Just after the magnitude 7.1 earthquake in Ridgecrest that was felt throughout Southern California, seismologists with Caltech and the U.S. Geological Survey put their earthquake early warning system to the test.
During a live briefing with reporters Friday, Dr. Lucy Jones noticed a system alert that said a quake was about to strike.
"We have an earthquake that's already begun up north, just slightly northwest, of the main shock," she said with about 26 seconds to go.
And sure enough, the aftershock hit when she said it would and prompted phones to ring.
"There it is."
The actual magnitude came in less intense than the 5.6 that the system anticipated, but it worked.
The ShakeAlertLA app, an early warning system that detects significant earthquake activity and is supposed to alert residents 48 seconds before the shaking hits, didn't send an an alert for the 6.4 quake on the Fourth of July.
Seismologists said the ShakeAlert computer at Caltech's seimic lab registered the warning, but the signal didn't go out to the public because it didn't meet the threshold for significant shaking.