Abstract
Abstract. Tsunami seldom strike the European Atlantic shores. The great Lisbon Earthquake of 1 November 1755 is the main destructive tsunamigenic event recorded. Since the mid-1990's, many simulations of propagation of tsunami waves from variants of the possible seismic source have been conducted. Estimates of run-up in Morocco are seldom included in publications, maybe for want of reliable historical data to control the simulations. This paper revisits some early accounts, transmitted as translations to European Chanceries, Scientific Societies and Newspapers. A critical analysis of the documents leads us to conclude that the Lisbon earthquake was overestimated because of amalgamation with a later Rifian earthquake. Then, the overestimation of the tsunami through worst interpretation of the scant data available appeared only reasonable, while the moderate measurements or interpretations were not given their due attention. In Morocco the amplitude of the tsunami (i.e. height at shoreline minus expected tide level) may not have exceed the measurement given by Godin (1755) for Cadiz, 2.5 m above the calculated astronomical tide, a crest-to-trough amplitude of 5 m at most. This age-old overestimation of both the earthquake and tsunami is detrimental to the evaluation of the risk for coastal people and activities.
Highlights
The Andaman-Sumatra earthquake and tsunami, on 26 December 2004 (Boxing Day), has awoken an unprecedented public awareness and anxiety on the risk of a tsunami hitting the coasts of any country: all over the world, coastal settlements or activities, such as fishing or industrial facilities, populated areas or holyday resorts, might prove very sensitive to such events, as well as the lives of the people involved.The Atlantic shores of Western Europe may not be among the most dangerous places on that account, tsunami being rather infrequent there
The tsunami caused by the Great Lisbon Earthquake, on the morning of 1 November 1755, remains the only destructive event described in some detail by historic documents, though an earlier event known to have occurred in Lisbon on 26 January 1531 (Baptista and Miranda, 2009), was mentioned in journals in 1756, both in France and in England (JHMT, 1756a; Wolsall, 1756, footnote)
The other local document at our disposal is a letter written on 5 November by Monsieur Soyris, trade agent of a company from Marseilles, based in Safi, but visiting in Marrakech, where he did not hesitate to initiate diplomatic contacts: this explains why his letter can be found in the Archives Nationales de France (Soyris, 1755; mentioned as Soyvia by Levret, 1991: Soyris was the name of a parish and seigneury in Quercy, a hamlet of Labastide-Murat, Lot Dept.)
Summary
The Atlantic shores of Western Europe may not be among the most dangerous places on that account, tsunami being rather infrequent there. The tsunami appears to have quickly lost its strength north of Lisbon, (Reid, 1914), either because the waves travelled in a rather oblique direction to the coasts there, or due to the shape and orientation of the accident responsible. It was destructive on the Portuguese coasts south of Lisbon, it struck rather heavily the bay of Cadiz, and further south, the shores of Morocco.
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