A newly identified tectonic structure off the coast of Portugal may help explain why the region has experienced powerful earthquakes far from known fault zones.

The discovery, detailed in the journal Nature Geoscience, suggests that a deep geological process beneath the seafloor could be setting the stage for future seismic events in the area.

Researchers utilized seismic imaging and data from ocean-bottom and land-based sensors to examine activity beneath the Horseshoe Abyssal Plain, a flat seafloor region southwest of Portugal.

Their findings revealed a dense, high-velocity structure deep below -- evidence of a block of ancient oceanic lithosphere beginning to break apart and sink into the Earth, a process known as delamination.

This type of geological activity has only previously been observed in continental plates. In this case, scientists believe water penetration triggered a chemical reaction called serpentinization, weakening the oceanic crust and allowing it to separate from the mantle more easily.

The result is a slow but ongoing descent of the plate's lower portion, which researchers link to the potential formation of a new thrust fault.

A groundbreaking study reveals that a rare oceanic plate delamination beneath the Horseshoe Abyssal Plain may explain Portugal's devastating earthquakes, including the infamous 1755 disaster, linking ancient tectonic processes to modern seismic risks. pic.twitter.com/pXH5FCg6lJ

-- Nyra Kraal (@NyraKraal) September 1, 2025

João Duarte, one of the study's authors, explained that two pre-existing fracture zones may have isolated the sinking lithospheric block. As the block moved northward, it likely led to the development of a major fault near the current zone of high-magnitude seismic activity.

That area includes the epicenter of the 1969 magnitude 7.9 earthquake and possibly the 1755 Lisbon earthquake, one of the deadliest in European history.

The team believes this process may represent the early stages of subduction -- the movement of one tectonic plate beneath another.

While subduction is known to cause some of the world's strongest earthquakes, the exact mechanics of how it begins have remained unclear. Duarte noted that once delamination starts, subduction may follow more easily under the right conditions.

This breakthrough supplies a new explanation for why large earthquakes continue to strike in a region not traditionally associated with active tectonic boundaries. It also opens the door for a better understanding of seismic risk in both Portugal and Spain.

Researchers say the findings not only shed light on historic earthquakes but could help improve forecasting and disaster preparedness.

Ongoing studies may also reveal similar geological processes in other parts of the world where tectonic activity has been difficult to explain.