Scientists have recognized a beforehand unknown seismic phenomenon that was probably accountable for Europe’s largest recorded earthquake that struck Lisbon in 1755, killing tens of hundreds.
Researchers led by the University of Lisbon discovered {that a} piece of tectonic plate sinking in an space of the Atlantic Ocean below the Iberian Peninsula was accountable for this 8.6 magnitude megaquake.
While this phenomenon, known as “lithosphere delamination”, is thought to trigger seismic occasions, it had beforehand solely been documented on continents.
The findings, printed this week within the journal Nature Geoscience, open up new views on earthquake danger prediction in Europe.
In the research, scientists used subtle quake mapping methods to grasp the construction of the Earth at nice depths.
They assessed an intensive dataset collected between 2007 and 2013 by 387 broadband land stations unfold all through the Ibero-Maghrebian area.
Scientists additionally included knowledge from 24 ocean-bottom seismometers deployed offshore Southwest Iberia.
Using such intensive knowledge, researchers meticulously constructed an in depth mannequin of seismic wave motion extending from the crust-mantle boundary to depths of 800 kilometres.
They detected a high-velocity anomaly as much as 250km deep beneath the Horseshoe Abyssal Plain, within the south-western area of the Iberian Peninsula, between the African and Eurasian plates, an space recognized for its intense seismic exercise.
This area was the scene of historic earthquakes such because the Lisbon earthquake of 1755 and the San Vincenzo earthquake of 1969, which had a magnitude of seven.9.
Scientists discovered that there are not any apparent floor indicators pointing to seismic dangers within the area, like twisted landscapes or underwater mountains.
However, they noticed {that a} portion of the oceanic floor is sinking into the Earth’s mantle, creating new faults.
In this course of, one plate can slide below one other if compression happens alongside the contact margin – one of many essential mechanisms by way of which the Earth’s floor is recycled into the mantle.
These options might clarify the origin of historic earthquakes of nice magnitude within the area, 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,” stated Chiara Civiero, a co-author of the research 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 stated.