Business Insider] Extending for 800 miles, the earthquake-prone San Andreas Fault in southern California has been relatively quiet lately.
Deep below the surface, along the fault, however, there are
hundreds of thousands of tremors called low-frequency
earthquakes, which occur routinely in areas where the
planet’s brittle crust is getting weaker and softer.
Gravitational tugs from the moon and the sun don't just
cause the rise and fall of the seas (high and low tides), they
also cause the surface of the Earth to go up and down as well,
stretching and compressing the planet’s crust in “solid Earth
tides.” These earth tides stress the faults deep inside the
planet, making it easier or more difficult for faults to slip
depending on the tidal phase.
Previous research has found that these Earth tides can
trigger tremors, but a new study published in Proceedings
of the National Academy of Sciences found
that these small, deep earthquakes within California’s San
Andreas Fault are more likely to occur during certain phases of
the tidal cycle.
In other words, when the tug of the moon or sun is pulling
in the same direction as the fault is slipping, it causes the
fault to slip farther, and faster.
Like with ocean tides, the strongest Earth tides occur when
the sun and moon are aligned, and the weakest occur when they are
90 degrees apart. Surprisingly, the number of low-frequency
earthquakes didn’t spike at the strongest point in the tidal
cycle, but instead when the tide was waxing
The researchers arrived at these results when they compared
the timing of 81,000 catalogued low-frequency earthquakes — not
larger than a magnitude 1 on the Richter scale — along
the San Andreas Fault between 2008 and 2015 to the two-week solid
earth tidal cycle, known as the “fortnightly tide.” Read More