The Lisbon earthquake of 1755: what it changed

An earthquake struck Lisbon on Saturday 1 November 1755, a little after half past nine in the morning. The hour comes from Joachim José Moreira de Mendonça, who was there and published in 1758 a Historia universal dos terremotos telling the day in detail. The sky had been cloudless for days. The ground starts gently, the movement builds, and by the second minute buildings are falling. Mendonça gives six or seven minutes of shaking, broken by two short lulls, under an underground roar many took for a heavy carriage at the gallop.

The exact hour was already being argued about on the spot. A merchant settled in Lisbon, whose letter to a friend in England was printed in London that same year, notes the disagreement, in his own English: “Some thinking it was before, others after ten of the Clock; but the greatest Number pronounce the Clock had not struck.” The Medeiros e Almeida museum, which holds an album of engravings of the ruins, keeps to 9.40.

Then the sea. At those shocks, writes Mendonça, the river drew back and uncovered a bed nobody had ever seen, then the water came back in very high mountains over every settlement along the shore: three great incursions, and some small ones. The merchant, who was on the great square before the palace, writes that “the Water of the River rose at once above twenty Feet perpendicular, and subsided again to its natural Pitch in less than a Minute’s time.”

Fire finished the city. It is All Saints’ Day: the altars are lit in every church, the cooking fires are lit in every house, and the roof timbers come down on them. The merchant counts five days of fire, the Medeiros e Almeida museum six. Mendonça, who went through the city street by street, reckons the fire devoured a third of Lisbon, where it was most crowded, in narrow streets lined with houses of four, five and six storeys.

His material reckoning is the most precise any contemporary attempted: one house in ten thrown down by the shock, more than two thirds of those left standing made uninhabitable, fewer than one in three still fit to live in. The effects, he adds, were felt over more than five hundred leagues.

Nobody knows, and nobody will. The merchant in Lisbon wrote it in 1755: “There is no Possibility of ascertaining Numbers.” He had heard the number killed estimated at about thirty thousand, and had heard the calculation carried by others much higher.

Mendonça is the only contemporary who says how he counted. Ask two or three people in each street about the missing in the neighbourhood; ask the parish priests what they reckoned for their parish; listen to the survivors of the deadliest churches; then reason about the religious communities, the clergy, the nobility, the courts and the trades. He arrives at about five thousand dead on the day itself, in the ruins, in the sea and in the fire, and as many during November among the injured and the sick. Ten thousand, then, and he calls himself more moderate than everyone else.

Around him the figures pile up. He notes the first printed letter on the earthquake, by Joseph de Oliveira Trovão, which gives seventy thousand dead; father António do Sacramento, in a consolatory exhortation, writes more than eighteen thousand; others reason in fractions, a third or an eighth of the inhabitants. Voltaire, in verse, writes of a hundred thousand wretches the earth devours. As for the count the king had the parish priests make, Mendonça never knew the total and supposed it enormous.

Recent work holds the range without closing it. Dora Santos Silva, in the review +Lisboa of the faculty of social sciences and humanities of the New University of Lisbon, speaks of estimates running from ten thousand to ninety thousand, with no geographical precision. The Medeiros e Almeida museum keeps to twelve thousand to forty thousand. A figure given alone, here, always hides the name of whoever advanced it.

No instrument recorded earthquakes in 1755: this one’s magnitude is reconstructed, not measured. The IPMA, the Portuguese institute of the sea and the atmosphere, places its origin offshore, southwest of Cape St Vincent, and estimates the moment magnitude at 8.7. The seismicity catalogue of Martins and Víctor, published in 1990 and presented in the IPMA’s book on the history of Portuguese seismology, gives it 8.5. The Medeiros e Almeida museum announces 8.5 to 9.

These gaps are not mistakes, they follow from the method. With no seismogram, magnitude is deduced from the damage described and from the extent of the area shaken, therefore from the testimonies, therefore from what they are worth. That is also why the parish replies, collected from 1756, are still in use.

The contemporaries were feeling for the origin. In 1760 John Michell put it under the ocean, somewhere between the latitudes of Lisbon and Porto, probably a little nearer the first, ten or fifteen leagues off the coast. He reasoned from the direction the shocks arrived from, which he took from Mendonça for Lisbon: the northwest.

The court was lodged at Belém, outside the city: Joseph I is unhurt, and the orders go out from there, signed by his secretary of state Sebastião José de Carvalho e Melo, the future marquis of Pombal. In 1758 a book presented to the king, the Memorias das principaes providencias que se derão no terremoto, arranges those decisions into fourteen providences, each followed by the documents that prove it.

