Deep tremors are caused by pulsing of the tides.
Ocean tides pulse twice daily as the moon revolves around the Earth. As the sun and moon change their relative positions in the sky, they also exert gravitational tug on the Earth that stretches and compresses crustal rocks over a fortnightly (14-day) cycle. Similar to ocean tides, Earth’s tides are strongest when the sun and moon are aligned, and weakest when they are 90 degrees apart.
The researchers examined about 81,000 low-frequency earthquakes from 2008 to 2015 along the section of the fault below the town of Parkfield and compared them to the tidal cycles. Study lead author Nicholas van der Elst, a seismologist at the U.S. Geological Survey in Pasadena, California, told Science News 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.”
They found that low-frequency earthquakes are most likely to occur when the fortnightly tide is rapidly increasing in size — not when it is at its peak, as might be expected.
“It's kind of crazy, right?” van der Elst commented to the Los Angeles Times. “That the moon, when it's pulling in the same direction that the fault is slipping, causes the fault to slip more — and faster.” This finding demonstrates the weakness of the fault, which is surprising, given that “there's 20 miles of rock sitting on top of it.”
Studying how these low-frequency earthquakes respond to the tides could eventually help scientists understand how major earthquakes are set off.
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