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The nation's first publicly available earthquake early warning mobile app was launched earlier this year as part of a pilot program designed to give Los Angeles County residents a few seconds of warning before the shaking.
So, why didn't users receive a notification from the ShakeAlert LA app when a magnitude-6.4 earthquake -- the region's strongest in 20 years -- rattled a widespread part of Southern California on Independence Day?
ShakeAlert warnings are issued for all quakes, including aftershocks, of magnitude-5.0 or greater in Los Angeles County. Thursday's earthquake was centered to the north in the Mojave Desert in Kern County and did not reach the shaking threshold in Los Angeles County.
"There were no glitches," said USGS seismologist Robert Graves.
More from NBC News:
Graves said the ShakeAlert system provided 48 seconds of warning to the seismology lab well before the shaking arrived at Caltech in Pasadena, just northeast of Los Angeles.
Los Angeles Mayor Eric Garcetti said in a tweet, "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. We hear you and will lower the alert threshold with @USGS_ShakeAlert."
Here's how the app works.
Once the app is downloaded, users are asked to enable notifications. ShakeAlert warnings are issued for all quakes, including aftershocks, of magnitude-5.0 or greater. The alert includes a sound and message that indicates the anticipated intensity level.
The early warning function only works in Los Angeles County. The app does not need to be open, but users must set the phone's location services to "Always On."
ShakeAlert LA also has resources to help prepare for an earthquake.
It's part of the early warning system being built for California, Oregon and Washington, which detects that an earthquake is occurring, quickly analyzes the data and sends out alerts that may give warnings of several seconds to a minute before strong shaking arrives at locations away from the epicenter. A few seconds is enough time to scramble for protection, slow trains, halt industrial processes, trigger back-up power generators and pause surgeries at hospitals.
Pilot programs involving select users have been underway for several years.
A new generation of ShakeAlert software was deployed in September, including improvements in reducing false and missed alerts. False alerts typically have occurred when a large quake elsewhere in the world is detected by a sensor and is mistaken for a local earthquake.
There's still work to do. The sensor network is only about 50 percent of the target size. Funding has been secured to complete the network in California in the next two years.