Early-warning system sent alert out 3 seconds before Southern California quake hit

When a 4.4 magnitude temblor hit the Southern California city of La Verne on Tuesday evening, the state's earthquake early-warning system successfully notified users a few seconds before the shaking started.

The system is being developed by the U.S. Geological Survey, and just a few users have access to warnings now. One user, seismologist Lucy Jones, said she was in Pasadena, about 25 miles away from La Verne, when she received a warning, and three seconds later, felt shaking. In April, the system was able to give a 10 second warning ahead of a 5.3 magnitude quake in Los Angeles; the amount of warning time is dependent on how close a person is to the epicenter.

The system works thanks to seismic sensors, which detect shaking. The shaking travels at the speed of sound through rock, which is slower than communications systems, meaning when a quake hits 100 miles away from Los Angeles, the warning reaches the city before the shaking is felt, giving people time to prepare, the Los Angeles Times reports. There are more than 850 quake-sensing stations in operation, but officials say at least 800 more are needed to cover the West Coast. It's estimated it would cost $38.2 million to build a full system, and $16.1 million annually for maintenance and operations.

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