The Etna earthquake of February 20, 1818

Most Etnean shocks are characterized by strong energy released in a very narrow area. The 20.2.1818 earthquake, on the countrary, is one of the few shocks that is characterized by the spreading of effects in quite a large area. In fact, this earthquake was felt in a large part of Sicily and in southern Calabria. The seismic event took place at the beginning of the 19th century, a period in which systematic studies on the volcano and its geodynamic phenomena had not started yet. Despite the extent of the phenomenon, only a few bibliographic references having scientific characteristic have been found, though it seems likely that further information might be obtained by looking for contemporary cronicles and reports in historical archives. Nevertheless, the information gathered allows a good characterization of the macroseismic field of the shock, using mostly the accurate descriptions given by an author contemporary to the event (3). The earthquake occurred at 18.20 (G.M.T.) and ruined several villages of the southeastern flank of Mt. Etna. Aci Consolazione and Aci Platani were nearly completely destroyed, while Aci S. Antonio, Acicatena, S. Giovanni Galermo, Mascalucia and Nico-losi were heavily damaged. The shock was also strongly felt in Catania and descriptions of falling objects and rotation of monuments and crosses are often reported. The more or less disastrous effects were distributed throughout the Etna area, with two maxima of intensity in Maletto and Piedimonte, where heavy damage and some house collapse occurred. Because of the great number of houses collapsed in many villages, there were 72 victims in total. The maximum was in Zafferana (3), due to the fact that they were grouped inside a church where the roof collapsed. A great number of phenomenon in the ground and in underground water were observed in the mesoseismal area. In particular, deep fractures, some metres long, were found in Aci Catena, S. Giovanni la Punta, Viagrande and Pozzillo. In a spring close to Acicatena, changes were observed in its regime, and the same thing happened in the salted thermal spring close to Paterno. Moreover, spouts of salted water and slight oscillations of the sea level were observed close to the mouth of the Simeto river. Both the above mentioned phenomena in the ground and the amount of damages described indicate intensity IX as the maximum intensity in the mesoseismal area. The isoseismal map (see figure) demonstrates that the earthquake was more or less felt in almost all Sicily. From Siracuse and Noto to Palermo, people panicked and clocks stopped. The area where the shock was felt reached southern Calabria and Malta as well (1). Several slight aftershocks took place until 2 March of the same year. In particular, the shocks of 21 and 28 February, were felt in the surrounding of Acireale. The focal depth, calculated using macroseismic methods, was about 5 km. The mesoseismal area of this earthquake is very irregular and rather wide (44.25 km2) in comparison to that of the majority of Etnean shocks. Considering the extent of the seismic event, it seems possible that a complex mechanism at the source, with the activation of more than one fault line, was responsible for this shock. Comparison between lobes of the complex mesoseismal area with the orientation of fractures and faults, obtained from field surveys, shows agreement and thus offers some support to this hypothesis.