The Great Lisbon Quake of 1755

Submarine landslides can be triggered by earthquakes, which are frequent along the Iberian margin. The most famous and destructive earthquake in the area occurred in Lisbon in 1755. Its effect on the seabed was so great that the event can be easily recognised in the layers of sediment on the seabed offshore Portugal.

On the morning of 1st November 1755, the people of Lisbon were hard at work. On the dockside, a large quay near the Customs House was host to about 600 workers, all busy sorting and packaging goods from foreign lands. Little did they know that their lives, and their city, were in grave danger.

The previous evening, strange plumes of dark yellow smoke had been observed, and the water in the wells began to develop a strange taste. Early in the morning livestock became unusually agitated and burrowing animals came out of their holes. Then, at 9:40 am, a magnitude 8.6 earthquake was unleashed offshore about 200 km to the south-east. The quay was sunk instantly by the first shock and all 600 people were reported to have perished. Several buildings within the city collapsed, trapping the people inside. After nine minutes the ground finally stopped shaking and the city began to contemplate recovery. However, their troubles were only just beginning.

Immediately after the earthquake, many inhabitants of Lisbon looked for safety on the sea by boarding ships moored on the river. But about 30 minutes after the quake, a large wave swamped the mouth of the River Tagus, sinking many of the boats and their passengers. The earthquake had generated a tidal wave (known as a tsunami) that swept towards the Portuguese coast. The tsunami wave grew as it reached the shallow waters of the Tagus River mouth, and by the time it reached Lisbon it was over five metres high. The waters crashed over the seawall and flooded 250 m into the city, drowning a further 900 people.

[click on images to enlarge]

A total of three waves struck the shore, each dragging people and debris out to sea and leaving exposed large stretches of the river bottom. In some areas of the city, the maximum height of the waves was estimated at 6 meters. Boats overcrowded with refugees capsized and sank. In the town Cascais, some 30 km west of Lisbon, the waves wrecked several boats and when the water withdrew, large stretches of sea bottom were left uncovered. In Setubal, 30 km south of Lisbon, the water reached the first floor of buildings.

The tsunami damage was greatest in Algarve, southern Portugal, where the tsunami destroyed some coastal fortresses and flattened houses. In some places the waves crested at more than 30 m. Almost all the coastal towns and villages of Algarve were heavily damaged, except Faro, which was protected by sandy banks. In Lagos, the waves reached the top of the city walls. For the coastal regions, the destructive effects of the tsunami were more disastrous than those of the earthquake.

However, for Lisbon the worst was yet to come.

The damage generated by the earthquake and the tidal wave had initiated a large number of fires across the city, caused by cooking fires and candles burning out of control. Many inhabitants fled from their homes and in the ensuing chaos these fires grew larger until they became an uncontrollable inferno. Narrow streets full of fallen debris prevented access to the fire sites. The public squares filled with people and their rescued belongings, but as the fire approached, these squares were abandoned, and the fire reached catastrophic proportions. The flames raged for five days, causing a huge swath of the city to be destroyed and 10,000 people were burnt to death.

The effects of the earthquake were not confined to Portugal. Severe shaking was felt in North Africa and there was heavy loss of life in Fez and Mequinez. Moderate damage was done in Algiers and in southwest Spain. Shaking was also felt in France, Switzerland, and Northern Italy and the tsunami caused heavy damage along the coasts of Portugal, southwest Spain, and western Morocco. Minor tremors were reported from as far away as Finland.The tsunami reached, with less intensity, the coast of France, Great Britain, Ireland, Belgium and Holland. In Madeira and in the Azores islands damage was extensive and many ships were in danger of being wrecked.The tsunami crossed the Atlantic Ocean, reaching the Antilles in the afternoon. Reports from Antigua, Martinique, and Barbados note that the sea first rose more than a meter, followed by large waves.

Most of our knowledge of the Lisbon event has been derived from historical documents, including scientific letters, poems, religious sermons, and answers to an inquiry distributed to all priests in the region by Marquez de Pombal, who was secretary of state at the time. A particularly graphic account of the earthquake, tsunami and fires was written by the Rev. Charles Davy - click here to read it.

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© Challenger Division for Seafloor Processes
August 2003
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