The Gravitational Pull Of The Sun And Moon Causes Californian Earthquakes

The earthquakes in California's San Andreas Fault, which is the major plate boundary fault in Southern California, are caused by the gravitational pull of the sun and the moon, according to a new study.

The study was printed in Proceedings of the National Academy of Sciences. It was led by Nicholas J. van der Elst, a seismologist from the U.S. Geological Survey and other colleagues, according to the Science Explorer.

The scientists have long been marveled if the gravitational pull of the sun and moon could trigger earthquakes on Earth. This was confirmed by the analysis of the 81,000 low-frequency earthquakes along the San Andreas Fault.

The team compared the 81,000 low-frequency earthquakes from 2008 to 1015 along the fault below the town of Parkfield to the tidal cycles. They discovered that the low-frequency earthquakes could occur when the fortnightly tide, which is the two-week tidal cycles, is rapidly increasing in size---not when it is at its peak, as might be expected.

Van der Elst explained that the daily tidal peaks seem to trigger the littlest, deepest tremors, whereas the larger spring tide sets off larger patches of slip higher up. He further explained that the moon, when it's pulling in the same direction the fault is falling, causes the fault to slip more --- and faster. He added that the finding demonstrates the weakness of the fault, which is astonishing, given that they're 20 miles of rock sitting on top of it, as noted by Los Angeles Time.

The team also said that the response of faults to the tidal cycle opens a window into the workings of plate tectonics. The quakes start out with magnitude less than 1.0 and occur near the earth's crust meets the mantle.

David Shelly, a seismologist from USGS said that if all of a sudden, they saw that the deep part of the fault was slipping a huge amount; it might suggest that there was an increased chance of having an earthquake come at the shallower part of the fault.