Abyssal Plain of the Horseshoe, a geological formation in the Atlantic Ocean not far from the Gorringe Bank seamount, located at the boundary between the Eurasian and African tectonic plates, was the epicenter of an approximately 8-magnitude earthquake on the Richter scale that shook Lisbon and other regions in 1969.
The fact that it is a flat geological structure without significant known seismic faults had posed “a mystery” to the scientific community regarding how such a region could trigger high-magnitude earthquakes. However, a study from the Faculty of Sciences of the University of Lisbon, published in ‘Nature Geosciences’, offers a new potential explanation.
No “obvious fault” had ever been discovered in that region that could explain an earthquake of the magnitude of the one in 1969, stated João Duarte, one of the study’s co-authors, a geologist, professor at the Faculty of Sciences of the University of Lisbon, and researcher at the Dom Luiz Institute.
The lack of finding “a fault large enough to generate an earthquake like the one in 1755,” believed to have had a magnitude close to 9 on the Richter scale, adds to the mystery driving the research.
João Duarte explained that the newly revealed insight which could explain the origin of both earthquakes is that “a portion of the tectonic plate is splitting apart,” in a process known as ‘delamination’.
This ‘delamination’ suggests that the plate is undergoing a horizontal fracture, akin to a rock being split by a blade, creating a fissure that causes the lower part to sink, reaching a depth of 200 kilometers towards the Earth’s mantle, whereas normally it should be around 100 kilometers.
The upper part of the plate remains in an unchanged horizontal position, making it impossible to detect geological changes at that site by observing the seabed, Duarte explained to Lusa.
This slow horizontal separation of the plate has been occurring for five to ten million years, identified using an “Earth ultrasound,” said the professor, explaining the use of seismic tomography and the sound captured from the seismic events to understand what happens below the ocean floor.
“We conducted a study that placed seismometers on the seabed for eight months to record small earthquakes. We realized that there was a ‘cluster’, a group of small quakes at a considerable depth, around 30 to 40 kilometers deep, which is somewhat unusual. Therefore, a combination of various observations suggests a process occurring there that is generating seismicity,” the researcher explained.
In addition to data observation and analysis, computational models were developed to simulate the ‘delamination’ process.
The friction created and the energy released by moving tectonic plates causes earthquakes. This geological feature’s ability to generate earthquakes, despite not being a seismic fault, is explained by the fact that the space created by the laminated cut in the plate does not remain empty.
“We know that the African plate is also moving very slowly, converging with the Eurasian plate. If you imagine two books with one starting to slide into the other, that’s how the plates are converging. In essence, the area where the plate begins to separate behaves somewhat like a sheet, starting to insert itself into the other plate. There is always a contact, but it’s a more horizontal contact. So, no hole or space remains. That space is later occupied by another rock,” João Duarte explained.
João Duarte hopes this study will lead to more detailed future investigations of the region. Given what is known, the ‘delamination’ process must be considered “in assessing seismic hazard and risk” in the country, already deemed a high-risk area due to the confluence of the two tectonic plates in this region.
The new generation of submarine cables, communication lines connecting both sides of the Atlantic and passing through the Azores and Madeira archipelagos and the Abyssal Plain of the Horseshoe, presents an opportunity.
“They will have seismic sensors, so as the cables pass through that area, it will be possible to better monitor and characterize the seismicity. We may also gather more data and records,” he stated.
Predicting earthquakes remains a chimera, but João Duarte believes that artificial intelligence might provide a breakthrough, even though learning from previous events and data poses a challenge since significant earthquakes are rare phenomena.
The key might lie in a more systematic study of minor earthquakes, which occur daily, even in Portugal, “with some hope” of understanding the seismic process and using statistics to infer and comprehend larger magnitude earthquakes.
[Updated at 15:40]
