How tides may tip small earthquakes into monsters

The three biggest earthquakes of recent years struck at a full or new moon, new research shows, when the moon’s tidal tug on the Earth was particularly strong.

A trio of seismologists from Japan measured tidal stresses – the amount the Earth’s crust bulges and shifts, depending on the moon’s effect – and found large earthquakes are more likely to be triggered during a spring tide.

Interestingly, there was almost no effect on small earthquakes. But the work, published in Nature Geoscience, could help pinpoint danger times when large earthquakes are more likely to strike.

Each day, myriad small earthquakes rattle the crust. Every now and again, one will bloom into a massive rupture capable of toppling entire cities and killing thousands of people.

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How these large earthquakes evolve is still a mystery – and how the moon is involved, even more so. 

We’re familiar with the moon’s effect on the oceans – tides rise and fall twice a day. The sun, too, plays a role in our tides. 

While it’s more a supporting role, when the moon and sun line up, their combined efforts produce an even bigger tide – which we know as the spring tide.

This celestial pull also affects the Earth’s crust. This is called tidal stress. But studies trying to find a link between tides and earthquakes has yielded mixed results. 

A 2004 paper found earthquakes of magnitude 7.0 or above tended to occur more frequently during spring tides, but it was a tendency that couldn’t be supported statistically – and when smaller earthquakes were included, the trend disappeared altogether.

But the amplitude or size of the tidal stress isn’t the same from spring tide to spring tide. 

So Satoshi Ide, Suguru Yabe and Yoshiyuki Tanaka from the University of Tokyo calculated tidal stress amplitude for the two weeks before the three most recent large earthquakes: the 2004 Indian Ocean earthquake which had a magnitude more than 9.0, the magnitude-8.8 Chile quake in 2010, and the 2011, magnitude-9.0 earthquake off the coast of Tohoku-Oki, Japan.

The first two occurred near a full moon, close to the maximum amplitude of tidal stress.

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The Japanese earthquake, though, happened at a neap tide, or the tidal phase with the least difference between high and low water. But the trio’s calculations showed tidal stresses were still very high at the time.

Smaller earthquakes didn’t show the same trend, although the authors note regional variation.

For instance, earthquakes around magnitude 5 in northeastern Japan did seem to be controlled, to a degree, by tidal stress.

So what’s happening?

No one knows for sure. Some seismologists think deep tectonic tremors at plate boundaries could be particularly sensitive to tidal stress changes.

They plate boundaries don’t lurch in an instant – they tend to take days of slow slipping before a big quake. And in those days, a small change in tidal stress might be enough to tip it over the edge from a small earthquake to a monster.

Knowing the tidal stress state in such regions can’t predict when a quake will strike, but it might be used to improve earthquake forecasting, the researchers write – particularly the extreme events.

During periods of high tidal stress, the probability that a magnitude-5 earthquake will grow to a magnitude-9 rises by a factor of six when compared to low stress conditions.