The Gravitational Tug of Sun and Moon Causes Earthquakes on the San Andreas Fault (+Video)

The gravitational tug between the sun and moon is not just a dance of high and low tides: It can also trigger a special kind of earthquake on the San Andreas Fault.

This phenomenon has fascinated scientists for years. Like sea levels, the surface of the Earth also goes up and down with the tides, flexing the crust and stressing the faults inside. Further study found that during certain phases of the tidal cycle, small tremors deep underground – known as low-frequency earthquakes – were more likely to occur.

When the moon is pulling in the same direction that the fault is slipping, causes the fault to slip more – and faster according to the new study. 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.

Studying how these low-frequency earthquakes respond to the tides can reveal new information about the San Andreas and what it might mean for larger earthquakes, researchers say. 

Scientists first discovered these deeper tremors on the fault about 10 years ago, along a particularly sensitive section in Parkfield, Calif., where the San Andreas transitions from its northern section, where it’s gently releasing tectonic energy, to its southern portion, which is locked and capable of producing a big one.

Researchers looked at about 81,000 low-frequency earthquakes from 2008 to 2015 along the Parkfield section of the fault and compared it to the two-week tidal cycle known as the “fortnightly tide.” They found that these earthquakes were most likely to occur during the waxing period, when the tide was getting bigger the fastest.

Like ocean tides, the strongest Earth tides occur when the sun and moon are aligned, and the weakest occur when they are 90 degrees apart. The same gravitational forces stretch and compress the Earth’s crust (though the rock moves less dramatically than seawater).

Some faults are more susceptible to tidal triggering than others, such as offshore faults like the Cascadia subduction zone off the Pacific Northwest coast, scientists said. Other characteristics of the fault, such as its orientation or how close it is to the Earth’s crust, also affect the tidal response.

It’s remarkable that the San Andreas even produces small earthquakes in response to tidal forces, researchers said, given that the fault is not oriented in a way that gets the full strength of the tides.

They also show that this part of the San Andreas is creeping, or slowly moving, almost all the time. 

Phys.Org
Read more here: http://tinyurl.com/h5h238v

The new study on the subject published Monday in the Proceedings of the National Academy of Sciences.

Fortnightly modulation of San Andreas tremor and low-frequency earthquakes
http://tinyurl.com/hu8efzd

Clips, images credit: USGS, ESO, ESA/HUBBLE & NASA