SAN JOSE – After a devastating earthquake hit the Hayward Fault in 1868, Bay Area physician J.D. Cooper shared what sounded like a half-baked idea at the time: Use wires and electric currents to sound alarm bells at the start of a major earthquake, giving a brief warning to people living away from the epicenter. The idea – or a variation of it – is likely to become reality in California within the next decade, many seismologists believe. And while 10 to 20 seconds’ warning might not sound like much, experts say it could make a difference. Commuter trains could be slowed to reduce the risk of derailments. Fire stations could open their garage doors and get their engines ready to roll. Interstate metering lights could all be turned to red to keep vehicles at a stop. Elevators could be programmed to open at the nearest floor to prevent people from getting trapped when the quake hits. Mexico City, although prone to devastating quakes, has the good fortune to be situated about 180 miles from a quake zone, allowing for about 100 seconds of warning before dangerous quakes rumble through town. Japan’s system is geared for earthquakes that originate offshore, also giving residents more warning time. But in California, where many cities are built directly atop numerous faults, warning times wouldn’t be greater than tens of seconds – tops. The warning systems don’t predict quakes before they happen, but instead sense a temblor’s motions and sounds alarms miles away, before the shaking has a chance to arrive. In Mexico City, old-fashioned seismographs wait for the peak ground shaking along the coast, estimate the size of the quake, then issue a warning if it is strong enough. The warnings are sent via radio signals, which are detected by special boxes in government offices and some residential areas. The signals are also sent to radio and TV stations, which relay the warnings. California researchers are working on a more advanced system that senses primary waves, the earliest of two types of energy or motions from an earthquake. These P-waves move much faster than secondary waves, which cause the greatest damage during a quake. California scientists are cautiously optimistic that within the first few seconds of an earthquake, high-tech observation stations could detect the P-waves, and a computer could then quickly estimate the expected magnitude of the quake. If it seems like a large quake – which usually can’t be confirmed until the S-waves arrive – a warning would be issued immediately. The warning wouldn’t be of much help at the epicenter, where only about two seconds separate the arrival of the P- and S-waves. But farther out, the slower S-waves take a little time to appear. During the 1989 Loma Prieta earthquake, which originated in the Santa Cruz Mountains, most of the fatalities occurred about 60 to 70 miles away in San Francisco and Oakland. “If we were to have a repeat of that earthquake, we would have about 20 seconds’ warning for Oakland or San Francisco,” UC Berkeley’s Allen said. While that isn’t long enough to help someone in a high-rise building or movie theater, it could give transit operators and first-responders a chance to take some action. “The more heads-up we can get, the safer we can make the system,” said Linton Johnson, a spokesman for the BART rail service in San Francisco. “You can’t stop a train on a dime, but slowing it down may very well limit the number of injuries in a massive quake.” But scientists concede that even if a system works as well as hoped, just as many buildings will be destroyed, and many lives will still be lost. “Earthquake early warning is not a panacea,” said Dave Wald, a seismologist with the U.S. Geological Survey in Golden, Colo. “Some people won’t get a warning.” Scientists are still trying to determine just how accurate and reliable the proposed system would be. By quickly estimating the size of an earthquake before accurate measurements are in and then sounding an alarm, the system could create unnecessary fear if the quake ends up being smaller than expected. But if the system waited until the true magnitude of a quake was known, there really wouldn’t be much warning at all. “It’s inevitable there will be some false alarms,” Allen said. “It’s also inevitable there will be some missed alarms. The question is: How many will there be?” There’s also the question of expense. Allen estimated California would need about 300 monitoring stations, costing from $10 million to $30 million. Despite the complications, Wald expects that a warning system will be – and should be – operational within the next decade. “There’s a lot more that needs to be worked out,” he said. 160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! There could even be a public alert system, the modern-day equivalent of an air-raid siren, but experts are debating whether that would promote safety – or panic. “I think it’s clear there’s benefit to having a warning system like this,” said Richard M. Allen, a professor at the seismological laboratory at the University of California at Berkeley, who is among those spearheading the efforts to create such a system. “And we’re developing one of the most accurate warning systems that exists.” Indeed, government and private university workers are now testing an early prototype for an earthquake warning system – one that works by measuring the fastest-moving waves from a quake, and sending an alert before the slower, more devastating waves hit. The concept isn’t new – earthquake early-warning systems are already up and running to some degree in Japan, Mexico, Taiwan and Turkey. But scientists say those systems wouldn’t be as effective in California because of the differences in geography and city design.