometric analyses in other site of the Avola area and to date landslides and fractures. This data can help to better define the seismic framework in Sicily having a strong seismic activity but poorly defined seismogenic sources. Acknowledgements. The research was supported by University of Catania PRA grants. References Argnani A., Armigliato A., Pagnoni G., Zaniboni F., Tinti S., Bonazzi C.; 2012: Active tectonics along the submarine slope of south-eastern Sicily and the source of the 11 January 1693 earthquake and tsunami. Nat. Haz. Earth Syst. Scie., 12, 1–9, doi: 10.5194/nhess-12-1-2012. Azzaro R., Barbano M.S.; 2000: Analysis of seismicity of Southeastern Sicily: a proposed tectonic interpretation. Ann. Geofis. 43, 171–188. Bianca M., Monaco C., Tortorici L., Cernobori L.; 1999: Quaternary normal faulting in southeastern Sicily (Italy): a seismic source for the 1693 large earthquake. Geophysical Journal International, 139 (2), 370-394. Caputo R.; 2005: Stress variability and brittle tectonic structures. Earth-Sci. Rev. 70, 103–127 Caputo R.; 2010: Why joints are more abundant than faults. A conceptual model to estimate their ratio in layered carbonate rocks. J. Struct. Geol., 32, 1257–1270. Costa J.E., Schuster R.L.; 1988: The formation and failure of natural dams. ����� ���� ��� ������ �������������� Geol. Soc. Am. Bull., 100,1054–1068.