Gravitational Tug of Sun and Moon Could Trigger Earthquakes Along San Andreas Fault

A new study revealed that the formation of the Earth, Sun and Moon that cause changes in Earth's tides can also trigger a special kind of Earthquake along the California's San Andreas Fault.

The study, published in the journal Proceedings of the National Academy of Science, is based on previous researcher suggesting that the gravitational tug of sun and moon is not only affecting the sea levels of Earth, but can also stretch and compress that Earth's crust, producing tremors and quakes. Furthermore, the new study showed that small tremors deep underground were more likely to occur during certain periods of the tidal cycle.

For the study, the researchers looked at about 81,000 low-frequency earthquakes from 2008 to 2015 along the Parkfield section of the San Andreas Fault and compared it to the two-week tidal cycle known as the "fortnightly tide." Low-frequency earthquakes are named due to the rumbling sound they make and not by the rate of its occurrence. These kinds of earthquake only occur about 15 to 30 kilometers below ground and are no larger than magnitude 1 in the Richter scale.

The researchers discovered that low-frequency earthquakes tend to occur during the waxing period of the tidal cycle, when the tides are getting bigger the fastest. It is already known that the strongest Earth tides occur when the sun and moon are aligned, while the tide is at its weakest state when the sun and moon are 90 degrees apart.

According to the researchers, low-frequency earthquakes show that some part of the San Andreas Fault is creeping, or slowly moving, almost all the time. With the help of the tidal forces, low-frequency earthquakes could provide more information about the deeper part of the fault.

"They tell us that the fault continues down below where the regular or typical earthquakes stop on the San Andreas, about 10 or 12 km (about 6 to 7 miles)," said David Shelly, a seismologist at USGS and helped write the study, in a statement. "And they tell us a lot of things about that deep part of the fault that before, we had no idea existed at all."

Their discovery could also be used to predict future earthquakes because whenever the deep part of the fault slips, the stress gets transferred in the shallow parts of the fault.