Earthquakes and tsunami in November 1755 in Morocco: a different reading of contemporaneous documentary sources

• Main Author: P.-L. Blanc
• Format: Article
• Language: English
• Published: Copernicus Publications 2009-05-01
• Series: Natural Hazards and Earth System Sciences
• Online Access: http://www.nat-hazards-earth-syst-sci.net/9/725/2009/nhess-9-725-2009.pdf
• ISSN: 1561-8633; 1684-9981

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.