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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 the Carnegie Institution for Science believes they have found a way to solve the mystery. The team’s results have been published in AGU Advances.
Two key words: fluids and diamonds
Earthquakes deep beneath the Earth’s surface — called deep focal earthquakes — occur between 186 and 435 miles (300 to 700 kilometers) underground, where, in theory, they shouldn’t happen because the type of friction needed for earthquakes to occur isn’t strong there near the surface. .
Earthquakes as we know them typically occur about 43 miles (70 km) below the Earth’s surface, where stress causes a fracture between two blocks of rock (or fault), causing them to slide over each other, the Carnegie team explains.
However, the overall temperature is much higher in the ground, which causes the rocks to deform and absorb stress more easily.
But high-focal earthquakes do happen, and until now, scientists weren’t sure how that might happen. Another recent study from the University of California, San Diego Jacobs School of Engineering suggests that shifting rock formations deep underground is associated with these types of earthquakes, and that the Kargini team’s focus isn’t far off.
Previous research hypothesizes that fluids play an important role in earthquakes of medium depth, from 43 to 435 miles (70 to 300 kilometers), but because this water cannot reach where high-focal earthquakes occur, scientists have largely ignored this cause.
That’s when the Carnegie team realized that “diamonds form in liquids.” And “if diamonds are there, there are liquids” and diamonds are there.
Water, diamonds and high-focus earthquakes
Diamonds found deep in the earth suggest that water must be somewhere in there, and since these diamonds can be lifted to the surface for examination, the team was able to study them. This provided scientists with a first-hand sample of deep underground, and how earthquakes would occur there, Science reports.
The team discovered that water and other minerals did not form deep in the Earth, but were transported there by sinking plates.
To test their idea, the scientists built advanced computational models to simulate the temperatures of submerged plates at great depths. Next, they noted any changes that occurred to the water-bearing minerals when they were placed at high temperatures, strong pressures, such as deep within the Earth’s surface.
They discovered that minerals could theoretically carry water to those depths, which explains how fluids descend there and may be part of why deep-focus earthquakes occur at such deep sites.
“The nature of deep earthquakes is one of the big questions in Earth sciences,” said Stephen Sherry of Carnegie Science. “We needed all of these four different disciplines (geochemistry, seismology, geodynamics, petrology) to work together to make this argument.”
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