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1755 Lisbon earthquake

Catastrophic earthquake that primarily affected Lisbon, Portugal

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A map from the USGS showing the location of the 1755 Lisbon earthquake
USGS · Public domain

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Chronologically, it was the third known large-scale earthquake to hit the city (following those of 1356 and 1531). Estimates place the death toll in Lisbon around 30,000–40,000. A further 10,000 may have died in Morocco.

The earthquake accentuated political tensions in Portugal and profoundly disrupted the Portuguese Empire. The event was widely discussed and dwelt upon by European Enlightenment philosophers, and inspired major developments in theodicy. As the first earthquake studied scientifically for its effects over a large area, it led to the birth of modern seismology and earthquake engineering.

Earthquake and tsunami

The earthquake struck on the morning of 1 November 1755, All Saints' Day. Contemporary reports state that the earthquake lasted from three and a half to six minutes, causing fissures 5 metres (16 ft) wide in the city center. Survivors rushed to the open space of the docks for safety and watched as the sea receded, revealing a plain of mud littered with lost cargo and shipwrecks. Approximately 40 minutes after the earthquake, a tsunami engulfed the harbor and downtown area, rushing up the Tagus river "so fast that several people riding on horseback ... were forced to gallop as fast as possible to the upper grounds for fear of being carried away." It was followed by two more waves. Candles lit in homes and churches all around the city for All Saints' Day were knocked over, starting a fire that developed into a firestorm which burned for hours in the city, asphyxiating people up to 30 metres (98 ft) from the blaze.

Lisbon was not the only Portuguese city affected by the catastrophe. Throughout the south of the country, in particular the Algarve, destruction was widespread. The tsunami destroyed some coastal fortresses in the Algarve and, at lower levels, it razed several houses. Almost all the coastal towns and villages of the Algarve were heavily damaged, except Faro, which was protected by the sandy banks of Ria Formosa. In Lagos, the waves reached the top of the city walls. Other towns in different Portuguese regions, such as Peniche, Cascais, Setúbal and even Covilhã (which is located near the Serra da Estrela mountain range in central inland Portugal) were visibly affected by the earthquake, the tsunami, or both. The shock waves of the earthquake destroyed part of Covilhã's castle walls and its large towers and damaged several other buildings in Cova da Beira, as well as in Salamanca, Spain. In Setúbal, parts of the Fort of São Filipe de Setúbal were damaged.

On the island of Madeira, Funchal and many smaller settlements suffered significant damage. Almost all of the ports in the Azores archipelago suffered most of their destruction from the tsunami, with the sea penetrating about 150 metres (490 ft) inland. Current and former Portuguese towns in northern Africa were also affected by the earthquake. Places such as Ceuta (ceded by Portugal to Spain in 1668) and Mazagon, where the tsunami hit the coastal fortifications of both towns hard, in some cases going over them and flooding the harbor area. In Spain, the tsunamis swept the Andalusian Atlantic Coast, damaging the city of Cadiz.

Shocks from the earthquake were felt throughout Europe as far as Finland and in North Africa, and according to some sources even in Greenland and the Caribbean. Tsunamis as tall as 20 metres (66 ft) swept along the coast of North Africa, and struck Martinique and Barbados across the Atlantic Ocean. A three-metre (ten-foot) tsunami hit Cornwall on the southern British coast. Galway, on the west coast of Ireland, was also hit, resulting in partial destruction of the "Spanish Arch" section of the city wall. In County Clare, Aughinish Island was created when a low-lying connection to the mainland was washed away. At Kinsale, several vessels were whirled round in the harbor, and water poured into the marketplace.

In 2015, it was determined that the tsunami waves reached the coast of Brazil, then a colony of Portugal. Letters sent by Brazilian authorities at the time of the earthquake describe damage and destruction caused by gigantic waves.

Although seismologists and geologists have always agreed that the epicenter was in the Atlantic to the west of the Iberian Peninsula, its exact location has been a subject of considerable debate. Early hypotheses had proposed the Gorringe Ridge, about 320 km (170 nmi; 200 mi) south-west of Lisbon, until simulations showed that a location closer to the shore of Portugal was required to accord with the observed effects of the tsunami. A 1992 seismic reflection survey of the ocean floor along the Azores–Gibraltar Transform Fault detected a 50-kilometre-long (31 mi) thrust fault southwest of Cape St. Vincent, with a dip-slip throw of more than 1 km (0.62 mi). This structure may have created the primary tectonic event.

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