San Andreas Earthquakes May Be Triggered By The Pull Of The Sun And The Moon


Robin Andrews

Science & Policy Writer


A false-color image of the San Andreas Fault, seen here running through the Crystal Springs Reservoir. NASA/JPL-Caltech

The Moon is more powerful than many give it credit for. Almost everyone is aware that it creates tides at the surface of the planet, but few know that it stirs tides deep within the Earth’s liquid core, perhaps even keeping our protective magnetic field stable as a result.

New research has now revealed that it could be doing something even stranger. Writing in the Proceedings of the National Academy of Sciences, this team of researchers think our pale guardian is causing the San Andreas Fault, one of the most dangerous faults in the world, to gradually move and rupture.


Researchers have picked up a fortnightly cycle of deep, small earthquakes occurring within San Andreas, with the most powerful of them occurring at very specific times. In fact, between 2008 and 2015, the team have detected 81,000 of them on an intersection between the slightly mobile northern section of the fault and the “locked and loaded” stationary southern section.

As with tides in the seas and oceans, these earthquakes are most potent when the Sun and Moon align – in other words, when their gravitational forces are acting in the same direction. Importantly, the fault at depth must be weak enough to allow our Moon, and to a lesser extent our conspiring local star, to pull it apart.

“It's kind of crazy, right? That the moon, when it's pulling in the same direction that the fault is slipping, causes the fault to slip more – and faster,” lead author Nicholas van der Elst, a United States Geological Survey geophysicist, told the Los Angeles Times.

In fact, it’s remarkable that the gravitational tug is actually causing these small earthquakes at all, given that San Andreas isn’t oriented in a way that gets the full strength of the tidal forces.



A lunar transit of the Sun, as observed by NASA's Solar Dynamics Observatory. NASA

These earthquakes register at no more than 1.0 on the moment magnitude scale, and anything around this value cannot be felt by humans – especially not 30 kilometers (19 miles) underground, the depth these lunar-induced tremors are occurring.

The San Andreas Fault is, overall, constantly moving, but segments of it have remained almost completely motionless for over a century, meaning that it’s been storing stress for an incredibly long period of time. When the fault does significantly rupture – something people refer to as the “Big One” – there will be many casualties, chaos, and a veritable catastrophe.

So by themselves, these quakes are completely harmless, but is their continued presence a good or bad thing?


Ultimately, it's probably not great. Although it seems like releasing small amounts of stress could help out in the long run, these earthquakes are so weak that they would either not make a difference at all, or they could end up triggering a major event.

In any case, these fortnightly cycles reveal fairly precisely where the fault is rupturing. It appears that whenever the deep part of the fault slips, the stress is transferred to the shallower part. This transferal takes time, so by knowing the rate at which stress is accumulating on the deeper fault, scientists may in the future be able to predict when the far more dangerous shallower section may rupture.


Aerial photograph of the San Andreas Fault in the Carrizo Plain Ikluft/Wikimedia Commons; GFDL


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