5-million-year-old fissure found off Portugal might clarify Lisbon’s main earthquakes

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For a long time, geologists have struggled to clarify the large earthquakes which struck Lisbon in 1755 and 1969. Now a fissure within the tectonic plate 200 kilometres off the coast of Cabo de São Vicente (Sagres) could lastly supply a clue to the forces behind earthquakes within the Portuguese capital.

This fissure, which has solely simply been found, has been forming for at the very least 5 million years, based on a research by the Faculty of Sciences of the University of Lisbon lately revealed within the journal Nature Geosciences.

The 1969 earthquake that shook Lisbon and different areas of Portugal and the 1755 ‘Great Earthquake’ originated within the Ferradura Abyssal Plain, a geological formation within the Atlantic Ocean not removed from the Gorringe Bank submarine mountain, on the border between the Eurasian and African tectonic plates.

As it’s a flat geological formation with no recognized seismic faults, the origin of those quakes and others like them in Lisbon has at all times puzzled scientists.

However, the invention of “a portion of the tectonic plate that is separating” in a course of often known as “delamination” could start to clarify this phenomenon, says João Duarte, co-author of the research, geologist, and professor on the University of Lisbon’s Faculty of Sciences, in addition to a researcher on the Dom Luiz Institute, talking to Lusa.

What does the delamination course of contain?

Delamination is happening as a result of the tectonic plate is present process a horizontal fracture, as if the rock had been separated by a blade. It opens up a fissure that causes the decrease half to sink. This decrease a part of the plate has already reached a depth of 200 kilometres into the Earth’s mantle, when it’s usually 100 kilometres.

The incontrovertible fact that the higher a part of the plate remained in an unchanged horizontal place made it troublesome to look at the seabed and due to this fact to find this geological change.

This phenomenon was solely found because of a sort of “Earth ultrasound”, defined João Duarte, by way of which it was attainable to see the method of plate separation that has been happening slowly for between 5 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 additionally used laptop fashions to simulate the so-called delamination course of.

How are earthquakes fashioned?

Earthquakes are attributable to friction and the discharge of vitality as tectonic plates shift. While the geological construction described within the research isn’t a conventional fault line, it nonetheless has the potential to set off seismic exercise. That’s as a result of the cut up occurring inside the plate doesn’t depart a void. Instead, the area is crammed, permitting stress to construct up and ultimately be launched as an earthquake.

Speaking to Lusa, João Duarte mentioned the set up of a brand new technology of undersea communications cables, connecting either side of the Atlantic and passing by way of the Azores, Madeira, and the Horseshoe Abyssal Plain, presents a chance to enhance earthquake monitoring.

The most up-to-date earthquake felt in Lisbon and the encompassing area occurred on 17 February 2025, with its epicentre positioned about 14 km southwest of Seixal. It registered a magnitude of 4.7 on the Richter scale.