Shake Alert app: The reason why some didn't receive the alert

Did you receive a shake alert notification this morning about the 6.4 earthquake? If not then you're not alone.

According to the City of L.A. those in the Los Angeles area didn't receive a notification because the shaking was below 4.5. The app only sends alerts if shaking is over 5.0 in L.A County.

Related: L.A. County earthquake early warning mobile app available for download

Following the quake several people took to Twitter to express their frustration and concern about not getting a notification. In a post online the City said "we hear you and will lower the alert threshold".

Seismologist Lucy Jones of Caltech says the shake alert computer system at Caltech's seismic facility registered a 48-second warning that shaking had begun.

Several aftershocks in the magnitude-3 range were reported by the USGS, as expected following a substantial quake. Jones said there was a better than 50-percent chance of a larger aftershock in the magnitude-5 area sometime this afternoon.

Several police agencies are reminding the public not to use 911 for earthquake questions unless they have injuries or dangerous conditions to report.

The last Southern California earthquake to exceed 6.4 was on Oct. 16, 1999, when a magnitude 7.1 quake struck in the Mojave Desert near Twentynine Palms Marine Corps Base. That quake knocked an Amtrak passenger train off its tracks and damaged two highway bridges.