Scientists detected the formation of a new tectonic fault under the Atlantic Ocean that may become a source of strong future earthquakes and tsunamis in Portugal. A study in Nature Geoscience described a high-velocity anomaly beneath the Horseshoe Abyssal Plain, about 200 kilometers southwest of continental Portugal, identified through seismic imaging from land stations and ocean-bottom seismometers and simulations testing different convergence scenarios among crustal structures. Researchers pinpointed a high-velocity anomaly at depths up to 250 kilometers between the African and Eurasian plates and observed a section of oceanic surface sinking into the mantle, creating new faults.
Phys.org reported that tomography data indicated the anomaly was a delaminating block of old oceanic lithosphere, meaning the usually rigid oceanic plate was splitting and the bottom part had started sinking into the Earth. The group’s models showed the ocean mantle began to decouple from the overlying crust due to serpentinization, a process in which water seeped into rock, hydrating it and turning it into hydrous serpentinite, enabling easier separation.
“During the study, a large number of small earthquakes were recorded at a depth of about 30-40 kilometers, which - scientists assumed - are the source of seismicity in this region of the Atlantic Ocean,” said João Duarte, a geologist at the University of Lisbon and the lead author of the study. “The upper part of the plate remains in an unchanged horizontal position, which has so far prevented the finding of the fissure during seabed observations,” said Duarte. “As a result of delamination, the lower part of the Eurasian Plate has already significantly sunk at least 200 kilometers towards the Earth’s mantle,” said Duarte.
Researchers integrated data from seafloor seismometers, teleseismic tomography, and sound recorded from earthquakes to examine processes below the seabed. The team assessed a dataset collected between 2007 and 2013 from 387 land stations across the Iberian Peninsula and northwestern North Africa and incorporated data from 24 ocean-floor seismographs off the southwestern Iberian Peninsula. Using these data, the researchers built a model of seismic wave propagation extending from the crust-mantle boundary to depths of 800 kilometers. To enhance their observations, the team created computational models to simulate delamination, which they said will be essential for improving models that predict crustal detachments. The models also indicated that delamination may have been preceded by two fracture zones that isolated a lithospheric block now sinking northwards, leading to a major thrust fault at the present-day location of high-magnitude seismicity.
Although the Atlantic Ocean is considered relatively quiet, with tectonic plates drifting slowly along a mid-ocean ridge, it has seen destructive earthquakes far from major fault zones. The Lisbon earthquake on November 1, 1755, was considered one of the strongest in modern world history and was linked to a deeper process in the Atlantic Ocean’s crust, possibly explaining the great earthquakes of Lisbon. The 1755 earthquake, with a magnitude close to 9 on the Richter scale, nearly destroyed Lisbon and caused a tsunami and fires that killed tens of thousands of people; historians estimated about 40,000 deaths. An earthquake with a magnitude of 7.9 occurred off the coast of Portugal in 1969, resulting in 25 deaths. After the 1969 earthquake, researchers began to suspect a subduction-like process despite the lack of a known subduction zone there. Recent events originated from the flat Horseshoe Abyssal Plain, which is not near any known major tectonic faults, and prior delamination had only been observed in continental plates.
“This discovery opens new avenues for understanding the evolution of very early stages of oceanic subduction with implications for plate tectonics,” said Chiara Civiero, a co-author of the study from the University of Trieste. “If even areas without obvious faulting on the surface, such as the Abyssal Plain of the Pétalo, can experience strong earthquakes, then there is a need to revise seismic risk models to include deep processes and structures that cannot be mapped with traditional methods,” said Civiero. Duarte said the delamination process should be factored into the characterization of seismic hazard and risk in Portugal, which could require a rethink of risk maps if the hypothesis was confirmed.
Scientists reported signs of mantle delamination southwest of Portugal, particularly near the Ferradura Abyssal Plain and the underwater mountain range Gorringe Bank. Researchers said the fissure had been forming for at least five million years, driven by the African Plate pushing toward the Eurasian Plate. The delamination process is known to cause seismic events and may be related to the start of subduction, which produced some of the deadliest megathrust earthquakes on Earth, such as the Indian Ocean earthquake in 2004 and the Tōhoku earthquake in 2011.
Scientists said this development could lead to stronger earthquakes and tsunamis in the future and raise risk across the Atlantic basin. Coastal cities from Lisbon to southern Spain and northern Morocco were urged to consider scenarios that had seemed unlikely because of the seismic risk at the confluence of two tectonic plates. The installation of a new generation of submarine communication cables through the Azores, Madeira, and the Ferradura area presented an opportunity to enhance earthquake monitoring.
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