How Earth's rotation speed might affect earthquakes - imagine an ice skater

Monday, 20 November 2017

More big earthquakes in 2018 as Earth slows slightly, theory suggests.

Two US scientists say it helps to think of an ice skater to try to understand how tiny changes in the time it takes the Earth to spin a full day might trigger large earthquakes.

Roger Bilham of the University of Colorado and Rebecca Bendick of the University of Montana have found a correlation between periods with larger than usual numbers of magnitude 7-plus earthquakes, and slowdowns of tiny fractions of a second in the Earth's daily rotation time.

The point of peak slowdown every three decades or so, lines up with the quake trend.

A collapsed building after last week's magnitude 7.3 earthquake centred near the Iran-Iraq border.

The slowdowns in rotation speed might be enough to give a kickstart to faults that were in any case ready to go. Most of the additional big quakes have happened near the equator.

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In the abstract of a presentation they made to Geological Society of America's annual meeting, the scientists said that five times in the past century, an increase of 25-30 per cent in the annual numbers of magnitude 7 and above quakes, had coincided with a slowing in the speed of the Earth's rotation.

The slowdown in rotation would start five to six years before the increase in the number of big earthquakes.

'The year 2017 marks six years following a deceleration episode that commenced in 2011, suggesting that the world has now entered a period of enhanced global seismic productivity with a duration of at least five years,' the abstract said.

As a way to think of what might be happening, Belham told BBC Inside Science: 'The mechanism we've come up with is that as the Earth slows down it's like a skater spinning on ice. As the Earth slows down it's equatorial diameter reduces,' he said.

'Its (the Earth's) waistline gets smaller, but its clothes, the tectonic plates on Earth, remain the same size, which means they get rumpled up.

'Where they get rumpled up is at the plate boundaries. So you've got to think of the Earth slowing down, the equator shrinking but the plates being unable to shrink, they're fairly rigid.

'They concentrate the shrinkage into the seismic zones where the earthquakes occur.'

Bendick said the change in the radius of the Earth was something like 2mm, meaning the distance around the Equator changed maybe 10-15mm.

'These tiny changes to the overall shape of the Earth are enough, if there are faults that are already ready to go … to kind of kick them over into failure,' she said.

The geophysical community was rightly sceptical, as that was the way science progressed. 'So far nobody's figured out why we're wrong, in my mind that's tantamount to saying, 'so far, so right',' Bendick said.

'We're just going into a phase when we expect an increase in the rate of magnitude 7 and greater earthquakes. So this forecast that we're making is very testable.

'We expect to have a larger than average number of large earthquakes over the next four or five years. So in four or five years we'll know whether this is right or not.'

The number of magnitude 7-plus earthquakes a year varied from intervals with as few as eight, to those with as many as 23.'So the difference is big.' There was an average of 14 a year 'but there are very few years that actually have 14 events'.

So far in 2017, the US Geological Survey shows seven magnitude 7-plus events, including one in New Caledonia on Monday.

GNS Science communications manager John Callan said the organisation was unable to provide meaningful comment on the research.

'It appears to be a conference presentation and very preliminary rather than peer reviewed research, so there is no detail for us to examine,' Callan said.

'It is true there have been periods of elevated rates of large earthquakes in the past 100 years. However, if you go looking for correlations with other natural phenomenon, you will almost certainly find some interesting matches.'

University of Canterbury tectonic geology lecturer Dr Tim Stahl said Bilham and Bendick were respected scientists in earthquake geology and seismic risk.

'I would look forward to seeing this study in a peer-reviewed journal, with some reporting of the statistical tests for the significance of the observed relationships and with further consideration of potential mechanisms.

'As with any conference abstract or presentation, it is difficult to judge the scientific merit of the claims being made until they have undergone rigorous peer-review and additional testing by other research groups,' Stahl said.

'It is important to note that the authors explicitly state in the abstract that the precise locations, times, or magnitudes of earthquakes cannot be predicted, even if their observations and interpretations are eventually confirmed by other researchers.'

Professor Mark Stirling, Chair of Earthquake Science, at Otago University agreed Bilham and Bendick were well known and regarded, he said there was no basis to support their conclusions at this time.

'The Otago Earthquake Science group does not support the primary conclusion of this article. We see it as yet another example of a fortuitous correlation between earthquake occurrence and an unrelated phenomenon.'