The researchers detailed their findings in a paper published May 26 in the journal AGU Advances.
What causes deep-focus earthquakes?
Deep-focus earthquakes are earthquakes that occur between 185 and 435 miles below the Earth's surface. First detected in the 1920s, these earthquakes continue to confound scientists to this day.
"The big problem that seismologists have faced is how it's possible that we have these deep-focus earthquakes at all," said co-author Lara Wagner.
Usually, earthquakes occur near the surface after stress builds up between two blocks of rock. When enough stress has accumulated, it causes this pair of rocks to suddenly slip and slide past each other, triggering earthquakes.
But intense pressures deep in the Earth create too much friction for this sliding to occur. The extreme temperatures also enhance the ability of rocks to deform and accommodate changing stresses.
"Once you get a few tens of kilometers down, it becomes incredibly difficult to explain how we are getting slip on a fault when the friction is so incredibly high," Wagner added.
Recent research found that a reservoir of water lurks more than 400 miles beneath the surface. This reservoir is three times the volume of what is found on the surface but is not an actual underground sea. Instead, all the water deep in the Earth is trapped inside rocks called ringwoodites.
These are bright, blue rocks formed under high temperatures and pressures in the Earth's mantle. The water they hold get squeezed out of them so that the rocks appear to sweat.
At the same time, studies suggested that water trapped underground plays a part in setting off intermediate-depth earthquakes, which occur between 45 to 185 miles beneath the ground. These studies showed that water released from minerals weakens the rock around the fault and allows slabs of rock to slip.
But scientists were not convinced that this could explain deep-focus earthquakes due to the notion that fluids couldn't make it far enough down into the planet's interior. Wagner and her colleagues challenged this notion in their study, noting that some rare diamonds originate in the same depths as deep-focus earthquakes.
"Diamonds form in fluids," Wagner explained. "If diamonds are there, fluids are there."
Searching for evidence of water in Earth's deep interior
Diamonds contain pieces of minerals that the researchers sampled to confirm the presence of fluids. These minerals, which are also known as inclusions, get trapped in diamonds during the latter's formation. They make jewelry less expensive but are invaluable to geologists since they provide an overview of the planet's deep interior.
In the diamonds the researchers studied, the inclusions had the distinct chemical signature of similar materials found in the Earth's oceanic crust. This indicated that the water and other materials found deep in the planet were not created there. Instead, they were carried down as part of a sinking oceanic plate.
The researchers then simulated the temperatures of sinking slabs of rock at much greater depths than had been attempted before. This was to show that the water-bearing minerals incorporated in these slabs would be able to retain water even when subjected to the intense heat and pressures of the Earth's deep interior.
In the simulations, the minerals in intensely heated tectonic plates failed to retain water. But those in cooler oceanic plates were able to carry water at the depths associated with deep-focus earthquakes. When the researchers compared their simulations to real-life seismological data, they found that these oceanic plates were also the ones experiencing deep-focus earthquakes. (Related: Gooey rocks deep within the San Andreas fault generate silent earthquakes, study finds.)
These findings led the researchers to conclude that the fluids that survive the trip down into the Earth's interior trigger or are related to deep-focus earthquakes.
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