A local resident inspects a crack in the earth after a 6.4 magnitude earthquake struck the area on July 4, 2019 near Ridgecrest, California.

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.

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.