WASHINGTON – Geophysicists who studied seismic activity over the past 117 years report 2018 could be shaping up as a year of monster earthquakes – especially around the equator.
The signal that causes the alarm from a historical pattern, they say, is the slowing of the Earth’s rotation.
The new study, presented at the annual meeting of the Geological Society of America, in Seattle, is published in Geophysical Research Letters by geologists Roger Bilham of the University of Colorado, Boulder, and Rebecca Bendick of the University of Montana.
It tracked the incidence of magnitude 7 or greater earthquakes worldwide since 1900, revealing through recent history, such major quakes occur about 15 times per year. Yet, over the last 117 years, there have been evenly spaced intervals in which the annual total jumped to between 25 and 30.
“Major earthquakes have been well recorded for more than a century and that gives us a good record to study,” said Bilham.
While the researchers caution that 117 years is a relatively short period of time in the history of the Earth, something else caught the attention of the researchers.
Quake-prone periods seem to follow by five or six years periodic slowdowns in the speed of the Earth’s rotation. And one of those slowdowns occurred five or six years ago.
What causes the slowdown of the rotation?
Bilham and Bendick explain that Earth is hardly a solid sphere. Not only is the planet surrounded by a gaseous atmosphere and 70 percent covered by oceans, but even the outer core some 1,200 miles deep is composed of mostly liquid iron and nickel. In fact, another new study earlier this year suggests water equivalent to all the surface oceans lies deep beneath the crust of the Earth. All of this non-solid matter is in motion in a pattern that oscillates somewhat predictably over time, like a bucket filled with water moving about in a repeating cycle.
“The correlation between Earth’s rotation and earthquake activity is strong and suggests there is going to be an increase in numbers of intense earthquakes next year,” Bilham said.
How does this movement change the speed of the Earth’s rotation?
The change is not dramatic. It can only be measured or recorded by atomic clocks. In fact, it only slows down the Earth’s rotation by about a millisecond per day. But that is enough of a change to cause the molten core to strain outward in accordance with Isaac Newton’s law that objects in motion tend to remain in motion unless acted on by some outward force.
That motion results in pressure through the rocks and plates and fault lines above the liquid mass. The researchers calculate that it takes five or six years for that energy to reach the outer layers of the planet where quakes occur.
Therefore, they surmise, five or six years after the slowdown in rotation is detected, the larger quakes occur more often.
“It is straightforward,” said Bilham. “The Earth is offering us a five-year heads-up on future earthquakes.”
Since the last slowdown in the Earth’s rotation was seen in 2011, the pattern has already been validated in 2017 with the 7.1 quake in Mexico City Sept. 19, the 7.3 quake in Iraq and Iran Nov. 12 and the 7.0 quake off New Caledonia Nov. 19.
The study also predicts the uptick in large quakes will be seen in and around the equator, the planet’s widest point, within a latitude of 30 degrees north or south. That’s because the equator rotates faster – up to 1,000 mph faster – than do the poles. Therefore, they suggest, the slowdown there would be more powerful.
“The inference is clear,” said Bilham. “Next year we should see a significant increase in numbers of severe earthquakes. We have had it easy this year. So far we have only had about six severe earthquakes. We could easily have 20 a year starting in 2018.”
What do other scientists think of the study?
“The correlation they’ve found is remarkable, and deserves investigation,” says Peter Molnar, a geologist also at Colorado University.
The evidence for some kind of link is compelling, says geophysicist Michael Manga of the University of California, Berkeley. “I’ve worked on earthquakes triggered by seasonal variation, melting snow. His correlation is much better than what I’m used to seeing.”