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


Journal title : Comptes Rendus - Geoscience
Volume : 340
Issue : 4
Pagination : 251-261
Publication date : 07/04/2008

Document type > *Article de revue

Keywords > documentary sources, earthquakes, tsunami

Research Unit > IRSN/DEI/SARG/BEHRIG

Authors > BLANC Paul-Louis

Publication Date > 07/04/2008

Summary

On the morning of November 1st, 1755, the town of Lisbon was ruined by an earthquake, supplemented by a tsunami of a 5 m height, engulfing downtown Lisbon and harbour, and a fire, lit by houses collapsing on kitchen fires, which raged for one week. At variance with Lisbon, the Spanish harbour and 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..." There, the classical estimate for the height of the tsunami wave is 19.5 metres. 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 original communications from members. The mean ground level in Cadiz is 11 metres above the mean sea-level. If the wave set-up had been anything close to 19.5 metres, the engulfment of Cadiz would have left its name to the November 1st, 1755, earthquake and tsunami, rather than the destruction of Lisbon. in Cadiz ,the impossibility to reproduce waves higher than 10 metres in the modelling experiments conducted in the last 10 to 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. The considerable risk to favour options which might bring a distortion towards output values, actually resulting from ancient misconceptions - just because such values appear to fit with the data ! - has been avoided.

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