Down to Earth

Down to Earth

Meet the earthquakes that happen 600km underground

An earthquake four times bigger than the 1906 San Francisco one struck off the coast of Siberia earlier this year, but more than 600km down. antiquationadmiration

A little more than 90 years ago, British geologist Herbert Hall Turner noticed some earthquake data that suggested a surprising explanation. The only way to explain it was if the quake had occurred hundreds of kilometres beneath the Earth’s surface, instead of the more commonly seen near-surface earthquakes.

Since Turner’s observations, deep earthquakes have fascinated seismologists. It is still unclear why they happen, but two studies just published in the journal Science, taking different approaches, conclude that they are probably a result of rapid changes in minerals at that depth.

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”, which causes the most devastating and dangerous earthquakes, goes down to about 15km beneath the surface.

As you go deeper, pressure and temperature both increase rapidly, so the nature of earthquakes changes. Rocks move slowly, speaking on geological time scales, when 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 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 University of California Santa Cruz took a step ahead to analyse 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 at that depth it was barely noticeable 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. In contrast, however, 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.

A slice through Earth. Jeremy Kemp

During the Okhotsk event, the Pacific plate of Earth’s crust was drawn down into the hot mantle that makes up much of the planet’s interior. What Lay found was that the seismic energy released in the event was so large that it caused fractures as great as 180km long below the surface. The rock ruptured at close to the speed of sound, which in the rock would be as much as 14,000 km/h.

But what caused such rapid rupture? Alexandre Schubnel of Ecole Normale Supérieure suggests an explanation, which hinges a the mineral making up the deep rock, called olivine. To be sure he designed lab experiments that could mimic 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 what those in the earth, the reason these experiments can be trusted is because the creaks and groans of minerals in a lab show similar characteristics as that of large earthquakes. So, even though Suchbnel’s idea is not new, it confirms experimentally suggestions made by researchers before. It opens the way to studying deep earthquakes in the safety and comfort of the lab.