Jakarta –
As Africa and Eurasia slowly collide, the rumbling of the Earth provides a seismographic picture of what was once part of our planet’s surface, which now lies upside down deep beneath the Mediterranean Sea.
Spain is particularly prone to rare, deep earthquakes, and a new study suggests that these overturning tectonic plates may have something to do with them.
“Since 1954, there have been five large deep-focus earthquakes located close together at a depth of more than 600 kilometers beneath Granada in Spain,” explained geologists Daoyuan Sun from the University of Science and Technology of China and Meghan Miller from the Australian National University, quoted from Science Alert.
Earthquakes at these depths are usually followed by significant aftershocks. But when Sun and Miller examined seismic data for the 2010 Spanish earthquake, no aftershocks were visible.
When tectonic plates push against each other, they often shift so that one tectonic plate slides under another in a process called subduction.
Sometimes these collisions destroy parts of sinking plates, lifting the Earth’s crust, creating mountains and uniting the fate of the two plates into one.
At other times, the grappling crust would remain separate but stacked, with one plate gradually sinking further toward Earth’s mantle. This is what happens at the boundary between the African and Eurasian plates, where the floor of the Mediterranean slowly sinks beneath the European continent.
Foto: Daoyuan Sun dan Meghan Miller
Subducted slabs form hydrous magnesium silicate in their uppermost layers when exposed to seawater. As the slab sinks, these silicates dehydrate and become more brittle, making them more susceptible to earthquakes and slowing seismic waves in a way that seismologists can detect.
The seismic waves during the 2010 Granada earthquake were very long lasting and had a late additional phase of activity. This can be explained by seismic waves moving more slowly at the bottom of the Alboran plate than at the top.
“Large amounts of water have been carried into the mantle transition zone, indicating a relatively cold slab,” Sun explained.
“Given the relatively young age of the seabed in the western Mediterranean, in order for the plate to remain cool, the subduction speed must be quite fast, for example a moderate speed of around 70 millimeters per year,” he explained.
It appears that the rapid rate at which the plates are sinking is helping this part of the Earth’s crust to overturn, taking pockets of water with it. This slab rollback process occurs when gravity helps pull the slab into a downward vertical rotation, like a dive.
The new study goes further by concluding that the region has completely overturned, landing the silicates face down and could explain the strange tectonic structure of the region and its occasional earthquakes more than 600 kilometers deep.
“This confirms that the slab beneath the Betics in southern Spain is subducted oceanic lithosphere,” the team wrote, explaining that this process formed the Beltic-Rif or Gibraltar arc that forms the western Mediterranean.
Watch the video “M 5.7 earthquake shakes Banten, felt as far away as Jakarta”
(rns/rns)
2024-03-03 04:07:58
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