First discovered in the 1920s, deep earthquakes have baffled the scientific community as to how they occur at such depths, and now, a team from Carnegie Institution for Science believes it's found a way to solve the enigma. The team's findings are published in AGU Advances.
Two keywords: Fluids and diamonds
Earthquakes deep beneath the Earth's surface — called deep-focus earthquakes — occur between 186 and 435 miles (300 to 700 km) below the ground, where, theoretically, they shouldn't happen because the kind of friction needed for earthquakes to occur isn't as strong down there as near the surface.
Earthquakes as we know them typically occur at around 43 miles (70 km) beneath the Earth's surface, where stress builds up a fracture between two blocks of rock (or a fault), which causes them to slide past each other, explains the Carnegie team.
Further down in the ground, however, the all-round temperature is much higher, causing the rocks to deform and accommodate stress more easily.
But, deep-focus earthquakes do happen, and up until now, scientists weren't sure how. Another recent study from the UC San Diego Jacobs School of Engineering points out that a transformation of the rock formation deep below the Earth is linked to these types of earthquakes, and the Cargenie team's focus is not far aligned.
Previous research posits that fluids play an important role in intermediate-depth earthquakes, from 43 to 435 miles (70 to 300 km) down, but because this water couldn't make it as far down as where deep-focus earthquakes take place, scientists largely ignored this reason.
That's when the Carnegie team realized that "Diamonds form in fluids." And "if diamonds are there, fluids are there." And diamonds are there.
Water, diamonds, and deep-focus earthquakes
Deep-Earth diamonds indicate that water must be somewhere down there, and as these diamonds could be brought up to the surface for inspection, the team was able to study them. This provided the scientists with a direct sample of our Earth's deep interior, and how earthquakes would occur there, reported Science.
The team discovered that water and other minerals weren't created in deep-Earth, rather they were transported there by sinking plates.
To test their idea, the scientists built advanced computational models to simulate the temperatures of sinking slabs at great depths. Then, they observed any changes that happened to water-carrying minerals when they were put in temperatures as hot, and pressures as strong, as deep beneath the Earth's surface.
They discovered that the minerals could theoretically carry water down to those depths, which explains how fluids would be brought down there and may be part of why deep-focus earthquakes can happen at such deep locations.
"The nature of deep earthquakes is one of the big questions in geoscience," said Steven Shirey from Carnegie Science. "We needed all four of these different disciplines (geochemistry, seismology, geodynamics, and petrology) to come together to make this argument."