Planet Earth appears to provide a long-range warning for some of its biggest earthquakes, according to a new study co-authored by University of Montana geoscientist Rebecca Bendick.

“What we find is there’s a particular pattern, five to six years after the Earth slows down its rotation, when we have more events than average,” Bendick said last week. “And we seem to have fewer events when the earth speeds up.”

After word of the study came out recently in the Guardian newspaper, Bendick said some equally predictable “sky-is-falling” reactions lit up her office phone. So a few details need clarification.

First, yes the Earth does speed up and slow down on a regular basis. Every day’s rotation varies a tiny amount depending on storms moving around the atmosphere and the depositing and melting of snow and ice near the poles. Those effects add up to larger cycles of fast and slow rotations spanning roughly 30 years. The changes typically involve just milliseconds a day.

Second, by “event” Bendick refers to earthquakes measuring 7, 8 or 9 on the Richter scale: Catastrophic quakes like the 2015 Gorkha earthquake in Nepal or the one about two weeks ago that killed 500 on the Iran-Iraq border happen on average about 14 times a year. Those tend to be concentrated in the planet’s most seismically active zones: the Pacific Rim, Java, Sumatra and Iran.

And by “warning,” Bendick suggested Montanans consider earthquake threats the same way they think about wildfire.

“We know around here that the likelihood of wildfire on a given day isn’t equal,” she said. “We know in November, it won’t happen today. But in June, it’s a good time to revisit your preparedness. Clear the brush. Move the wood pile. Talk to your daughter about where to meet if there’s a fire in the Rattlesnake.”

University of Colorado-Boulder researcher Roger Bilham and Bendick presented their paper to the annual meeting of the Geological Society of America in October. They compared the historical records of major earthquakes and the pattern of the planet’s rotational speed changes and found a strong correlation.

“We are not saying specific earthquakes are caused by this pattern,” Bendick said. “That’s like climate scientists don’t say a particular hurricane was caused by climate change. The probability of the event went up.

"The last time we had a major slow-down was in the late 1990s, and the result was a large number of events. The highs are around 23 a year. At the low point, we may have eight a year.”

As the earth speeds up, it bulges a bit at the equator. As it slows, it gets slimmer.

Meanwhile, the planet’s continental plates and their webs of fault lines constantly grind against one another. Those opposing land masses can load energy for years or centuries. They also amass tension from interactions between the planet’s outer mantle and molten core. When the tension gets too great, the resulting snap is an earthquake.

The rotational changes appear to make it more likely that a building earthquake will be triggered.

“The warning point comes when it starts slowing down — that gives us a five-year advance warning,” Bendick said. “When the earth spins at its slowest, and the equator is at its narrowest, that’s the mechanism.”

Bendick compared the 30-year cycle to a rollercoaster ride. The earth’s rotation goes fastest as the cars shoot down the track. As they reach the bottom of the curve, they slow down: That’s the start of the five-year warning period. As they start slowly climbing the next hill, the period of most-active earthquake probability arrives.

“The latest cycle of deceleration of earth rotation started in 2012,” Bendick said. “That means five years is now, and we should be entering the period of peak earthquake activity for the next two to three years.”

That doesn’t count quakes like the magnitude 5.8 temblor that broke dishes and cracked chimneys in Lincoln last July. Bendick said the pattern might reflect those smaller quakes, but the archives are too dispersed to efficiently catalog and study.

And Bendick made no guesses about the potential for Yellowstone National Park’s supervolcano or Seattle’s “Cascadia quake” to make appearances in that period.

“In the earthquake science community, if you say something (may) happen in 30 or 60 years, people ignore that,” Bendick said. “It’s too long a time horizon to worry about. We think: That’s all well and good, what about Thanksgiving dinner?

"This study is a reason for saying now would be a good time to think about this. If it turns out totally wrong, no harm done. You should be thinking about natural disaster preparedness anyway.”

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Natural Resources & Environment Reporter

Natural Resources Reporter for The Missoulian.