Deepest earthquake ever detected should have been impossible

The Bonin Islands form part of the geologic arc known as Izu-Bonin–Mariana Arc. The arc is located above the subduction zone where the Pacific Plate is slowly sinking beneath the Philippine Sea Plate.
Scientists discovered the Earth's deepest earthquake, measuring 467 miles (751 km) below its surface.

This depth places the earthquake in the lower mantle where seismologists thought it would be impossible to cause an earthquake. Because rocks bend and deform more under extreme pressures than they are to fracture with an abrupt release of energy, However, minerals do not always behave exactly as expected. Pamela Burnley, a professor of Geomaterials at University of Nevada Las Vegas, was not involved in this research. They may remain in their old configurations even at higher pressures, which would allow them to transform into more earthquake-resistant states.

Burnley stated that just because they should change doesn't necessarily mean they will. The earthquake could reveal that Earth's boundaries are more fuzzier than we often give them credit for.

Crossing the border

First reported in Geophysical Research Letters in June, the quake was an aftershock to the 7.9-magnitude earthquake that rocked the Bonin Islands off the coast of Japan in 2015. Eric Kiser, a University of Arizona seismologist, detected the earthquake using Japan's Hinet array of seismic stations. John Vidale, a University of Southern California seismologist, stated that the array is the most powerful system currently in use for detecting earthquakes. Because the quake could not be felt at the surface of the earth, sensitive instruments were required to locate it.

Vidale said that other researchers still need to confirm the depth of the earthquake, but the finding appears reliable. Vidale stated that the researchers did a great job and that it seems likely to be right.

The 2015 earthquake that struck Japan was the strongest ever. It reached down into Earth's mantle. (Image credit: Shutterstock)

This makes the earthquake quite a shocker. Most earthquakes are small and originate within the Earth's crust or upper mantle, which is located within the Earth's first 62 miles (100km) below the surface. The crust is only 12 miles (20km) thick and the rocks are brittle and cold. Burnley stated that these rocks can only bend slightly before breaking under stress. This is similar to a spring-coiled energy release. Higher pressures and deeper in the crust make the rocks more susceptible to breaking. High pressures pushing on fluid-filled pores within the rocks can cause earthquakes. Burnley stated that rocks can also break apart under these conditions.

These types of dynamics can explain quakes down to 249 miles (400km), which is still in upper mantle. Even before 2015 Bonin aftershocks, quakes were still being observed in the lower mantle. They have been measured at 420 miles (670km) Burnley stated that those quakes are long mysterious. Fluids no longer trigger because the pores that hold water in rocks have been closed.

She stated, "At that depth we think all the water should be driven away, and we are definitely far, far from where we would find classic brittle behavior," This has been a problem for us all."

Minerals that change

Problems with earthquakes greater than 249 miles deep are due to the way minerals react under pressure. A large portion of the planet's crust is composed of olivine, a shiny and green mineral. The pressures at 249 miles below caused the olivine atoms to change into a new structure. This blue-ish mineral is called wadsleyite. Wadsleyite is rearranged into ringwoodite another 62 miles (100km) further. The mantle is 423 miles (680km) deep, and ringwoodite begins to break down into two minerals: bridgmanite (or periclase). Although geoscientists cannot probe the Earth that deep, they can use laboratory equipment to create extreme pressures at the surface. Geophysicists can detect signs of seismic waves by studying vibrations from large earthquakes, because different minerals phase's seismic waves behave differently.

This transition marks the end and beginning of the upper mantle. It is not the names of these phases that are important, but how they behave. Burnley said that it is similar to graphite or diamonds. Both are made from carbon, but in different ways. Graphite, which is stable on the Earth's surface and diamonds deep within the mantle, are two forms of carbon. Both behave differently. Graphite can be soft, gray, and slippery while diamonds can be extremely hard, clear, and durable. Olivine becomes more flexible and less likely break when it is exposed to higher pressure. This can trigger earthquakes.

Geologists have been puzzled by earthquakes in upper mantle since the 1980s. They still aren't sure why they happen. Burnley and Harry Green, her doctoral advisor, came up with a possible explanation. The 1980s saw the discovery that the olivine mineral phases weren't as neatly defined. For example, in certain conditions, olivine could skip the wadsleyite stage and go straight to ringwoodite. Under enough pressure, the mineral may actually fracture instead of bend right at the transition between olivine and ringwoodite.

Burnley stated that if there wasn't any transformation in my sample, it wouldn’t break. It would break if it had transformation and was squishing it simultaneously.

Burnley and Green published their findings in 1989 in Nature. They suggested that earthquakes below 249 mi could be explained by the pressure in the transition zone.

The mineral olivine is a major component of Earth's mantle. (Image credit to underworld111/Getty Images).

Going deeper

However, the new Bonin earthquake is much deeper than the transition zone. It originated at 467 miles below the surface, in a location that should have been in the lower mantle.

Heidi Houston, a geophysicist from the University of Southern California, suggested that there may be a problem with the boundary between the upper mantle and the lower mantle. A subduction zone is an area around Bonin Island where a slab oceanic crust dives beneath a slab continental crust. This kind of thing can cause a warping effect.

Houston said that it was a complex place and we don't know where the boundary between the upper mantle and lower mantle is." Houston spoke to Live Science.

According to the paper's authors, the subducting crust could have settled on the lower mantle sufficiently to place the rocks under extreme stress. This would have generated enough heat and pressure to cause an unusual break. Burnley believes that the most likely explanation is due to minerals acting strangely or badly. She said that the continental crust that plunges towards the Earth's center is cooler than surrounding materials. This means that minerals might not be warm enough for the necessary phase changes to occur at the pressure they are expected to.

Burnley stated that graphite and diamonds are good examples. Although diamonds don't form spontaneously at Earth's surface (they aren't stable), they won't turn into graphite if they're stuck into engagement rings. This is because carbon atoms require a certain amount energy to rearrange and at Earth's surface temperatures that energy isn’t available. (Unless the diamond is zapped with an Xray laser.

Burnley suggested that something similar could happen in depth with olivine. It could be subject to enough pressure to become a non-brittle mineral, but if it is too cold, such as because there is a huge slab of chilly continental crust around it, it may remain olivine. This could explain why an earthquake might originate in the lower crust. It's not as hot as scientists think.

Burnley stated that "My general thought is that if the material's temperature is low enough to cause enough stress to suddenly release it in an earthquake," Burnley added.

Houston stated that no matter what the cause, the quake is unlikely to happen again. About half of subduction zones in the world experience deep earthquakes. The kind of large earthquake that precedes this one occurs only every two to five year on average.

She said, "This is a very rare occurrence."

Original publication on Live Science