My 48th page... “Big earthquakes are going to happen again”

A new tectonic fault could be emerging beneath the Atlantic Ocean, raising the risk of powerful earthquakes and tsunamis that could ripple across the basin. That’s according to a new study published this week in Nature Geoscience.

For centuries, scientists have puzzled over why Portugal has suffered huge earthquakes despite lying far from the world’s major fault lines. 

On 1 November 1755, Lisbon was devastated by a magnitude 8.7 quake that killed tens of thousands and sent tsunami waves as far as the Caribbean. More recently, a magnitude 7.8 tremor struck off Portugal’s coast in 1969, killing 25 people. 

“One of the problems is that these earthquakes occurred on a completely flat plain, far from the faults,” Prof João Duarte, a geologist at the University of Lisbon and lead author of the study, told BBC Science Focus. 

“After the 1969 earthquake, people started to realise that something strange was going on, because it had the signature of a subduction zone, yet there isn’t one there.”

Subduction zones – where one tectonic plate dives beneath another – are responsible for the planet’s most devastating 'megathrust' quakes, such as the 2004 Indian Ocean and 2011 Tōhoku disasters. But the Atlantic has long been considered relatively calm as its plates slowly drift apart along a mid-ocean ridge.

Duarte’s team pieced together seismic records and computer models of the Horseshoe Abyssal Plain, a stretch of deep seafloor southwest of Portugal. They found evidence that the mantle – the hot, dense layer beneath Earth’s crust – is peeling away in a process called delamination.

Abstract

Subduction of oceanic lithosphere and delamination of continental lithosphere constitute the two predominant mechanisms by which the Earth’s surface is recycled into the mantle. Continental plate delamination typically occurs in collisional orogens by the separation of the lithospheric mantle from the overlying lighter crust, aided by weak layers within continental lithosphere. By contrast, oceanic lithosphere is generally considered to be sufficiently rigid to inhibit delamination. Here we show from seismic imaging and numerical simulations that delamination of oceanic lithosphere is occurring offshore Southwest Iberia. Specifically, seismic tomography reveals a high-velocity anomaly that we interpret as a delaminating block of old oceanic lithosphere, a process that we reproduce with numerical simulations. We propose that this process was triggered by plate convergence and assisted by a thick serpentinized layer that allows the lithospheric mantle to decouple from the overlying crust. We suggest that such oceanic delamination may facilitate subduction initiation, a long-unsolved problem in the theory of plate tectonics, and may be responsible for some of the highest-magnitude earthquakes in Europe, including the M8.5–8.7 Great Lisbon Earthquake of 1755 and the M7.9 San Vincente Earthquake of 1969.

This engraving depicts the 1755 Lisbon earthquake. A combination of the earthquake, tsunami and subsequent fires almost completely destroyed the Portuguese capital - Credit: Getty

Such unpeeling is almost unheard of in the oceanic crust, which usually behaves like a “crème brûlée”, as it has a rigid buoyant layer sitting atop a squishier one beneath. 

But here, water appears to have seeped into the rock over millions of years, chemically weakening it and allowing chunks of mantle to sink into Earth’s depths. 

The findings suggest we may be witnessing the birth of a new subduction zone in the Atlantic – one that could eventually pull Africa, Europe and the Americas back together into a future supercontinent.

For now, the more immediate concern is seismic hazard.

“Big earthquakes are going to happen again,” Duarte said, warning that the impacts of these could devastate unprepared coastal regions across the Atlantic. 

Date: Published: August 30, 2025 at 4:00 pm

Fontes/Links:

https://www.sciencefocus.com/news/mega-earthquake-hotspot-atlantic

https://www.nature.com/articles/s41561-025-01781-6/figures/1

https://www.nature.com/articles/s41561-025-01781-6

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