If a devastating earthquake were about to strike, would you want to know about it 30 to 60 seconds before it hit?
That might not be enough time to run out and buy quake insurance, but it is enough to take some precautions and possibly save a few lives, according to scientists who are looking into the possibility of setting up an earthquake warning system in California.
Southern California seems particularly well suited for such a system because the most dreaded of faults, the San Andreas, is some distance from the most densely populated areas. Sensors along the San Andreas could telegraph a warning that a catastrophic quake has struck up to a minute and a half before the deadly shock waves hit the population centers.
The time lag could be less--depending on which segment of the San Andreas breaks loose--but some warning could be given even if the quake hits along the part of the San Andreas that is closest to the metropolitan areas.
The San Andreas is believed capable of generating the largest earthquake credible for this region, but smaller quakes closer to developed areas could actually do more damage, and the advance warning would be much shorter for closer faults.
However, a number of scientists believe that Southern California should seriously consider developing a system that will give some warning, even if very brief, of a catastrophic quake. It would be the first earthquake warning system in the country.
"A lot of people are playing around with it," said Thomas H. Heaton, chief scientist of the U.S. Geological Survey's seismological laboratory in Pasadena and one of the principal backers of the idea. The Southern California system envisioned by Heaton would be similar to a system that has been in effect in Japan for 20 years, but it would be much broader in scope.
Heaton sees an automated system that would instantly shut down some key facilities to reduce losses when the earthquake hits. However, at this point no one is sure what those facilities might be, and a feasibility study authorized by the state Legislature has not been funded.
Others carry the possibilities further. Brian Tucker, geophysics officer of the California Division of Mines and Geology, believes the warning could even include flashing red lights in schools to give children enough time to get under their desks before the quake hits.
Tucker is the man who would carry out the feasibility study if it is ever funded. The cost for the study was originally estimated at $200,000, "but we could learn something for less," he said.
"I prefer to call it a cost-benefit study," Tucker said, "because there's no question that it is feasible. The only question is whether the benefit would be greater than the cost."
When an earthquake strikes, low-intensity waves are sent out from the epicenter, traveling at about four miles per second. They can be detected by seismometers some distance from the fault, even though the ground motion from those first waves is so slight that "you wouldn't even feel it," Heaton said.
Meanwhile, "the fault is unzipping itself" as the quake ruptures down the fault zone at about half that speed, Heaton said. It is this movement down the fault that generates the powerful shock waves that can cause buildings to collapse many miles away.
Heaton said news of the first movement could be transmitted electronically from the epicenter "to areas that may be strongly shaken when seismic energy propagates to them."
Civil Defense Warnings
That electronic impulse could trigger existing civil defense warning systems, such as sirens. Tucker believes that it also could be used to turn on audio cassettes at radio stations, thus broadcasting a warning and telling people to stay calm.
He suggested that anyone wanting to use the system could tie into it electronically and that automatic responses could be programmed according to the user's needs.
The time between the warning and the arrival of the shock waves would depend on the distance between the fault and the system's user.
Geologists believe that a major earthquake of magnitude 8 on the Richter scale will strike along the southern San Andreas Fault in the next 10 to 30 years.
The largest earthquake to hit Southern California in the last two centuries struck in 1857. The epicenter was below the Central California community of Parkfield, and the quake ruptured southward along the San Andreas.
If that quake were to be repeated, an early warning system could detect that it was on its way about a minute and a half before the shock waves hit Los Angeles, Heaton said.
The warning time would shrink to "probably about a minute" if the Salton Sea segment on the southern San Andreas ruptured instead of the Parkfield leg, since the southern segment is much closer to Los Angeles, he added.
If the epicenter was near Palm Springs, the warning would be reduced to "maybe 10 or 20 seconds."