A little more than 90 years ago, British geologist Herbert Hall Turner noticed some data that suggested something unexpected. The only way to make sense of the data was if an earthquake occurred hundreds of kilometers beneath the Earth’s surface.

Since Turner’s observations, deep earthquakes have fascinated seismologists. It is still unclear why they happen, but two studies recently published in Science use different approaches yet reach the same conclusion. These quakes are probably a result of rapid changes in minerals that propagate at up to 14,000 km/hour (nearly 8700 mph).

Such deep earthquakes do not have immediate consequences for humans. But they hold clues about destructive quakes in the Earth’s shallower crust, making it important to understand them.

Not just superficial

Most earthquakes occur in the stiff, brittle outer shell that includes the Earth’s crust. This “seismogenic zone” causes the most devastating and dangerous earthquakes, but it only goes down to about 15km (roughly nine miles) beneath the surface.

As you go deeper, pressure and temperature both increase rapidly, so the nature of earthquakes changes. Rocks move slowly on geological time scales, pushed or pulled by different forces acting on them. At depth, they appear to flow like soft toffee rather than break like peanut brittle.

This is why Turner’s observations of earthquakes more than 600km (372.8 miles) below the surface were puzzling. If the rocks flow slowly, then there shouldn’t really be any sudden shocks that cause an earthquake. Rather, there should be gentle, continuous readjustments to stress.

Suggestions have been floated in the past about what triggers such earthquakes. But Thorne Lay of the University of California at Santa Cruz managed to analyze a deep earthquake that occurred this year on May 24 in the Pacific Ocean beneath the Okhotsk plate. At a magnitude of 8.3, it was four times greater than the 1906 San Francisco earthquake. Indeed, it was the biggest ever recorded at a depth of more than 600km. A near-surface earthquake of the same magnitude could’ve been very destructive, but this was barely noticeable, at least at the surface above.

Recent analysis of an earthquake at Bhuj, India, in 2001 suggests it shared similarities to the Okhotsk event, although it was just 16km deep. At that depth, it caused terrible devastation, including an estimated 20,000 deaths. “There may be things we don’t understand about more shallow earthquakes that we can learn from studying these deep earthquakes,” said Bob Myhill of the University of Bayreuth.

During the Okhotsk event, the Pacific plate was drawn down into the hot mantle that makes up much of the planet’s interior. Lay found that the seismic energy released in the event was so large that it caused fractures as great as 180km (111.8 miles) long near the depth of the earthquake. The rock ruptured at close to the speed of sound, which would be as much as 14,000 km/h under those conditions.

A change of structure

But what caused the rapid rupture? Alexandre Schubnel of Ecole Normale Supérieure suggests an explanation, one that hinges on one of the minerals that make up the deep rock, called olivine. To be sure, he designed lab experiments that could mimic the deep earth.

Schubnel found that, above a critical temperature and pressure, olivine changes into another mineral called spinel. Under stress, this sudden change creates fractures, much like those seen in the earthquake. The mineral change releases stress instantaneously, in just the same way as stress was relieved in the deep earthquake under the Pacific Ocean.

There is one critical difference however. To make the experiments easier, the olivine used by Schubnel in the lab contained the element germanium instead of silicon. Germanium-olivines are known to behave slightly differently than silicon-olivines, and this may make a lot of difference 600km below the surface.

Still, while the mini-earthquakes seen in the lab were a million billion times smaller than those in the earth, the creaks and groans of minerals in the lab show similar characteristics as that of large earthquakes. So, even though Schubnel’s idea is not new, it gives us experimental confirmation of suggestions made before by researchers. Plus, it opens the way to studying deep earthquakes in the safety and comfort of the lab.

Science, 2013. DOI: 10.1126/science.1240206 and 10.1126/science.1242032 (About DOIs).

Simon Redfern is professor of mineral physics at the University of Cambridge. This article was first published on The Conversation.