People in Los Angeles are wondering why their phones didn't warn them before the ground started shaking on Thursday.
But officials say it was not a failure.
The ShakeAlertLA app, which is connected to the US Geological Survey's network of early warning sensors, is set to alert cellphone users when the system senses an earthquake of magnitude 5.0 or greater, or with potentially damaging shaking, within Los Angeles County.
While Thursday's quake was greater than magnitude 5 in Kern County, in Los Angeles the shaking was below 4.5, the city said.
"The system actually worked as designed," Rob Graves, a seismologist at the US Geologic Survey, told reporters.
After receiving questions from residents about the lack of push alerts from the app, the city of Los Angeles said officials will be making adjustments.
"We hear you and will lower the alert threshold with @USGS_ShakeAlert," the city tweeted.
Earlier this year, residents downloaded the app hoping it would give them a heads-up before a big quake hits. USGS said the warnings could go out seconds, or even tens of seconds, before people start to feel the shaking.
The alert would read "Earthquake! Earthquake! Expect strong shaking. Drop, cover and hold on. Protect yourself now!"
Los Angeles was the first US city to make the app available to the public, but the US Geological Survey has said it plans to deliver it to the entire West Coast once the pilot project is completed.
The app is a collaboration between Los Angeles, the US Geological Survey, the Annenberg Foundation and AT&T, CNN's parent company. It's available in English and Spanish in the Apple and Google Play stores.
CNN's David Williams contributed to this report.
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