Massive Earthquakes Could Be Linked to Tiny Changes in Earth's Molten Core

San Andreas Fault
The San Andreas Fault. Ian Kluft/CC

Wouldn't it be nice to have a better idea of when the next earthquake is going to hit? Of course it would. While predicting where the next earthquake will hit is nearly impossible, scientists think they may now have a better, general sense of how frequent quakes might be—and the next five years might bring more very serious ones than usual.

That prediction is based on tiny differences in the length of the day on Earth, which may act as an early warning signal for earthquake-prone years, according to a news article published in Science, about findings presented at the Geological Society of America meeting in Seattle earlier in October.

The findings, which have not yet been published, were based data from some of the strongest earthquakes that happened in the last century—those that were a 7-magnitude quake and above. The devastating earthquake that hit Mexico City earlier this year had a 7.1 magnitude recorded.

For some reason, the Earth seems to slow down about five years before there are more highly-damaging earthquakes than usual, Roger Bilham of the University of Colorado in Boulder told Newsweek. The planet began to slow down in 2012.

Mexico City Earthquake Damage Interior
View of an office in a neighboring building to that in Alvaro Obregon 286, which collapsed during the recent 7.1-magnitude earthquake and where rescuers are still working on the recovery of victims, in Mexico City on September 28, 2017 STR/AFP/Getty Images

In geological time, 100 years is like the blink of an eye. But humans have only been able to measure earthquakes since the early 1900s, and early seismometers couldn't necessarily pick up all the earthquakes that were less than magnitude 7. This means the researchers were only able to detect just over a dozen earthquakes.

Earthquakes happen because the crust is not all one piece. There are cracks—fault lines—and the Earth is not made up of just crust; underneath are layers of viscous, liquid-like rock and metal. As pieces of crust are floating around, they can wind up grinding up against each other or sliding over or under one another. That grinding and slipping is what causes an earthquake.

So Bilham proposes that maybe—and it's still just a maybe—the difference in the speed of Earth's rotation could be causing more earthquakes. It makes some sense; when a car or a subway stops abruptly, people who aren't hanging on or buckled in may continue to move at the car or subway's previous velocity. That may also be true for tectonic plates, in a far less extreme way.

But what is making the Earth slow down? It could be subtle fluctuations in the movement in Earth's core.

The core is a chaotic place, Bilham said. "It's very unstable. And these instabilities, over the course of decades, are able to influence the rotation rate of the Earth. So what it means is that the core may well be responsible for modulating earthquakes at the Earth's surface. It doesn't produce them," he noted, "but it looks as though the core may indirectly influence the motion of the plates by changing the rotation rate."

However, Bilham noted, the math isn't quite adding up. There must be other factors besides changes at the core contributing to this pattern, too.

One thing, at least, is clear: there are more milliseconds during a day in this century than there were a few hundred or millions of years ago. Variations in the length of a day on the order of a few extra milliseconds here or a few fewer there have also been observed since the 1960s.

"The striking thing we've noticed is that the deceleration of the Earth is five years ahead of the earthquakes," Bilham said. "If it were the other way around, it would be boring."

It would be less interesting because we already know that earthquakes can feed into these decelerations. "We know that earthquakes can change the rotation rate of the Earth, on a very, very tiny level," said Brian Koberlein, an astrophysicist at the Rochester Institute of Technology.

The moon has the most influence on these changes, but earthquakes do, too. "If you have something like an earthquake, where a chunk of the ground will shift, that will very slightly change the shape of the Earth," Koberlein said. That change in the Earth's shape can have an impact on the rate at which it's spinning—the same way that a figure skater can change the rate at which he or she is moving during a jump or spin by pulling in or extending their arms.

Bilham will be watching to see if his prediction for the next five years comes true. "Five years is a great amount of warning," he said. " Earthquake prediction is known to be either impossible or beyond our means. But here is the Earth saying—perhaps—you're in for something more. It's now time to do something about it."