A destructive earthquake occurred two days before the mainshock at 21:00 local time, centered in the Val di Noto. It had an estimated magnitude of 6.2 and a maximum perceived intensity of VIII–XI on the Mercalli intensity scale. Intensities of VIII or higher have been estimated for Augusta, Avola Vecchia, Floridia, Melilli, Noto Antica, Catania, Francofonte, Lentini, Scicli, Sortino, and Vizzini.
Augusta lies well outside the main zone of severe shaking; its extensive damage is probably due to its construction on unconsolidated sediments.
From the shape and location of the area of maximum damage, this earthquake is thought to have been caused by movement on the Avola fault.
Some buildings collapsed in Catania, Vizzini, and Sortini. There were an estimated 200 deaths in both Augusta and Noto.
The earthquake lasted for four minutes, according to contemporary accounts. The estimated magnitude of 7.4 is taken from the extent and degree of the recorded damage, with a very large area that reached X (Extreme) or more on the Mercalli scale. The maximum shaking reached XI in the towns of Buscemi, Floridia, Melilli, Occhiola and Sortino.
The source of the January 11 earthquake is debated. Some catalogs give an onshore epicenter without any direct association with a known structure, while others propose that the source was offshore due to the associated tsunami, involving either rupture along a normal fault, part of the Siculo-Calabrian rift zone, or rupture along the subduction zone beneath the Ionian Sea. An analysis of the distribution of tsunami run-ups along the coast suggests that a submarine landslide triggered by the earthquake is the most likely source, in which case the tsunami provides no constraint on the epicenter. A landslide origin is supported by the observation of possible landslide bodies along the Hyblean-Malta escarpment.
The number of deaths recorded at the time in official sources were about 12,000 in Catania (63% of the population), 5,045 in Ragusa (51%), 3,500 in Syracuse (23%) 3,000 in Noto (25%), 1,840 in Augusta (30%) and 3,400 in Modica (19%).
The total death toll in the same source was recorded as 54,000 with other sources referring to totals of about 60,000.
In 1853, Mallet recorded 93,000 deaths, in his catalog of earthquakes.