Past periods of slow rotation over the last 100 years have coincided with more earthquakes than average, according to research presented last month at the annual meeting of the Geological Society of America.

“The numbers of earthquakes that have occurred each year in the past century are well known. The changes in Earth’s rotation rate are also well known,” study co-author Roger Bilham, a geophysicist at the University of Colorado Boulder, said in an email to Live Science. “All we have done is to compare these two well-known lists of numbers and report an interesting and useful relationship.”

The basic idea is that as the Earth’s spin slows slightly, the equator shrinks. However, tectonic plates don’t shrink as easily, meaning the edges of the plates get squeezed. Although this amount of squeezing isn’t huge, it does put added stress on plate boundaries that are already under stress, where earthquakes are more likely, Bilham said.

Historical pattern

Bilham and his colleague, Rebecca Bendick, a geophysicist at the University of Montana in Missoula, looked at the history of earthquakes of magnitude 7 or greater since 1900.

On average, there were about 15 major earthquakes per year since 1900. However, during certain periods, the planet sees between 25 and 35 earthquakes greater than magnitude 7 in a year. When the team looked more closely, they found those periods coincided with times when the Earth spins more slowly, meaning the days get slightly longer. Changes in Earth’s rotational speed can be caused by weather patterns like El Niño, ocean currents and currents in the molten core of the planet. When fluids speed up, the solid Earth must slow down, Bilham said.

Because NASA tracks the length of the day to the microsecond, these slowdowns in Earth’s spin can be predicted five years in advance, Bilham said.

Based on that data, the Earth is entering a period of prolonged slower rotation. As a result, next year could see more quakes, if past data is any indication. Whereas an average year might see about 15 earthquakes of magnitude 7 or greater, the next four years could see closer to 20 earthquakes of that size, Bilham said.

“Knowing that earthquakes will be more plentiful in five or six or seven years is useful because if a city planning department is considering retrofitting buildings to make them earthquake-safe now, or in 10 years’ time, the knowledge that more earthquakes are on their way may make them act now, rather than later,” Bilham said.

However, this effect will likely impact only faults that are already under stress and at high risk of rupture.

“We have no information on where these earthquakes will occur, except that they will occur at the world’s plate boundaries,” Bilham said.

Possible effect

“It’s a very interesting possible effect,” said Amos Nur, a geophysicist at Stanford University in California, who was not involved in the research. “Even though the rotation rate changes so little, the size of the mass [of the Earth] and the inertia are so great, you don’t need a huge change in rotation to have a change in stress.”

Scientists still have only a poor understanding of what triggers earthquakes, and have no way to predict earthquakes, so it would be impossible to definitively pin any given earthquake on Earth’s rotation, Nur said. Still, there are ways to validate the researchers’ basic idea, Nur said.

“The next step would be to go back and try to model what happens to the stress inside the Earth,” when its rotation changes, Nur told Live Science. “It’s not ridiculous. It’s quite feasible.”

Source: Live Science