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Since the time of Isaac Newton, scientists have wondered if the gravitational pull of the sun and moon might be strong enough to trigger earthquakes and tremors on Earth. An analysis of 81,000 low-frequency earthquakes along the San Andreas Fault now confirms these suspicions.

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The new paper, published in Proceedings of the National Academy of Sciences, shows that small, deep earthquakes within California’s San Andreas Fault—the primary plate boundary fault in southern California that extends for 800 miles—are more likely to occur when the tide is waxing. A team led by Nicholas J. van der Elst from the U.S. Geological Survey chronicled tens of thousands of quakes along the fault during a repeating two-week tidal cycle, known as “fortnightly tides.”

Given the complex dance of the sun and moon relative to the Earth’s surface, different parts of the Earth are subject to different tidal cycles. The classic 12-hour and 24-hour tides are well observed along the coast lines, but other long period tides exist as well, including the fortnightly tide—a recurring tidal phase that varies in magnitude over the course of its 14-day cycle. A fortnightly tide happens when the Earth-moon “bulges” are combined with the Earth-sun “bulges,” resulting in a twice monthly change in the range of the tides.

The San Andreas Fault, on the Carrizo Plain. (Photo: Doc Searls/Flickr)

The idea that the sun and moon might be pulling and pushing the Earth’s crust like an accordion, has captivated scientists for hundreds of years. Alexis Perrey, a 19th century French seismologist, suspected a correlation between the moon and seismic activity on Earth, and devoted much of his life’s work to the subject.

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Perrey struggled to find empirical evidence to support his assertion, but his followers would have better luck. Today, it’s well established that “solid Earth tides,” as they’re called, can trigger tremors (very slight earthquakes that cause little to no damage) virtually anywhere that tectonic tremors are found. Scientists have also found evidence (though that evidence is scant) that solid-Earth tides can trigger more substantial earthquakes. But these gravitationally triggered events are only observed in very select environments, such as mid-oceanic ridges and shallow thrust faults.

For the new study, van der Elst and his team considered the potential for tidal cycles to trigger slow, deep, and relatively weak tremblors known as low-frequency earthquakes (LFEs). They compared the phase of the solid-earth tide with the timing of 81,000 cataloged LFEs that occurred along the fault from 2008 to 2015.

And they discovered that the fortnightly tide cycle triggers earthquakes along the San Andreas fault, but only under certain conditions. The number of LFEs spiked as the two-week cycle was still waxing—not at its maximum peak when the gravitational pull from the sun and the moon is at its strongest.

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The researchers theorize that the LFEs are triggered when the stress imposed by the solid-earth tide exceeds the strength of the fault at a given point, resulting in a fault slip. The gravitational pull of sun and moon slightly lifts and lowers the earth’s crust, stretching and compressing crustal rocks. Sometimes this is enough to cause a quake.

This research strengthens the suspected links between tidal forces and earthquakes, while providing a new way of studying plate tectonics. It could even help seismologists predict when earthquakes and tremors might happen.

[Proceedings of the National Academy of Science]