Could California's long-dreaded "Big One" be triggered by a full moon?
Perhaps, says a new study out Monday that claims large earthquakes are more likely during unusually high tides, which occur during full and new moons.
High tides, which typically occur twice a day, are caused when ocean water is moved by the gravitational pull of the moon. But twice a month, during a full or new moon, tides are especially high because the moon, earth and sun all line up together. (These twice-monthly tides are known as "spring" tides.)
Big quakes can occur when this additional weight of tidal water strains geological faults, according to the study. Though this theory is not new, this is the first study to display a firm, statistical link.
“The probability of a tiny rock failure expanding to a gigantic rupture increases with increasing tidal stress levels,” the study said.
Precisely how large earthquakes occur is not fully understood, but scientists say they may grow via a cascading process where a tiny fracture builds up into a large-scale rupture. If so, the authors’ results imply that the likelihood of a small fracture cascading into a large earthquake are greater during high tides.
The study was led by Satoshi Ide, a seismologist at the University of Tokyo and appeared in the peer-reviewed British journal Nature Geoscience.
Ide found that some of the most devastating recent earthquakes, such as the 2004 Sumatra quake that killed 230,000 people and the 2011 quake in Japan that killed 15,000, both hit during periods of high tide. In fact, his research team determined that nine of the 12 biggest quakes on record happened near or on days with full or new moons.
The scientists found no clear correlation between high tides and small earthquakes.
The study could help improve earthquake forecasting, the authors say, in places that are especially vulnerable to high seismic activity
"Scientists will find this result, if confirmed, quite interesting," said University of Washington seismologist John Vidale, who was not part of the study. He cautions that "even if there is a strong correlation of big earthquakes with full or new moons, the chance any given week of a deadly earthquake remains miniscule," making predictions rather unhelpful.
The study's other authors are Suguru Yabe and Yoshiyuki Tanaka, also of the University of Tokyo.