31/10/2020
🇵🇹 Earthquake of Lisbon, All Saints Day 1755 🇵🇹
One of the most important moments in Portugal’s history, the earthquake that devastated Lisbon on November 1st 1755 has stayed forever in the memory of the Portuguese people.
At one time, Portugal ruled the world. Its explorers discovered the Cape of Good Hope, created colonies around the Indian Ocean, reached China and Japan, founded the city of Nagasaki, and claimed much of South America. The glory days of the Portuguese empire lasted from 1415 to 1750, and left a rich heritage in Portugal. But geology was the nation’s Achilles heel.
Portugal’s capital, Lisbon, midway along the Portuguese coast, is not close to a plate boundary and is not a place to expect a major earthquake. History decided differently. The Lisbon earthquake of 1755 is among the worst disasters to hit Europe in modern times. It devastated the Portuguese empire. Still, many articles, newspapers and even some scientific literature overstate the magnitude, number of casualties, and impact on society; scientific papers comment on the fact that some statements appear exaggerated. Let’s look at the facts.
The earthquake
It came from the Atlantic Ocean, during the celebrations of All Saints day. In Lisbon, Sunday, 1 Nov 1755 was a sunny day, warm for the time of the year. Around 9:35am, gentle shaking started, as from heavy carts, and a rumbling sound came from the ground, lasting about 1.5 minute. A minute later, there was a sudden violent shock: strong shaking lasted 2.5 minute, causing panic. Another minute later, there was another shock, perhaps stronger than the previous one; this time the shaking lasted 3.5 minutes, slowly becoming less violent. These nine minutes left Lisbon in ruins, its population destitute from the immense destruction. All major churches in Lisbon collapsed, killing many of the congregations. Over 80% of the houses were damaged. Deep fractures were left in the city. The first aftershock came at 11am. The aftershocks continued for months and some were themselves violent.
Lisbon was not the only place that was badly affected. There was major damage in the Algarve; Faro was almost completely destroyed. Damage in southern Spain and Morocco was extensive. North of Lisbon the damage was considerably less. The shaking was felt across Spain, France and Italy, to about 2500 km away.
There is a detailed and precise report on the earthquake from Cadiz, in southern Spain, 350km southeast of Lisbon. The same three phases were observed with a similar duration (perhaps the last phase lasted a bit longer), but the third phase was notably less violent than the second, whereas in Lisbon they were more comparable.
Tsunami
The population of Lisbon fled to safety, away from buildings. Many went for the harbour. Around 10am, some 30 minutes after the quake, a mountainous wave came up the Tagus river. Within 5 minutes the water rose to a height of 5 to 6 metre; the wave penetrated 250 metres inland impacting especially the western areas of Lisbon. Many boats overturned. A large pier with many people sheltering on it sank and completely disappeared, probably due to liquefaction.
The first wave was followed by 2 or 3 more, over 1-2 hours. The first major aftershock occurred in between these waves. In the midst of the destruction, there was one lucky break: the tsunami hit an hour before low tide, so that the base water level was low and the tsunami was slowed down by the outgoing tide and flow of the river.
On the coast, the harbour of Setubal was swamped by the tsunami with water reaching the first floor of buildings, 6 metre high. Further south, the harbour of Lago was ruined and had to be moved afterwards due to the sediment. Here the waves came over the city walls, 11 meter high: the water traveled 2.5 km in-land. Blocks up to 4500 kg were deposited on the coast. Portimao was badly hit. Faro escaped lightly, protected by a sandbank in front of the city (but this was scant consolation as the city had been destroyed by the earthquake instead). Elsewhere in the Algarve, the highest waves reached 20 metre, perhaps higher. In Agadir, Morocco, the water also came over the city walls. There was tsunami damage on the island of Terceira on the Azores. 3.5 meter high waves reached Antigua 10 hours after the earthquake. There was damage in Recife, Brazil. In Cornwall, UK, the waves were 3 meter high.
Some reported tsunami heights seem overstated. For instance, for Cadiz, in Spain, a wave height of 17-19 metre is often stated. But the town survived unscathed: as it is 11 metre above sea level, the tsunami was less than that, probably 5-10 metre. Some reports mention very high tsunamis in Morocco, but this is disputed: the wave heights may have been around 5 metres instead.
In the majority of places, the reports state that the water initially came up: there are far fewer descriptions of the water initially receding. That may be related to the fact that it was already low tide, and an initial outflow may not have been noticed.
Elsewhere in Europe, to the north, the earthquake triggered resonance waves (seiches) on lakes, rivers and harbours. They reached 60 cm amplitude in Loch Ness, and were observed from Switzerland to Finland including the Netherlands and Stockholm, and possibly even along the coast of Greenland.
In the UK, large waves were seen along the south coast but little actual damage was reported. Computer models indicate that the region between Penzance and Lizard Point, near the tip of Cornwall, would have been worst affected; elsewhere wave heights were less than 2 meters. The extensive continental plateau around the British Isles disperses the waves and provides a degree of protection against tsunamis. But Spain and Portugal lack this protection.
Fire
After the earthquake, fires broke out, some from cooking fires and candles, and some set by looters. These quickly raged out of control and large areas of downtown Lisbon but also surrounding districts burned down. Some buildings which had escaped major earthquake damage, including the Royal Palace and the Patriarchal Cathedral were instead gutted by the fires. Lisbon burned for 5 days. In medieval times, fires were a major hazard for cities as the narrow streets and inflammable building materials made them impossible to control. Once a fire started, chances were that the entire town would burn down, with many casualties. Better control of building materials later improved the situation but city-wide fires remained common.
Lisbon was destroyed by earth, water, and fire, but most of the physical damage was done by fire.
