John Vidale, director of the Southern California Earthquake Center, said the system worked. He said he and others at the center at USC heard the alarm go off before they felt the shaking, "maybe 10 seconds ahead, enough for us to puzzle about it before the shaking hit."
"It was pretty widely felt across the USC campus. We only felt about 3 to 4 seconds, kind of a vibration, kind of long enough for it to sink in that it was an earthquake, before it stopped," Vidale said.
The earthquake early-warning system is under development by the U.S. Geological Survey and is only available to a limited array of testers, but it is expected that more people will be eligible to test the system later this year.
It works on a simple principle: The shaking from an earthquake travels at the speed of sound through rock — which is slower than the speed of today's communications systems.
For example, it would take more than a minute for a magnitude 7.8 earthquake that starts at the Salton Sea and travels up the state's longest fault, the San Andreas, to shake Los Angeles, 150 miles away. An early warning system would give L.A. residents crucial seconds, and perhaps even more than a minute, to prepare.
It got a significant boost in the federal budget signed into law in March, defying an earlier proposal by President Trump to end federal funding for the program.
As part of the $1.3-trillion budget bill approved by Congress and signed by Trump, officials approved $22.9 million for the project. That more than doubles the $10.2 million it got in the previous year's budget.
A seismic early warning system for the West Coast has been under development for years by the USGS, the nation's lead earthquake monitoring agency, but the project has remained short of funds.
It's estimated that building a full system covering the West Coast will cost at least $38.2 million, with about $16.1 million annually to operate and maintain it.
The USGS has said it planned to begin issuing limited public alerts from the system by the end of this year, as long as funding wasn't cut. Southern California is one area where the network of seismic sensors is dense enough at present to begin early warnings.
For the system to go live all along the West Coast, more sensors need to be installed in Washington, Oregon and sparsely populated areas of Northern California. More than 850 earthquake-sensing stations are online, but about 800 more are needed, officials said. Too few sensors could mean, for instance, that Los Angeles would experience delays in warnings from an earthquake that starts in Monterey County and barrels south along the San Andreas fault.
Along the West Coast, facilities including airports, oil refineries, pipelines, schools, universities, city halls and libraries are already testing or planning to test the system.
Hospitals in California are testing audible notifications, broadcast from fire alarm equipment, so staff can take steps such as surgeons engaged in operations removing scalpels from patients.
Condominium towers testing the system have been similarly rewired to give residents time to drop, cover and hold on before shaking arrives.