Many in the Bay Area got a scare this October morning: a piercing alarm emanated from cellphones just before 9:30 AM. The message effectively said brace yourself, an earthquake is coming.

But then for many, nothing happened.

Here's audio from a live report on KTVU...

"So the ShakeAlert system actually did what it was doing within about five seconds after the earthquake started, and so that means that our word started arriving to phones within about eight seconds," Robert De Groot, head of the USGS ShakeAlert early warning system.

He says not a lot of information is available yet on how the alert system worked, but that a ShakeAlert was sent to Northern California residents.

The USGS says the quake was centered about 3 miles away from Isleton and about 8 miles from Bethel Island.

The alert reported it as a 5.7-magnitude earthquake before the USGS downgraded it  minutes later.

The cellphone ShakeAlert came the day before the Great ShakeOut, the yearly test for emergency systems. As a part of this, MyShake app users will get an earthquake test alert on Thursday.

It's also a day after the 34th anniversary of the deadly Loma Prieta earthquake in 1989.

If you've turned off Amber or other alerts in your settings, then you may have not received the loud warning as so many others across the Bay Area did.

For now, no major damage is being reported in Oakley or Isleton, close to the epicenter of what is now being classified as a 4.2=magnitude earthquake.