The list gives the order of the urgencies, and there is nothing abstract about it. Avoid the plague threatened by the corruption of the corpses, since no living were left to bury them. Avoid famine. Treat the injured and the sick left in the streets. Bring back the inhabitants who had fled. Prevent theft and punish thieves, and patrol the river so the loot did not leave by sea. Bring in troops. House the people. The fourteenth and last gives the means of rebuilding the city.

The king added a precaution of another order: to obtain from the pope that Saint Francis Borgia be held protector against earthquakes, with a first-class office. And the phrase always quoted, bury the dead and care for the living, is not Pombal’s: the scholar Filomena Amador returns it to D. Pedro de Almeida, marquis of Alorna, and gives it whole, with its third clause, close the ports. Amador also notes that Carvalho e Melo wanted the natural causes of the earthquake explained to the population.

Lisbon was not rebuilt as it had been. The Medeiros e Almeida museum sums up the result: the city came out of the rubble and the ashes as an orthogonal city, organised and modern, willed by the marquis of Pombal. It is the quarter since called the Baixa Pombalina.

The book published by the IPMA in 2021 sets out what that building site forced people to invent. Serial production and principles of prefabrication first, to build fast and cheaply. Then a structure able to resist vibration elastically, so that buildings could sway without breaking: the famous gaiola construction, the cage, a three-dimensional timber frame set inside the masonry.

Two decisions count as much as the cage. The height of the buildings was studied and stopped at three or four storeys, where the most crowded streets had had four, five and six before. And the works gave Portugal its first written building regulation, which prescribes framed houses.

Lisbon is not alone in its century. Stefania Stellacci, Nicola Ruggieri and Vasco Rato set the post-1755 Baixa Pombalina against the case baraccate of southern Italy after 1783: two eighteenth-century cases of anti-seismic heritage, compared in their structural principles and their mechanical behaviour. That comparison says best what the gaiola was: a technique thought out against the next earthquake.

The phrase comes from an inquiry. In 1756 a questionnaire of thirteen questions on the signs and manifestations of the earthquake went out to the parishes of the whole kingdom, from Monção in the north to Vila Real de Santo António in the south. It asks the hour and the duration, whether the push came more from one side than another, whether the tide first fell or rose, whether there was a fire and how long it lasted.

This is not a damage survey, it is an attempt to understand a natural phenomenon: knowing whether the shock had a direction meant being able to draw conclusions for the rebuilding. The IPMA’s book takes up the judgment of the seismologist Montessus de Ballore, quoted by Pereira de Sousa in 1919: that questionnaire had a genuinely scientific character, odd enough for its time.

Its answers had a second life. Pereira de Sousa used them at the beginning of the twentieth century for his study of historical seismicity, and they are read again today within plate tectonics, to estimate risk. The IPMA’s book holds 1755 to be the beginning of modern seismology and keeps two contributions: Kant, for his interpretation of effects felt very far away, and John Michell, who set out before the Royal Society in London in 1760 the earthquake as an elastic wave travelling through the Earth.

One precision, because it often gets lost. “The first earthquake studied scientifically” describes that inquiry and what it allowed, not a field survey: in the chronology of the same book, the first scientific study carried out on the spot on earthquake damage is Mallet’s, at Naples, in December 1857.

Because it put in question the idea that the world is well made. Filomena Amador presents it as probably the natural phenomenon with the greatest scientific and philosophical impact in human history; the IPMA’s book takes up the expression intellectual earthquake for Enlightenment Europe.

Voltaire fired first, in 1756, with his Poème sur le désastre de Lisbonne, subtitled an examination of the axiom that all is well. It contains a hundred thousand wretches the earth devours, and this line, aimed at the living as much as the dead: “Lisbonne est abîmée, et l’on danse à Paris”, Lisbon is destroyed, and in Paris they dance. In Candide he would have an auto-da-fé ordered by the university of Coimbra to prevent earthquakes.

Rousseau answered him on 18 August 1756. He moves the fault towards human beings: nature, he writes, had not gathered there twenty thousand houses of six or seven storeys, and if the inhabitants had been spread out more evenly and housed more lightly, the damage would have been much less and perhaps none at all. Two centuries before anyone spoke of risk, that is already the idea that a disaster is partly built.

At Königsberg, on the Baltic, Kant does not comment, he investigates at a distance. He gathers everything that reaches him and publishes, between January and April 1756, three texts on the earthquake. He holds it an unpardonable arrogance to read a divine punishment into it, because that means claiming to interpret God’s designs according to human convenience. The commonest explanation at the time was precisely God’s punishment.