Abnormal atmospheric pressure patterns over the Pacific Ocean and western United States during the summer and fall may trigger earthquakes in California, and may have done so with fatal results in July 1986 and last October, a scientist says.
"I've found tantalizing but not completely conclusive evidence of a relationship between sea-level pressure patterns and their associated winds and moderate and larger earthquakes," said meteorologist Jerome Namias.Namias spent 30 years as chief of the National Weather Service's extended forecast division in Washington and is now affiliated with Scripps Institution of Oceanography in La Jolla. His work in long-range weather forecasting is widely respected.
The statistical correlation between earthquakes and air pressure is weak, difficult to explain in scientifically respectable terms and "definitely a far-out idea," said Namias in a recent telephone interview.
But he speculated that if atmospheric pressure patterns are related to quakes, it may be because winds associated with the patterns raise offshore sea levels slightly, putting more pressure on the earth.
"We know so little about why earthquakes occur when they do that we've got to keep an open mind, even on what seems like a far-out kind of idea," said geologist Clarence Allen of the California Institute of Technology.
Allen said that while he guesses the correlation will turn out to be coincidental, "it's too early to rule out these kinds of things. And Namias is certainly a very respected scientist."
Forces from unsual air pressure patterns and higher sea levels are smaller than the moon's tidal influence on Earth and its oceans, and seismologists still debate whether lunar gravity triggers quakes, said Tom Heaton, scientist in charge of the U.S. Geological Survey's Pasadena office.
"If we can't prove that tides do it, it seems a long shot that (any pattern of) atmospheric pressure does it," Heaton said.
Namias said he studied Caltech's list of California quakes measuring 4.5 or more in the Richter scale since 1947.
He found the quakes tended to occur more often a month to six weeks after the onset of an abnormally strong gradient between very high sea-level atmospheric pressure over the eastern half of the North Pacific and very low, surface-level pressure over the interior portion of the western third of the nation.
The unusual pattern existed prior to the 5.9-magnitude Whittier Narrows quake and its aftershocks, which rocked Southern California last October, killing eight people, injuring more than 200 and causing $358 million in damage, Namias said.
He said the same pattern preceded three hefty jolts in July 1986: the 5.9-magnitude July 8 Palm Springs quake, which caused $5.3 million in damage; the 5.3-magnitude July 13 Oceanside quake, which killed one person, injured 28 others and caused $720,000 in damage, and the 6-magnitude July 21 Chalfant Valley quake near Bishop, which caused $436,500 in damage.
The abnormal pattern involved barometer readings of roughly 30.1 inches of mercury offshore and roughly 29.75 over the interior, Namias said.
Namias said the unusual large-scale pattern is unrelated to local weather or to what some people contend is "earthquake weather," unseasonably hot, dry, windy conditions such as those last October.
Namias said he first noticed the quake-air pressure pattern correlation years ago, but even after the 1986 quakes he decided "I'd keep my mouth shut on the topic" because he knew seismologists would be dubious. He changed his mind after last October's shocks.