Gravity. You might know it as the force that explains how all clumps of matter came to be. You might know it as John Mayer's nemesis. You might know it as the thing that causes Earth's oceans to slosh back and forth twice daily.
The tidal pull of the Moon is also strong enough to cause some degree of movement in Earth's crust. That fact has caused serious scientists and cranks alike to speculate, for at least a century, about whether the tides can also cause earthquakes. And a new study published today in Nature Geoscience suggests this might be true—at least for the largest quakes.
Tides happen because the Moon's orbit is way slower than the Earth's rotation. Any given location on Earth generally passes under the Moon: high tide. "Stress goes up and down two times a day, and that’s the kind of stressing we normally look for," says John Vidale, a seismologist at the University of Washington who reviewed this paper. But the sun also exerts a—much weaker—tidal influence. When the sun and Moon line up, their gravitational influences combine, forming what's known as a spring tide.
Spring tides typically happen twice a month, during a full or new Moon. The new study cross-referenced earthquake records from three massive databases with tide charts. It found little to no correlation between spring tides and small quakes. But bigger quakes—your 7, 8, 9 magnitude shakers—started lining up with the moon cycles.
Now, hold up: This doesn't mean moon charts can predict quakes. For one, the pattern is barely statistically significant. "This really relies on having enough quakes to resolve the pattern," says Vidale. He says the records simply don't contain enough high magnitude quakes. "Sadly, it may take another hundred years of data to nail down the pattern solidly."
Even if scientists did have enough data to nail down the correlation, they still don't know enough about individual faults to predict which could go critical in a spring tide. "We can't measure all the stresses on the faults, and we don't know their geometry," Vidale says. Seismologists have been preoccupied for decades with looking for clues that might help them predict a quake. "The longer we look without finding any, the less likely it is they exist," he says.
Not to say that this study is useless. The fact that spring tides might affect only really big quakes could tell seismologists new things about how quakes happen. And that might help them improve their estimates for the danger of living near any particular fault line. But likely not. The high-risk zones are well mapped, and the odds of recurrence are pretty well established. If you live in an earthquake zone, preparation is your best bet. Get your home seismically certified. Put together a quake kit. Practice those "jump in a doorframe" reflexes. Or just move to a place where the Moon's gravity doesn't have any fault lines to flex.