Fonte: Nat Geo
Subduction zones form where tectonic plates collide, with one plate diving beneath the other and into Earth’s mantle. Sometimes these collisions are gradual, but often they occur in big lurches that can trigger quakes. Because subduction zones are generally on seabeds, earthquakes in these zones can set off tsunamis, like the killer wave that devastated Japan last month. For millions of years the African plate, which contains part of the Mediterranean seabed, has been moving northward toward the Eurasian Plate at a rate of about an inch every 2.5 years (a centimeter a year). Now studies of recent earthquakes in the region indicate that a new subduction zone may be forming where the plates are colliding along the coasts of Algeria and northern Sicily (see a map of the region). “Formation of a new subduction zone is very rare,” said study leader Rinus Wortel, a geophysicist at Utrecht University in the Netherlands.
Europe and Africa Switching Geologic Roles
According to Wortel, the opposite situation existed about 30 million years ago, when the African plate was diving under the Eurasian plate along a sizable subduction zone in the western Mediterranean. There, the dense rocks of the African seabed were being thrust beneath the European plate. Over millions of years, Africa moved so far north that none of the plate’s seabed was left in the western Mediterranean. All that remained were the rocks of the continent itself, which were lighter than the seabed and wouldn’t subduct, Wortel said. But the two plates have continued to converge, building up tectonic stresses. Part of this stress was taken up by the buckling of the Eurasian plate into the Alps, Caucasus, and Zagros mountain ranges. (Related: “Alps Glaciers Gone by 2050, Expert Says.”) Now, based on the locations and motions of recent earthquakes along the plate borders, Wortel and colleagues think subduction is starting up again—but this time with Europe being thrust under Africa. The new subduction zone, announced at a recent meeting of the European Geosciences Union in Vienna, is an exciting find, Wortel said, because such regions tend to exist for long time periods, geologically speaking.
Earthquake Risk Underestimated in the Med?
Other scientists are intrigued by the possible subduction zone—but they remain cautious. “I didn’t hear the talk, but it’s perfectly plausible,” Seth Stein, a geophysics professor at Northwestern University in Evanston, Illinois, said in an email. For instance, other parts of the Mediterranean region—such as mainland Italy—have seen tectonic changes in the past two million years, he said. Still, deciphering these changes is a very complex process, said Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Laboratory at Oregon State University in Corvallis. “I’d have to spend a week with the data to have any opinion that was worth anything,” he said by email. Study leader Wortel added that the formation of a subduction zone is a slow process: “These processes happen at the time scale of several million years,” he said. For example, he said, most established subduction zones are marked by giant undersea trenches. A similar trench should eventually form in the Mediterranean—but certainly not overnight. Wortel does believe that the new data may mean the earthquake risk in the western Mediterranean has been underestimated—and may be increasing. (See related pictures of the aftermath of a magnitude 6 earthquake in Italy in 2009.) “It is usually not considered to be a region of enormous seismic activity—not of the giant magnitude we experienced in Japan last month,” Wortel said. That may simply be due to historical complacency. “Even though something has not occurred in a hundred years, it does not mean there is no risk.” In fact, he noted, 70,000 people were killed in Messina, Italy, in 1908 when a magnitude 7.1 earthquake produced a tsunami with waves reported at 40 feet (12 meters) high. And one of the most devastating earthquakes in European history occurred somewhere west of the Strait of Gibraltar in 1755, sending a giant wave into Lisbon, Portugal, and killing—by some estimates—as many as a hundred thousand people.