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The Sun And Moon Cause Earthquakes On The San Andreas Fault

Not a reassuring sight. (Photo by Doc Searls via the Creative Commons on Flickr)
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According to the Encyclopedia Britannica, Ra, also known as the god of the sun, was thought by Egyptians "to travel across the sky in his solar bark and, during the night, to make his passage in another bark through the underworld."

OK. Well, researchers say he can also cause earthquakes in the San Andreas fault, so there's that too. As reported at the L.A. Times, researchers looked at 81,000 low-frequency earthquakes from 2008 to 2015 and concluded that the tides—namely the fortnightly tide—have a significant effect on the number of tremors that occur deep underground in the San Andreas Fault.

The fortnightly tide arrives every two weeks, when the sun and moon align and exert its tug on the Earth, according to Science Magazine. Though, as noted at Gizmodo, researchers found that the low-frequency earthquakes were most likely to happen as the fortnightly tide was in its waxing period, not at the maximum peak when the gravitation pull of the sun and the moon is strongest. Researchers theorize that the tremors happen when the stress exerted by the tide outmatches the strength of the fault at a specific area. A "slip" is "initiated when stress exceeds the local fault strength," researchers explained in the paper.

What's the big significance of these findings? U.S. Geological Survey seismologist David Shelly told the Times that the findings suggests that, despite the "20 miles of rock" sitting on top, there's a deeper level of the San Andreas Fault that is "much weaker than we would expect." It's also significant because, while scientists have known that the gravitational dance of the sun and the moon compress the Earth like an accordion, the idea that the tides can produce earthquakes isn't quite as well-understood. The U.S.G.S's findings provide a clearer glimpse on the activities that are going on down below. The research also highlights the fact that there are different levels of activity underneath. As noted by Science Magazine, the research shows that daily tides produce small tremors at the deepest regions of the fault, while the fortnightly tide leads to stronger tremors that happen a little higher above.

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So, does this mean that the sun and the moon will conspire to bring about The Big One? Will we be living in an Escape From L.A. scenario thanks to the tides? Not so fast, say researchers. While they've determined that the fortnightly tide causes a lot of tremors, researchers say that they "don't quite know yet what it's going to mean in the long term, whether it'll result in some sort of warning that an earthquake is coming. We're going to monitor it for a lot longer."

Very reassuring. And hey, more than a dozen small earthquakes have shaken up Central California in the past 24 hours. So we're just gonna get under our desk right now. Good bye.