ShakeAlert fails to send earthquake warning for big SoCal quake

California is still in the testing stages of using ShakeAlert, an app that lets residents know a large earthquake has begun in their area. California is still in the testing stages of using ShakeAlert, an app that lets residents know a large earthquake has begun in their area. Photo: USGS Photo: USGS Image 1 of / 24 Caption Close ShakeAlert fails to send earthquake warning for big SoCal quake 1 / 24 Back to Gallery

The future of earthquake preparedness on the West Coast is supposed to be ShakeAlert, an app that warns residents that a significant earthquake is about to start shaking. But many Southern California residents were disappointed that they never received an alert before or during the 6.4-magnitude earthquake that rattled Searles Valley in San Bernardino County Thursday.

At a press conference held by the USGS and Caltech about an hour after the Ridgecrest quake, Dr. Lucy Jones, a research associate at the Seismological Laboratory of Caltech, confirmed ShakeAlert had not sent notifications to users.

There's good news along with the bad, however. Jones said the alert system did function as expected in the lab — it did detect the earthquake. The disconnect happened with the alerting system, which did not then send out any alerts to citizen users.

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Although Jones said it is still too soon to say why ShakeAlert didn't send that notification — and that developers would be looking into the problem in the coming days — the official City of Los Angeles Twitter account tweeted the problem was with the app's notification threshold.

"The #ShakeAlertLA app only sends alerts if shaking is 5.0+ in LA County. Epicenter was 6.4 in Kern County, @USGS confirms LA's shaking was below 4.5," the city account tweeted. "We hear you and will lower the alert threshold with @USGS_ShakeAlert."

ShakeAlert cannot "predict" an earthquake. Instead, it reacts to shaking that has already begun, giving people some time to protect themselves before the brunt of the earthquake hits.

"ShakeAlert is not earthquake prediction," reads the app's website, "rather a ShakeAlert indicates that an earthquake has begun and shaking is imminent."

Today's July 4 earthquake hit at 10:33 a.m. local time at a depth of about 5 miles. Its preliminary magnitude is 6.4, making it the strongest earthquake to hit Southern California since 1999.

Katie Dowd is the SFGATE Senior Manager. Email: katie.dowd@sfgate.com | Twitter: @katiedowd