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.
“The base of the plate is separating like the sole of a shoe peeling off,” Duarte said. “That was the first Eureka moment when I thought, ‘aha, there’s something there’. The second was when the computer models also showed delamination was happening.”
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.
“If you see on the forecast that it’s going to rain tomorrow, you take an umbrella,” he continued. “You don’t need to know exactly what minute it will start raining because you are prepared.
“With earthquakes, it’s the same thing; we don’t know when a major one will hit, but we know that one will, so we need to be prepared for that.”
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About our expert
João Duarte is an assistant professor in tectonics at the University of Lisbon and president of the Tectonics and Structural Geology Division of the European Geosciences Union. His research has been published in journals including Geophysical Research Letters, Nature Communications and Geology.