The quake was too small to cause much damage but was felt over a wide area.
Seismologist Lucy Jones told reporters at Caltech on Tuesday night that the system sent out a warning to her location in Pasadena three seconds before the shaking began to be felt there. The amount of warning time depends on how close the person receiving the alert is to the earthquake’s epicenter.
And residents who live near the epicenter said the quake packed a punch. Vickie Carillo was sitting with her son on the couch watching “Jaws 2” when they felt the shaking start.
“It was like if somebody had grabbed it and was shaking the house,” she 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 it later this year.
The system already has proved successful. The alert gave scientists at USC a 10-second warning before waves arrived from a 5.3 temblor that hit near the Channel Islands in April. And in 2014, the prototype early-warning system gave San Francisco eight seconds of warning before a 6.0 magnitude earthquake hit Napa.
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 would 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.
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 steps can be taken 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.
Some office buildings have also been wired to automatically bring elevators to the nearest floor, preventing people from being trapped after an earthquake.
Times staff writer Rong-Gong Lin II contributed to this report.
3:15 p.m.: This story was updated to make clear the three second warning was for users in Pasadena, where Caltech is based.
10:10 a.m.: Updated with image of warning alert for Napa quake.
9 a.m.: This article was updated with details about previous alerts made by the system.
This article was originally published at 7 a.m.