The cause of Earth’s deepest earthquakes has remained a mystery until now. New research by a team of Carnegie scientists provides evidence that fluids play a key role in deep-focus earthquakes which occur between 300 and 700 kilometers below the planet’s surface.
Most earthquakes occur down to about 70 kilometers below the Earth’s surface.. They happen when stress builds up at a fracture between two plates – known as a fault – causing them to suddenly slide past each other.
However, deeper into the Earth, the intense pressures create too much friction to allow this type of sliding to occur and high temperatures increase the ability of rocks to deform to accommodate changing stresses. Scientists have been able to identify earthquakes that originate more than 300 kilometers bellow the surface.
Ongoing work over the past several decades has shown that water plays a role in intermediate depth earthquakes – those that occur between 70 and 300 kilometers below the Earth’s surface. In these cases, water is released from minerals, which weakens the rock around the fault and allows the blocks of rock to slip.
But scientists could not explain deep-focus earthquakes because it was believed that water could not go down that far. The depths of rare deep-Earth diamonds changed this thinking because diamonds form in fluids.
These diamonds capture pieces of minerals from the surrounding rock called inclusions which had the distinct chemical signature of oceanic crust. This means that the water and other materials were carried down as part of a sinking oceanic plate.They were also able to show that the slabs that could theoretically carry water to these depths were also the ones producing the previously unexplained deep earthquakes.
https://phys.org/news/2021-05-deep-earth-mysterious-earthquakes.html