Science & Technology



Do moon phases produce big earthquakes? Study debunks that idea

Rong-Gong Lin II, Los Angeles Times on

Published in Science & Technology News

In an interview in October, USGS research geophysicist Ken Hudnut explained why earthquakes are impossible to predict. To show how a fault gathers seismic stress that eventually ruptures into an earthquake, he showed a model of bricks sitting on sandpaper -- equivalent to the two sides of the fault.

The bricks are attached to a rubber band connected to a handcrank, which, when it is moving, is like the accumulating seismic stress of plate tectonics. (In Southern California, the Pacific plate, where downtown L.A. sits, is moving northwest, while the North American plate is moving southeast.)

As Hudnut moved the handcrank, friction would keep the brick steady on the sandpaper, until at one point the accumulating force from the pulling rubber band was unbearable, and the brick would suddenly move -- analogous to an earthquake. But when the movement happened wasn't predictable. It was random.

There are other myths out there, such as the one in which hot, sunny "earthquake weather" somehow makes seismic events more likely; it doesn't. Earthquakes happen underground, and the weather has no effect on their timing.

Hough said she decided to work on this study to rigorously test an idea that seismologists have long stated -- that earthquakes aren't more likely to happen on certain days of the calendar year or the cycle of the moon.

There are sometimes weird coincidences. For instance, in California, June 28 is the anniversary of a couple of memorable earthquakes: the magnitude 7.3 Landers earthquake that struck the Mojave Desert in 1992 (and the subsequent 6.5 Big Bear aftershock hours later); and the magnitude 5.6 Sierra Madre earthquake in 1991 that killed two people.

The next day, June 29, is the anniversary of the magnitude 6.8 Santa Barbara earthquake of 1925.

But those coincidences don't mean anything.

"One analogy: if you had a classroom of 36 kids, on average, you'd expect to see three birthdays every month. You'd probably have a couple of kids on the exact same birthday," Hough said, a result that does not hold some kind of larger meaning.

For her study, out of the more than 200 earthquakes she studied, if 20 or 30 of them happened on the full moon, "that would've actually been significant." But that's not what the results showed.

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