The tsunami in Cadiz on 1 November 1755: A critical analysis of reports by Antonio de Ulloa and by Louis Godin

On the morning of 1 November 1755, the town of Lisbon was ruined by an earthquake, supplemented by a tsunami, inundating the lower town and harbour, and by a fire, lit by houses collapsing on kitchen fires, which raged for one week. At variance with Lisbon, the Spanish harbour town of Cadiz was considered as miraculously saved, despite the fears reported in the descriptions: “the sea-flood raised fears that the town might be submerged.” A classical estimate for the height of the tsunami wave there is 18 m. The study of a restricted selection of primary documentary sources demonstrates that the tsunami was much weaker. It is difficult to assess how the now classical records have been altered with reference to the original letters, but one of the reasons is that the secretaries of the scientific institutions only used to put in the print abridged, or worse, synthesized versions of the communications from members. The average ground level in Cadiz is 11 m above mean sea level. If the wave set-up had been about 19 m, the engulfment of Cadiz would have left its name to the 1 November 1755 earthquake and tsunami rather than to the destruction of Lisbon. In Cadiz, the impossibility to simulate waves higher than 10 m in the modelling experiments conducted in the last 12 years does not result from a flaw in the models: to the opposite, it has to be borne to the credit of M.A. Baptista and her scientific partners. They avoided the considerable risk to favour options that might bring a distortion towards output values, actually resulting from ancient misconceptions – just because such values appear to fit the data.