October 21, 1998
"We found that earthquakes are not closely related to tides," said John Vidale, professor of earth and space sciences at UCLA, who conducted the most extensive study ever on the question. "The relation is very weak, at best. Earthquakes follow their own schedule."
Vidale reports on more than 13,000 earthquakes over 25 years, from 1969 to 1994, along the San Andreas fault near Parkfield and the Calaveras fault in Northern California. He studied when the earthquakes struck and calculated the stresses on the faults at the times they occurred, and at random times. The findings are reported in the October issue of the Journal of Geophysical Research, the premier scholarly journal in solid earth geophysics.
While some, especially amateur scientists, had suspected a strong link between lunar tides and earthquakes, Vidale found that when lunar tides "favor" earthquakes, the rate of quakes is higher by at most two percent, perhaps less. This small correlation is statistically insignificant, he said.
Vidale's research supports the theory that earthquakes are preceded by a "preparatory phase" that begins days in advance and accelerates substantially in the final few minutes or seconds before the actual earthquakes. This theory, developed by Jim Dieterich of the U.S. Geological Survey and others, gives some hope to those who think earthquakes can be predicted, although Vidale said that this "preparatory phase" is not well understood and may not be observable until just seconds before an earthquake, if at all. Even if this preparatory phase could be identified in advance, it would not necessarily indicate whether the approaching earthquake will be small, moderate or strong, he cautioned.
Such a "preparatory phase" may be strong enough to negate the lunar tides, and it could explain why the tides play so small a role in triggering earthquakes, Vidale said.
The amount of stress released in major earthquakes is much stronger than the tidal stresses and may have been built up for hundreds, or even thousands, of years, Vidale said. Because lunar tides change the stress on fault lines much more rapidly than tectonic plate movements, some people believed that earthquakes should occur when the tides pull the ground and add additional stress.
Vidale's co-authors on the article are Duncan Agnew, a professor at UC San Diego's Institute of Geophysics and Planetary Physics and an expert on calculating tides; and Malcolm Johnson and David Oppenheimer, geophysicists with the U.S. Geological Survey. The research was funded by the National Science Foundation and the Southern California Earthquake Center. The Journal of Geophysical Research is published by the American Geophysical Union.