San Andreas Earthquakes Could Be Caused By Gravitational Pull of Sun and Moon

Tides caused by the sun and moon's gravitational pull could cause earthquakes along the San Andreas Fault, scientists said.

According to scientists, low-frequency earthquakes - which are small tremors deep underground - are more likely to occur as tides are strengthening or waxing.

Tides, the rise and fall of the seas, are primarily caused by the gravitational tug of the sun and moon on Earth. But this pull does not only affect the seas but also stretches and compresses the Earth's crust.

"The moon, when it's pulling in the same direction that the fault is slipping, causes the fault to slip more - and faster," Nicholas van der Elst, a seismologist and geophysicist at the U.S. Geological Survey's Earthquake Science Center in Pasadena, California and lead author of the study, said in a report by Phys.org.

"What it shows is that the fault is super weak - much weaker than we would expect - given that there's 20 miles of rock sitting on top of it."

In the study, which was published in the Proceedings of the National Academy of Sciences, the researchers catalogued 81,000 low-frequency earthquakes that struck along the San Andreas Fault in California between 2008 and 2015. Low-frequency earthquakes are often no bigger than magnitude 1 on the Richter scale.

According to the scientists, tidal strength varies over a two-week or fortnightly cycle. The strongest tides occur when the moon and sun are aligned (spring tides), while the weakest tides occur when the sun and moon are perpendicular to each other with respect to Earth (neap tides).

The scientists found that the number of low-frequency earthquakes did not increase during the strongest point of the fortnightly cycle. Instead, it peaked as the tide was strengthening or waxing.

According to van der Elst, these types of quakes were more likely to occur on days when tides were "larger than the previous day's tides by the greatest amount."

"That tells you something about how fast the fault is loaded - how long it takes for the fault to recharge before you can trigger these earthquakes on it, how quickly this patch of fault is accumulating stress," van der Elst told Live Science.

The researchers said that the study could somehow contribute to a better understanding of earthquakes and when and where the big earthquakes are likely to occur.

"The hope is that looking at low-frequency earthquakes that happen deep in the fault will ultimately shed light on how shallow parts of the fault accumulate stress," van der Elst said.