ShakeAlertLA: Here's why a 6.4 earthquake didn't trigger an alert from Los Angeles' early warning system

LOS ANGELES -- The ShakeAlertLA app is an earthquake early warning system that detects significant earthquake activity and is supposed to alert residents 48 seconds before the shaking hits. But after Thursday's 6.4 earthquake hit, many people who had downloaded the app were wondering why they didn't receive a notification.

Seismologist Dr. Lucy Jones said the ShakeAlert computer system at Caltech's seismic lab registered a 48-second warning that shaking had begun, but the warning signal did not go out to the public.

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When the ShakeAlert system works correctly, sensors placed by the U.S. Geological Survey in certain fault lines detect the earthquakes. Alerts, that work much like an Amber Alert, are then sent to mobile phones across the area.
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So what happened? Robert Graves, with the USGS, said the the early warning system actually worked as designed. He explained why scientists at Caltech received a warning but not the public.



"The app, with the city of Los Angeles, is set up with various thresholds. The first threshold is a magnitude 5 or greater in L.A. County; it will send out an alert. This earthquake was outside of L.A. County, so it didn't satisfy that condition," Graves said.



"The other condition is intensity, greater than 4. The intensity in L.A. was 3 and lower," he said. "So, the app performed as designed. It's just that the intensity levels were below that threshold that had been set by the app."
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A statement on Los Angeles' official Twitter account said: "The #ShakeAlertLA app only sends alerts if shaking is 5.0+ in LA County. Epicenter was 6.4 in Kern County. We hear you and will lower the alert threshold with USGS."'

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Graves confirmed that officials will be reviewing whether the threshold should be lowered or adjusted for the public delivery system.

Experts point out that while 48 seconds may not sound like a significant amount of time, it's enough to stop a surgery or other medical procedure in progress, divert a plane landing, pull over a vehicle or get into a safer position if a person is inside a building or at home.
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