Big earthquakes, such as the ones that devastated Chile in 2010 and Japan in 2011, are more likely to occur during full and new moons — the two times each month when tidal stresses are highest.
Earth’s tides, which are caused by a gravitational tug-of-war involving the Moon and the Sun, put extra strain on geological faults. Seismologists have tried for decades to understand whether that stress could trigger quakes. They generally agree that the ocean’s twice-daily high tides can affect tiny, slow-motion tremors in certain places, including California’s San Andreas fault and the Cascadia region of the North American west coast.
But a new study, published on 12 September in Nature Geoscience
, looks at much larger patterns involving the twice-monthly tides that occur during full and new moons. It finds that the fraction of high magnitude earthquakes goes up globally as tidal stresses rise.
Satoshi Ide, a seismologist at the University of Tokyo, and his colleagues investigated three separate earthquake records covering Japan, California and the entire globe. For the 15 days leading up to each quake, the scientists assigned a number representing the relative tidal stress on that day, with 15 representing the highest. They found that large quakes such as those that hit Chile and Tohoku-Oki occurred near the time of maximum tidal strain — or during new and full moons when the Sun, Moon and Earth align.
For more than 10,000 earthquakes of around magnitude 5.5, the researchers found, an earthquake that began during a time of high tidal stress was more likely to grow to magnitude 8 or above.