Moon no factor in big quakes

UNITED STATES: Huge earthquakes are not significantly influenced by the moon, a new study says.

The study, conducted by US Geological Survey seismologist Susan Hough, looked at earthquakes of magnitude 8 or greater over the past four centuries.

And a review of more than 200 earthquakes demonstrated there is no connection between the phase of the moon and the time when huge seismic events of magnitude 8 and greater strike.

‘‘That’s obviously a big earthquake myth: that big earthquakes happen on the full moon,’’ Hough said. Her study was published yesterday in the journal Seismological

Research Letters, a publication of the Seismological Society of America.

Hough said the myth can gain more attention when a large earthquake strikes on a full moon or when scientific studies show a weak influence on earthquake rates by tidal or other forces.

‘‘In recent years, there have been a couple of nice studies that show that tidal forces do modulate earthquake rates slightly.

‘‘It makes sense: The tides create stress in the solid earth, and not just the oceans. And in some cases, that small force can be ‘the straw that breaks that camel’s back’ and nudges the fault to produce an earthquake.’’

But it’s also important to understand that ‘‘this isn’t of any practical value for prediction’’, Hough said.

‘‘A recent study ... for example, concluded that very large earthquakes, with magnitudes close to 9, tend to occur near the time of maximum tidal stress,’’ Hough said in her study, adding that researchers ‘‘point out, however, that the relationship is not clear-cut and does not hold when low-magnitude events are included in the analysis.’’

Indeed, other scientists who have authored studies on the impact of tides with earthquakes have been careful to point out that many earthquakes will still happen when tidal stress is low, and note that the studies don’t mean that the public can get a warning about the exact date, time and location of the next big earthquake.

But sometimes reports of those studies, Hough said, ‘‘turn into headlines that say the moon causes earthquakes.’’

Exactly when and where earthquakes strike is a random process. The primary driving force behind earthquakes is the movement of tectonic plates.

Last 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 Los Angeles 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.

For her study, out of the more than 200 earthquakes Hough studied, if 20 or 30 of them happened on the full moon, ‘‘that would have actually been significant’’. But that’s not what the results showed. –