Geologists believe we may be witnessing the birth of a new subduction zone.
Researchers have long puzzled over a plain, featureless area off the coast of Portugal. The seemingly-boring area stood out in 1969 when it triggered a massive earthquake that generated a tsunami. This was highly unusual — earthquakes don’t just happen in random areas. Most often, they take place in tectonically active areas, at the edges of tectonic plates. The correlation is so good that if you’d look at a global map of large earthquakes (see below), it looks like a map of tectonic plates.
So why then did a 7.9 earthquake shake the coast of Portugal? João Duarte, a marine geologist from the Instituto Dom Luiz at the University of Lisbon, believes he has the answer. According to a recent study published by Duarte, the tectonic plate off Portugal’s coast might be peeling away from its top.
Actively tectonic
The Earth might seem static from our point of view, but from a geological perspective, it’s very active. Our planet’s crust is split into rigid plates which are always in motion to each other, at a rate of a few centimeters per year — which, in millions of years, can dramatically change the surface of the Earth.
Naturally, when the plates are moving, they will sometimes be pushing against each other. If one plate is heavier than the other, it will slide beneath it — a process called subduction. We’re quite familiar with subduction as we’ve observed it and its effects in several parts of the world, but we’ve never actually seen it start. Until now.
Suspicions of a potential subduction-related peeling event started after the 1969 earthquake, but it wasn’t until 2012 that researchers got a good view of the area, using seismic wave analysis (which works somewhat similar to an ultrasound). In 2018, Chiara Civiero, a postdoctoral researcher at University of Lisbon’s Instituto Dom Luiz, and her colleagues published a high-resolution peek into Earth in this region, and confirmed the discovery of the unusual blob.
Now, Duarte found new evidence to support this theory in a seemingly innocuous geological layer, one which allows water to percolate (infiltrate) through. This water transforms the minerals inside the plate, transforming them into softer minerals, producing just enough weakness to allow the bottom of the plate to peel away.
“Now we are 100-percent sure it’s there,” Duarte told Nationl Geographic. Other researchers found that above this deep body, which stretches 155 miles below the surface, tiny quakes seemed to tremble.
“It’s a big statement,” Duarte says of the conclusions, acknowledging that he and his team still have work to do. “Maybe this is not the solution to all the problems. But I think we have something new here.”