1755 Lisbon Earthquake - The Earthquake and The Tsunami

The Earthquake and The Tsunami

The Azores-Gibraltar Transform Fault, which marks the boundary between the African (Nubian) and the Eurasian continental plates, runs westward from Gibraltar into the Atlantic. It shows a complex and active tectonic behavior, and is responsible for several important earthquakes that hit Lisbon before November 1755: eight in the 14th century, five in the 16th century (including the 1531 earthquake that destroyed 1,500 houses, and the 1597 earthquake when three streets vanished), and three in the 17th century. During the 18th century, earthquakes were reported in 1724 and 1750.

In 1755, the earthquake struck on the morning of 1 November, the holiday of All Saints' Day. Contemporary reports state that the earthquake lasted between three and a half and six minutes, causing fissures 5 meters (15 feet) wide to open in the city centre. Survivors rushed to the open space of the docks for safety and watched as the water receded, revealing a sea floor littered with lost cargo and shipwrecks. Approximately 40 minutes after the earthquake, a tsunami engulfed the harbour 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. In the areas unaffected by the tsunami, fire quickly broke out, and flames raged for five days.

Lisbon was not the only Portuguese city affected by the catastrophe. Throughout the south of the country, in particular the Algarve, destruction was rampant. The tsunami destroyed some coastal fortresses in the Algarve and, in the 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 of different Portuguese regions, like Peniche, Cascais, and even Covilhã which is located near the Serra da Estrela mountain range in central inland Portugal, were affected. The shock waves of the earthquake destroyed part of Covilhã's castle walls and its large towers. 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 m inland.

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

Although seismologists and geologists had 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 theories had proposed the Gorringe Ridge until simulations showed that a source closer to the shore of Portugal was required to comply with the observed effects of the tsunami. A seismic reflection survey of the ocean floor along the Azores-Gibraltar fault has revealed a 50 km-long thrust structure southwest of Cape St. Vincent, with a dip-slip throw of more than 1 km, that might have been created by the primary tectonic event.