image: To the west, it is possible to recognize two main structures: the Gloria Fault, which is generally considered the main trace of the present-day plate boundary, and the seemingly less active Tydeman Fracture Zone (although this segment of the plate boundary is often referred to as diffuse1,48). Both these structures have experienced high-magnitude earthquakes. To the east, their linear morphological traces gradually give way to thrust fault systems that uplift the seafloor, forming two major bathymetric highs, the Coral Patch Seamount and the Gorringe Bank, that delimit the Horseshoe Abyssal Plain (see also Extended Data Fig. 1). This region has unleashed some of the major historical and instrumental earthquakes in the Atlantic region. Although the precise location of the historical earthquakes is unknown, probable locations are given. For the instrumental earthquakes, the circles roughly correspond to the uncertainties in their locations1,4,5,6,7,16. A more detailed structural map of the Horseshoe Abyssal Plain and surrounding areas is provided in Extended Data Fig. 2. The black arrow indicates the Africa–Eurasia convergence direction in the SW Iberia region for a fixed Eurasian plate48. The West Iberian margin is a classic example of a hyperextended magma-poor margin in which the continental and oceanic crusts are separated by a stripe of exhumed serpentinized mantle (light green)12,24,49. The projection of the seismic anomaly of Fig. 2a is depicted in darker green. Basemap created with the ArcGIS® software by Esri using data available from GEBCO (https://www.gebco.net/). Seismicity data are available at http://www.isc.ac.uk.
Credit: Duarte, J.C., Riel, N., Civiero, C. et al.
A new study published in Nature Geoscience, by researchers from IDL – Instituto Dom Luiz, at the Faculty of Sciences, University of Lisbon, has gathered seismic evidence that could explain a long-standing mystery: what caused the 8.7 mega earthquake that struck Portugal on the 1st of November 1755, causing widespread destruction and a tsunami that killed tens of thousands.
Almost 200 years later, in 1969, another 7.9 earthquake occurred in the same region. Due to the availability of instruments at the time, this event was better documented, and its characteristics have been used to understand the seismic activity in the region. Scientists have been able to pinpoint its origin to the flat Horseshoe Abyssal Plain, and the data point to the occurrence of an unusual process.
It is known that there are two main mechanisms through which Earth’s surface is recycled into the mantle: subduction of oceanic lithosphere and delamination of continental lithosphere. Subduction zones are regions where one tectonic plate dives beneath another and are responsible for the most devastating earthquakes. The fact that both the 1755 and 1969 mega earthquakes occurred far from any known subduction zone has puzzled scientists for decades.
“One of the problems is that these earthquakes occurred on a completely flat plain, far from the main known faults,” João Duarte, a geologist at the University of Lisbon and lead author of the study.
“After the 1969 earthquake, people started to realize that something strange was going on, because it had the signature of a subduction zone, yet there isn’t one there.”
Based on seismic tomography, using land and ocean bottom seismometers (OBS) data, together with numerical models, the authors were able to gather evidence for what may be the first case of oceanic plate delamination, where the lithospheric mantle – a dense layer that together with the crust makes up the Earth’s plates – peels away.
The tomography models seemed to suggest “it was like the base of the plate was separating like the sole of a shoe peeling off,” Duarte said. “That was the first Eureka moment when I thought, ‘aha, there’s something down there’. The second was when the computer simulations reproduced the process of oceanic delamination.”
This research sheds new light on the long-term enigma of why this region is producing some of the strongest earthquakes in Europe’s history and suggests that a new subduction system may develop in the region, which, in time, may lead to the closing of the Atlantic Ocean. Understanding what is happening below the flat Horseshoe Abyssal Plain will be fundamental to mitigating the seismic risk to which the Atlantic margins are exposed.
Journal
Nature Geoscience
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Seismic evidence for oceanic plate delamination offshore Southwest Iberia
Article Publication Date
27-Aug-2025