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

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

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

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.

These smaller earthquakes, therefore, could theoretically be releasing small but meaningful amounts of built-up stress on the fault. This potentially means that when the Big One happens, it will be somewhat less severe than it would be without the Sun and Moon’s pull. Breathe a sigh of relief, then – we won’t have to destroy the Moon after all.

In addition to this, 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.

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

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