Large earthquakes appear to more likely when the tides are at their peak.
By Nathan Collins
(Photo: Jewel Samad/AFP/Getty Images)
Most of the terrible things we associate with the moon—werewolves, upticks in crime, trouble sleeping, psychiatric crises, armageddon—are really just myths. Except, according to a new study, earthquakes, which might really be more likely during full moons.
To be precise, big earthquakes are more likely at high tide, especially under full or new moons, when the tides get extra high, researchers Satoshi Ide, Suguru Yabe, and Yoshiyuki Tanaka report today in Nature Geoscience.
Ocean tides are caused by the gravitational pull of the moon and the sun. Because gravitational forces grow smaller with distance, the moon and sun pull harder on the side of the Earth facing their direction. As a result, the sun and moon stretch the planet slightly, creating high tides on the near and far sides, with low tides in between. (Picture spinning around in circles, with a Slinky held out in front of you, and you’ll have some idea of the stretching that’s going on.) The tides reach their zenith when the Earth, moon, and sun are all in a line, a condition known as syzygy, and better known to us as full and new moons.
Big earthquakes are more likely at high tide, especially under full or new moons.
But tides don’t just affect oceans. They affect the entire Earth, putting twice-daily stresses on both sea and land, which could, in principle, trigger the occasional earthquake. The question is, do such things actually happen?
According to Ide, Yabe, and Tanaka, the answer is yes. The team first gathered about 40 years’ worth of data from around the world on earthquakes of magnitude 5.5 and higher, then computed tidal stresses on the corresponding faults during the 15 days prior to each earthquake. Finally, they ranked each of the 15 days according to the maximum stress on that day.
Earthquakes were generally more common on high-stress days—often full or new moons—and there were more high-magnitude earthquakes on the highest ranked days than others. Nine of the 12 largest earthquakes recorded since 1976, all magnitude 8.2 or higher, for example, occurred on top-five tidal stress days, while only one occurred on a very low stress day.
The team also examined the probability that smaller earthquakes would grow into larger ones, using a standard model of the relationship between the number and size of earthquakes. On relatively high-stress days, they found, there’s a roughly six-fold increase in the chance a magnitude 5.0 will grow into a 9.0, compared with low-stress days.
“Thus, knowledge of the tidal stress state in seismic regions can be used to improve probabilistic earthquake forecasting, especially for extremely large earthquakes,” Ide, Yabe, and Tanaka write.
