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Every two weeks, on the full and new moons, the sun, moon and Earth fall along a nearly straight line. The combination of gravitational forces in this arrangement creates large swings in the tides. But the celestial¹ alignment² affects more than the oceans—it also tugs³ on Earth’s crust, adding to the stress on faults. This makes it more likely that major earthquakes will strike at these times, according to a new study.

The idea isn’t new, but scientists have had a hard time testing the earthquake-tide relationship. For instance, three of the largest earthquakes in recent years happened when tidal stress was high. But those big ones are rare and the link seems to break down for smaller events.

So the researchers crunched⁴ a bunch of numbers: by looking at more than 10,000 medium and large earthquakes, they found that the proportion of large events increased when tidal stress was high.

The results do not imply that every full or new moon will bring an earthquake—obviously. What the findings mean is that high tidal stress during new or full moons may up the chances that an earthquake will grow bigger than it otherwise might have been.

“Even magnitude 9 earthquakes start like magnitudes 1 or 2.” Satoshi Ide, the University of Tokyo seismologist who led the study. “So, at the initial stage, it’s very difficult to distinguish small or large earthquakes. It starts similarly, but it grows at some probability to large scale. That probability is controlled by some other effect.”

And one of those effects might be the tides. Ide points out that tides in the Earth’s crust are known to cause tiny tremors⁵ along deep faults. “Earthquake faults are locked everywhere, but that kind of slow deformation⁶ unlocks very small part of the fault…so it doesn’t make large failure there, but once earthquake rupture⁷ starts…it can propagate very far and become very large earthquake.”

The study was published in the journal Nature Geoscience. [Satoshi Ide, Suguru Yabe and Yoshiyuki Tanaka, Earthquake potential revealed by tidal influence on earthquake size–frequency statistics]

Ide is now doing a similar analysis on earthquakes that all happened in the same geologic⁸ setting, like subduction zones. Perhaps that research will confirm whether heavenly bodies can take smaller shakes and supersize them.

—Julia Rosen