The gravitational pull between the moon and the sun not only leads to high and low tides on earth but also causes a particular kind of earthquake on the San Andreas Fault.
Examining 81,000 low-frequency earthquakes on the San Andreas Fault has made scientists reach this conclusion.
Nicholas J. van der Elst from the US Geological Survey explained that "small and deep earthquakes within the fault" happen during the waxing of the tide. The San Andreas Fault in California extends for 1,300 kilometres.
This is one phenomenon that has intrigued people for years. Experts have been puzzled that the Earth, like sea levels, goes up and down along with the tides. It is an activity that "flexes the crust and stresses the faults inside."
"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. What it shows is that the fault is super weak - much weaker than we would expect - given that there're 20 miles of rock sitting on top of it," Nicholas told Los Angeles Times.
Comparing information by analysing 81,000 low-frequency earthquakes from 2008 to 2015 to the two-week tidal cycle named the "fortnightly tide," scientists found that the fortnightly tide cycle tends to trigger off earthquakes on the San Andreas Fault.
USGS seismologist David Shelly said: "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 six to seven miles]. And they tell us a lot of things about that deep part of the fault that before, we had no idea existed at all."
Low-frequency earthquakes happen when the stress due to solid Earth tide exceeds the fault strength at some point, leading to a fault slip. Due to the gravitational pull of the moon and the sun, there is a dipping as well as a lifting in the earth's crust, which compresses as well as stretches crustal rocks, and finally, results in an earthquake.
Hence, there appear to be links between tidal forces and earthquakes. Plate tectonics can be studied in this manner, and can also help seismologists to predict the occurrences of tremors or earthquakes.
The study was published in Proceedings of the National Academy of Sciences.
