Scientists have identified a previously unknown seismic phenomenon that was likely responsible for Europe’s largest recorded earthquake that struck Lisbon in 1755, killing tens of thousands.
Researchers led by the University of Lisbon found that a piece of tectonic plate sinking in an area of the Atlantic Ocean under the Iberian Peninsula was responsible for this 8.6 magnitude megaquake.
While this phenomenon, called “lithosphere delamination”, is known to cause seismic events, it had previously only been documented on continents.
The findings, published this week in the journal Nature Geoscience, open up new perspectives on earthquake risk prediction in Europe.
In the study, scientists used sophisticated quake mapping techniques to understand the structure of the Earth at great depths.
They assessed an extensive dataset collected between 2007 and 2013 by 387 broadband land stations spread throughout the Ibero-Maghrebian region.
Scientists also incorporated data from 24 ocean-bottom seismometers deployed offshore Southwest Iberia.
Using such extensive data, researchers meticulously constructed a detailed model of seismic wave movement extending from the crust-mantle boundary to depths of 800 kilometres.
They detected a high-velocity anomaly up to 250km deep beneath the Horseshoe Abyssal Plain, in the south-western region of the Iberian Peninsula, between the African and Eurasian plates, an area known for its intense seismic activity.
This region was the scene of historic earthquakes such as the Lisbon earthquake of 1755 and the San Vincenzo earthquake of 1969, which had a magnitude of 7.9.
Scientists found that there are no obvious surface signs pointing to seismic risks in the region, like twisted landscapes or underwater mountains.
However, they observed that a portion of the oceanic surface is sinking into the Earth's mantle, creating new faults.
In this process, one plate can slide under another if compression occurs along the contact margin – one of the main mechanisms through which the Earth's surface is recycled into the mantle.
These features could explain the origin of historical earthquakes of great magnitude in the region, researchers say.
“This discovery opens up new avenues for understanding the evolution of the very early stages of oceanic subduction with important implications for plate tectonics,” said Chiara Civiero, a co-author of the study from the University of Trieste.
“If even areas without obvious surface faults, such as the Horseshoe Abyssal Plain, can be subject to strong earthquakes, there is a need to revise seismic hazard models to include deep processes and structures that cannot be mapped using traditional methods,” Dr Civiero said.
