Remains of the historic tsunami that succeeded the famous Lisbon earthquake (1755) have been identified in Martinique by archaeologists and geophysicists.
This etching from 1755 shows the ruins of Lisbon in flames and a tsunami submerging ships in the port.
It has long been the archetype of the tsunami. This morning of the 1 st November 1755, all the inhabitants of Lisbon, Portugal, - one of the richest capitals of the XVIII th century - celebrate All Saints Day in the gold-filled churches of its prosperous colony of Brazil. Suddenly, in less than 10 minutes, three huge earthquakes, whose magnitude is now estimated between 8.5 and 9 on the Richter scale, destroy in an apocalyptic crash almost the entire port city, killing tens of thousands of inhabitants. The magnitude of the disaster is such that Voltaire evokes the drama in his Candide (see box 1). But the intensity of the earthquakes whose epicenter was at sea hundreds of km south of Cape St Vincent (Portugal), immediately triggered a tsunami which radiates throughout the Atlantic Ocean (see box 2). The waves surge for thousands of kilometers until they reach the Caribbean.
Calculations of the hourly time taken by the transoceanic tsunami of the 1 st November 1755. © NOAA's National Geophysical Data Center (NGDC)
"Tsunami Hunter »
Geophysicist Jean Roger has been interested in tsunamis and this disaster in particular for several years. And the story it tells was detailed in an article submitted to the online journal Natural Hazards and Earth System Sciences, as reported by the weekly EOS, published by the American Geophysical Union . In collaboration with a team of experts including archaeologists and geophysicists, he thinks he has found traces of this extraordinary cataclysm in the Caribbean, in the city of Fort-de-France in Martinique. "Descriptions of its passage are attested in a dozen historical documents in Newfoundland, Brazil, and Martinique explains the young geoscience specialist contacted by Sciences et Avenir.
It was Emmanuel Moizan of the National Institute for Preventive Archaeological Research (INRAP), who alerted Jean Roger in 2013, then working at the University of Antilles-Guyane. The archaeologist indeed studies buildings of the XVIIth-XVIIIth century in a central district of Fort-de-France and discovers a strange deposit of black sand. He invites the geophysicist and Valérie Clouard, director of the Martinique Observatory of the Institut de Physique du Globe de Paris (IPG), to come and examine this 7 to 9 cm thick layer, superimposed on white sand of 1 .25 cm from the degradation of a coral reef in the bay of Fort-de-France. Everything indicates that it is a relic of the powerful cataclysm of 1755 described in the archives!
Based on dates made on pottery, INRAP archaeologists manage to estimate that this layer of black sand was deposited between 1726 and 1783. origin of this deposit can only be the tsunami of 1755" , explains Jean Roger. “No other significant climatic event that can be retained was included in the historical documents of the city for these periods “, adds Valérie Clouard, also joined by Sciences et Avenir. It remained to determine the origin of this black sand and how it had arrived there... The investigations, carried out by these researchers throughout the island and the neighboring archipelagos, will last two years.
And the established scenario is now the following:after having traveled in 7 hours the 5700 km which separated the coasts of Portugal from that of the Caribbean island, a tide of 3m to 4m high swept over Martinique. "You have to imagine this rise in sea level as a tidal bore phenomenon “says Valérie Clouard. As it energetically entered Fort-de-France, the sea literally swept over the mouth of the Madame* River, the only place to have a small black sand beach from the rocks of the Mount Pelée volcanic complex. This black sand was then transported by the flood to the interior of the city, where these sediments were then deposited to a thickness of 7 to 9 cm (see map ). A second archaeological site has since been discovered by Emmanuel Moizan (INRAP) with this same deposit. "All of these testimonies agree with our digital models and historical descriptions “, explains Jean Roger. “Not to mention similar data related to the 1755 tsunami, recently described by an American team in Anegada, one of the British Virgin Islands concludes the director of the Martinique Observatory.
Distribution of flows from the 1755 tsunami entering Fort-de-France, at the time criss-crossed by numerous canals that no longer exist today. Their presence was linked to the drying up of the mangroves on which the city was built. © Jean Roger/Valérie Clouard/Emmanuel Moizan
Excerpt from Candide by Voltaire
“...No sooner have they set foot in the city mourning the death of their benefactor, than they feel the earth tremble under their feet; the sea rises and boils in the port, and breaks the vessels which are at anchor. Whirlwinds of flame and ash cover the streets and public squares; the houses crumble, the roofs are knocked down on the foundations, and the foundations scatter; thirty thousand inhabitants of all ages and sexes are crushed under ruins …”.
Ruins of Lisbon:the survivors lived in tents on the outskirts of the city as shown in this German engraving from 1755. © Public domain
A tsunami (from Japanese:tsu, "port" and nami "wave") is not the consequence of weather-related elements (winds, etc.) but a wave caused by a rapid movement of a large volume of water (ocean or sea). This movement is generally due to an earthquake, an explosive-type underwater volcanic eruption or a large-scale underwater landslide. A meteorite impact can also be the cause, as well as an underwater atomic explosion. Tsunamis, officially named since 1963, are among the most destructive disasters in history. Over the past four millennia, at least 279 events have been recorded. totaling more than 600,000 victims worldwide. The 2004 tsunami in the Indian Ocean is the deadliest disaster of the last 30 years, with more than 250,000 victims (source BRGM).