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Does the Moon’s Phase Cause Earthquakes?

One researcher offers a succinct answer

(Amarit Opassetthakul)
smithsonian.com

There’s a popular belief that earthquakes are more frequent when the moon is close to full. The explanation is that a full moon has the strongest tidal pull, which supposedly places more stress on the Earth’s crust and increases the chances that a fault may slip. But are the two actually related? As Nicole Mortillaro at CBC News reports, a new study answered the question with one word: "No."

For the study, published in the journal Seismological Research LettersU.S. Geological Survey researcher Susan Hough matched the dates and lunar phases of 204 magnitude 8 or higher earthquakes dating back to the 1600s. She found that the incidence of earthquakes had no relationship between the position of the moon or the sun relative to Earth. "The data are completely random," Hough says in a press release.

As Hough tells Shannon Hall at The New York Times, the idea is “not some wild, crazy idea." Scientists have wondered whether there might be some connection between the moon and quakes since the 1800s, reports Rebecca Boyle at The Atlantic. In 2004, USGS analysis suggested there was a very small increase in the number of earthquakes in deep ocean basins during low tide. And in 2016, a study by researchers at the University of Tokyo found that some of the largest recent earthquakes—including the 2004 Sumatra quake, the 2010 quake in Chile and the 2011 Tohoku-Oki off the coast of Japan—all occurred during close to times of peak tidal stress. But the study's authors were careful to not infer that one necessarily caused the other. ​

Hough’s data seems to suggest that is likely just coincidence. “I think the lore persists for a number of reasons,” she tells Mortillaro. “One is that people find patterns in random data, like seeing animals in the clouds. When a big earthquake occurs on a full moon, or the shortest day of the year, people tend to ascribe significance to the coincidence. When big earthquakes occur that don't fit a pattern, it tends to slide right past us.”

As Hough tells Rong-Gong Lin II at the L.A. Times, many incidents that people point to as potential evidence for trends in earthquakes (like quakes happening on the anniversaries of previous quakes) are really just coincidences. “One analogy: if you had a classroom of 36 kids, on average, you’d expect to see three birthdays every month,” Hough explains. “You'd probably have a couple of kids on the exact same birthday.” However, she points out that is just coincidence and doesn’t have some larger meaning.

Honn Kao a researcher at the Geological Survey of Canada tells Mortillaro that Hough’s research doesn’t quite settle the question. More research needs to be done on earthquakes of a lesser magnitude, Kao says.

Hough says that it is possible that tidal forces created by the moon could have an impact on earthquakes, but the effect is so small it’s not useful to researchers. “It makes sense that tidal forces might in some cases be the proverbial straw that breaks the camel's back,” Hough tells Mortillaro. “But the studies note that the modulation is small, such that the effect is of no practical use for prediction.”

It would take a much larger catalog of earthquake data to make that sort of connection, Hough tells Ryan F. Mandelbaum at Gizmodo. “What we really need to do is wait 4,000 years and redo the exercise. Short of that, there are more sophisticated statistical tests that one could do, but in the end, the catalog is the catalog, and clever statistics are not going to overcome that limitation,” she says.

The study is notable for another reason, as Mandelbaum points out. It amusingly short abstract. One word is provided to answer the question posed in the title: “Do large (magnitude ≥ 8) global earthquakes occur on preferred days of the calendar year or lunar cycle?” Hough’s answer: "No."

That terse little abstract is a callback to a 1974 paper on earthquakes whose title asked “Is the sequence of earthquakes in Southern California, with aftershocks removed, Poissonian?” That abstract simply reads “Yes.”

Hough says that answering the simple question posed in the title was the goal of her paper. “That was the point of the exercise, in fact: to boil lunar/tidal triggering down to the questions that most people think about,” she wrote in an email to Mandelbaum. “Once it was clear there is no evidence for a significant correlation, the abstract wrote itself =)”

Hough says that she doesn’t expect her study to squash the myth that the moon triggers earthquakes, but hopes it will help a little. “Sooner or later there is going to be another big earthquake on a full moon, and the lore will pop back up,” she says in the release. “The hope is that this will give people a solid study to point to, to show that over time, there isn't a track record of big earthquakes happening on a full moon.”

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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