The link between Earth’s rotation and seismic activity was highlighted last month in a paper by Roger Bilham of the University of Colorado in Boulder and Rebecca Bendick of the University of Montana in Missoula presented at the annual meeting of the Geological Society of America.
“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 told the Observer last week.
In their study, Bilham and Bendick looked at earthquakes of magnitude 7 and greater that had occurred since 1900. “Major earthquakes have been well recorded for more than a century and that gives us a good record to study,” said Bilham.
They found five periods when there had been significantly higher numbers of large earthquakes compared with other times. “In these periods, there were between 25 to 30 intense earthquakes a year,” said Bilham. “The rest of the time the average figure was around 15 major earthquakes a year.”
The researchers searched to find correlations between these periods of intense seismic activity and other factors and discovered that when Earth’s rotation decreased slightly it was followed by periods of increased numbers of intense earthquakes.
Bilham and Bendick found that there had been periods of around five years when Earth’s rotation slowed by such an amount several times over the past century and a half. Crucially, these periods were followed by periods when the numbers of intense earthquakes increased.
“It is straightforward,” said Bilham. “The Earth is offering us a five-year heads-up on future earthquakes.”
This link is particularly important because Earth’s rotation began one of its periodic slowdowns more than four years ago. “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.”
Exactly why decreases in day length should be linked to earthquakes is unclear although scientists suspect that slight changes in the behaviour of Earth’s core could be causing both effects.
In addition, it is difficult to predict where these extra earthquakes will occur – although Bilham said they found that most of the intense earthquakes that responded to changes in day length seemed to occur near the equator. About one billion people live in the Earth’s tropical regions.
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