As the African and Eurasian tectonic plates slowly crashed into each other, part of the Earth's crust today lies upside down deep beneath the Mediterranean.
An overturned plate of Earth's crust lies deep beneath the Mediterranean. Photo: BobHemphill
Spain is prone to unusually deep earthquakes. New research published in the journal The Seismic Record suggests that the cause may be related to tectonic plate flips, Science Alert reported on February 29. Since 1954, there have been five large earthquakes close together at a depth of more than 600 kilometers beneath the Spanish city of Granada, according to geologists Daoyuan Sun of the University of Science and Technology of China and Meghan Miller of the Australian National University. Earthquakes of such great depth are often accompanied by strong aftershocks. But when Sun and Miller examined seismic data from the 2010 earthquake in Spain, they found no aftershocks.
When two tectonic plates collide, they often shift, causing one plate to slide beneath the other in a process called subduction. Sometimes, this collision destroys the submerged portion of the plate, pushing up the crust to form mountains, fusing the two plates into one. In other cases, the plates remain separate and pile up on top of each other, with one eventually sinking into the Earth’s mantle. This is what happens at the boundary between the African and Eurasian plates, as the Mediterranean floor sinks beneath Europe.
Subducting plates form water-bearing magnesium silicate in their upper layers when they come into contact with ocean water. As the plate sinks, the silicate loses water and becomes more brittle, making it susceptible to earthquakes and slowing down seismic waves in a way that seismologists can detect. The seismic waves from the 2010 Granada earthquake lasted unusually long and had an extra phase of activity at the end. This can be explained by the fact that seismic waves travel more slowly at the bottom of the Alboran Plate than they do above it.
"A large amount of water is brought to the mantle transition zone, which shows that the plate is relatively cold," Sun explains. "Given the relatively young age of the seafloor in the western Mediterranean, for the plate to remain cold, the subduction rate must be quite fast, about 70 millimeters per year."
It appears that the rapid rate at which the tectonic plate is sinking has caused the Earth’s crust in this region to flip, taking a pocket of water with it. The overturning occurs when gravity pulls the plate downwards vertically. The new study concludes that the plate flipped completely, leaving the silicate portion facing down, leading to the region’s unusually complex tectonic structure and earthquakes at depths of more than 600 kilometers.
An Khang (According to Science Alert )
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