Five-million-year-old fissure discovered off Portugal could explain Lisbon's major earthquakes

For decades, geologists have struggled to explain the massive earthquakes which struck Lisbon in 1755 and 1969. Now a fissure in the tectonic plate 200 kilometres off the coast of Cabo de São Vicente (Sagres) may finally offer a clue to the forces behind earthquakes in the Portuguese capital.

This fissure, which has only just been discovered, has been forming for at least five million years, according to a study by the Faculty of Sciences of the University of Lisbon recently published in the journal Nature Geosciences.

The 1969 earthquake that shook Lisbon and other regions of Portugal and the 1755 'Great Earthquake' originated in the Ferradura Abyssal Plain, a geological formation in the Atlantic Ocean not far from the Gorringe Bank submarine mountain, on the border between the Eurasian and African tectonic plates.

As it is a flat geological formation with no known seismic faults, the origin of these quakes and others like them in Lisbon has always puzzled scientists.

However, the discovery of "a portion of the tectonic plate that is separating" in a process known as "delamination" may begin to explain this phenomenon, says João Duarte, co-author of the study, geologist, and professor at the University of Lisbon’s Faculty of Sciences, as well as a researcher at the Dom Luiz Institute, speaking to Lusa.

What does the delamination process involve?

Delamination is occurring because the tectonic plate is undergoing a horizontal fracture, as if the rock were separated by a blade. It opens up a fissure that causes the lower part to sink. This lower part of the plate has already reached a depth of 200 kilometres into the Earth's mantle, when it is normally 100 kilometres.

The fact that the upper part of the plate remained in an unchanged horizontal position made it difficult to observe the seabed and therefore to discover this geological change.

This phenomenon was only discovered due to a kind of "Earth ultrasound", explained João Duarte, through which it was possible to see the process of plate separation that has been taking place slowly for between five and 10 million years.

"We carried out a study that placed seismometers on the seabed for eight months to record small earthquakes. We realised that in that area there was a 'cluster', a group of small earthquakes at great depth, around 30 to 40 kilometres deep, which is a bit abnormal," the researcher explains.

"And so there is a combination of various observations here that point to a process taking place there that is generating seismicity."

The researchers also used computer models to simulate the so-called delamination process.

How are earthquakes formed?

Earthquakes are caused by friction and the release of energy as tectonic plates shift. While the geological structure described in the study isn’t a traditional fault line, it still has the potential to trigger seismic activity. That’s because the split occurring within the plate doesn’t leave a void. Instead, the space is filled, allowing stress to build up and eventually be released as an earthquake.

Speaking to Lusa, João Duarte said the installation of a new generation of undersea communications cables, connecting both sides of the Atlantic and passing through the Azores, Madeira, and the Horseshoe Abyssal Plain, presents an opportunity to improve earthquake monitoring.

The most recent earthquake felt in Lisbon and the surrounding region occurred on 17 February 2025, with its epicentre located about 14 km southwest of Seixal. It registered a magnitude of 4.7 on the Richter scale.