The Catania earthquake of February 4, 1169

The 1169 Sicilian earthquake is one of the oldest shocks of great magnitude, for which it was possible to gather some reliable information. It took place in the southeastern part of the island, whose seismicity is characterized by strong energy releases occurring after long quiet periods. The study of this earthquake can add useful information to the knowledge of the seismicity of this important area. Further investigations for data concerning older shocks encounter enormous difficulties due to lack of original descriptions because of missing documents or destruction of original texts. The study of the 4.2.1169 earthquake, presents great difficulties as well. The majority of contemporary authors did not give detailed descriptions of villages and respective damages due to the shock with scientific purposes. Rather, they were impressed by the extent of the phenomenon which was considered due wrath of God because of human evil. In some cases the information found comes from letters written by local notables to squires and nobles, in which they talk of the phenomenon together with other news and personal considerations. Another problem that occurred often in this study was the difficulty of locating villages indicated by various authors. In many cases this problem was solved with further research in order to determine the present name of the place otherwise, a rough location of the area has been used. Bibliographic research was done in libreries and archives of Catania and Siracusa, collecting descriptions of both contemporary and subsequent authors. This earthquake seems to have struck all eastern Sicily and part of southern Calabria with considerable damage. The limits of the area in which it was felt cannot be obtained from the poor descriptions available, but it seems comparable to those of 1693 earthquake (19). There is a lack of information regarding foreshocks and aftershocks, and contradictory information about the time the shock took place. It seems very likely that the earthquake occurred after sunset as mentioned by (1) and (3), which is in accordance with the fact that the majority of people killed in Catania were inside the cathedral. All the most important villages and cities of Val di Noto, Piana di Catania, and Val Demone were heavily damaged. Catania and Lentini were completely destroyed, as was Modica. Serious damage occurred to a number of castles and villages between Catania and Piazza Armerina and to some castles close to Caltagirone. Messina suffered from a tzunami and a tzunami wave went up the Simeto river destroying the village of Casal Simeto, which, since then, has ceased to exist. Several springs dried out and many new ones appeared, but no details about their locations are given; only for Fonte Aretusa and Tavi (close to Siracusa and Assoro, respectively), are there good descriptions. An eruption took place on Etna on the same date of the shock and there are descriptions of collapse of a great part of the top of the volcano (3) (20), but no useful details are given in order to distinguish if this event is linked to eruptive activity or to an effect of the earthquake. Slight damage also occurred in Reggio Calabria, but descriptions are very poor. A detailed report of descriptions of damage with indications of places where original texts can be found is presented separately. Considering the quality of data collected, it is extremely difficult to interpret them in terms of intensity in order to obtain observation points for drawing isoseismals. In Tab. 1, a list of localities with a summary description of damage is reported. It seems more suitable, considering the poor quality of data obtained, to asses relative degrees of seriousness of effects according to the following empirical scale: A = Total destruction. B = Damage to villages, permanent phenomena on ground and effects on undergound water. C = Felt with slight damage. In order to deal with more familiar values, a rouah estimation of described effects in terms of intensity by M.S.K. scale will be attempted. This attempt must be considered only as indicative. Thus, the degrees XI, X and VIII can be very roughly compared with class A, B and C respectively. The tentavive isoseismal map obtained is shown. The difficulties above described did not allow the drawing of a good isoseismal map from which reliable conclusions can be obtained regarding the main source characteristics of the shock. Nevertheless, from the map obtained, some considerations can be drawn: the 1169 earthquake, similar to 1693 one, has an epicentral area which seems to be located in the sea, close to the coast line between Catania and Siracusa, as the effects of tzunami widely described seem to confirm. The maximum intensity attained a value at least comparable with that of 1693 shock (I - XI M.S.K.). Both earthquakes seem to take origin in the same tectonic structures located in the hyblean foreland and they are evidence of the high seismicity of this area, which undergoes a remarkably high risk, considering its great industrialization and urbanization.