Strongest Earthquakes Are Not Linked To Moon Phase, Time Of Year: Study

The question of whether earthquakes and their intensity are linked to the moon in some way has cropped up again. And the new study which delved into the subject finds an answer that contradicts some older studies, as well as "earthquake lore."

Susan Hough, a scientist with the U.S. Geological Survey, looked at data for 204 earthquakes, each of magnitude 8 or stronger, that have occurred since the 1600s, and found no evidence that rates of these large earthquakes were influenced by the phase of the moon or even the Earth’s position relative to the sun.

Based on a study she published in the journal Seismological Journal Letters, Hough said any patterns in earthquake cycles that have been linked by observers to the phases of the moon "are no different from the kinds of patterns you would get if the data are completely random," according to a statement released Tuesday by the Seismological Society of America.

For the study, Hough chose powerful earthquakes to avoid the clustering effect, which refers to a number of earthquakes that happen in a relatively short time in the same region, many of them as aftershocks of one big earthquake. A big earthquake is less likely to be an aftershock.

She found that certain days in the lunar cycle did, in fact, throw up some earthquake clusters. But to see if the patterns she found were of any significance, Hough randomized the earthquake dates and found similar patterns in the random data as well.

"When you have random data, you can get all sorts of apparent signals, just like when you flip a coin, you sometimes end up with five heads in a row," Hough said by way of explanation.

If anything, some of the unusual patterns she found in the unchanged dataset showed the opposite of an earthquake-moon phase link. The maximum number of earthquakes on a single day (in terms of the lunar cycle) was 16, and this was the seventh day after a new moon, or in the middle of the cycle, when the lunar tides would be at their minimum.

However, there is the fact of solid Earth tidal stresses — not tides, but ripples through the Earth caused by the sun and the moon — which Hough said could be one of the factors that make a small contribution to earthquakes.

A study published in September 2016 made exactly that point, after analyzing over 10,000 earthquakes that were stronger than 5.5 in magnitude. In that study, which said the effect is significant but small, researchers concluded "that large earthquakes are more probable during periods of high tidal stress," and also that "an earthquake that began during a time of high tidal stress was more likely to grow to magnitude 8 or above."

Referring to studies of that sort, Hough said: "There is in some cases a weak effect, where there are more earthquakes when tidal stresses are high. But if you read those papers, you'll see that the authors are very careful. They never claim that the data can be used for prediction, because the modulation is always very small."