Earthquake weather is a myth. But those creepy nights when a bright moon is peeping in your bedroom window might indeed be a sign that the ground is about to shake.

New research suggests that moon- and sun-driven high tides can trigger and enhance certain kinds of temblors along the mighty San Andreas Fault.

“It's a really exciting discovery,” says the lead author of the study, Nicholas J. van der Elst of the U.S. Geological Survey in Pasadena.

“The sun and moon exert a gravitational tug on Earth that stretches and compresses crustal rocks,” a summary of the research states. “This cyclic stressing can promote or inhibit fault slip, particularly at the deep roots of faults.”

But this tidal effect, which usually happens twice a month during the highest tides that correlate with the new and full moons, won't necessarily apply to the Big One we've all been waiting for, Van der Elst says.

The study, recently published in the journal Proceedings of the National Academy of Sciences, looked at more than 80,000 small earthquakes along specific points of the San Andreas that would be more susceptible to tidal pressure, which pushes tons of extra water against the Earth's surface, according to Van der Elst. “When there's a big earthquake on the San Andreas,” he says, “you won't be able to conclude the extra tug from the tides put it over the top.”

The critical portions of the San Andreas Fault, which runs from the Gulf of California to the Bay Area and serves as the dividing line as the Pacific Plate moves north and attempts to become an island, are too strong to be affected by the tides, Van der Elst has argued.

The areas where temblors were influenced by tidal pressure were more “shallow,” he says, and only produced small, off-the-radar shakers. He compared these baby quakes to the temblors associated with fracking. 

However, Van der Elst says he hopes that further study of these tidal effects can lead to the Holy Grail of earthquake research — prediction.

With these tidal quakes, “The fault has seen levels of stress that it hasn't seen for a few days or a week,” he said. “It tells us the fault has to recharge in order to produce more earthquakes. We can actually estimate how the fault is being recharged down there.”

It's possible that a network of tide-influenced quakes could someday show some relationship to major temblors.

“There is hope that, if we watch it enough, we could forecast larger earthquakes,” Van der Elst says.

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