The United States Geological Survey said the ShakeAlert system worked as planned when the 5.1 magnitude quake hit the South Bay Tuesday.
About 95,000 people across the Bay Area got an alert from the MyShake app a few seconds before the shaking started.
It was developed by the UC Berkeley Seismology Lab using the ShakeAlert infrastructure.
“We’re trying to focus on areas that could experience damage,” said Dr. Jennifer Strauss of the MyShake app.
Get a weekly recap of the latest San Francisco Bay Area housing news. Sign up for NBC Bay Area’s Housing Deconstructed newsletter.
But many others with the same app said they didn't get an alert and Strauss said it has to do with people’s phone settings.
“So people need to have location services turned ‘always on,’ not just while using the app and/or set a homebase location, and has not paused notifications to the app or offloaded the app in the case of iOS,” said Strauss.
Phones using a VPN also won’t get the alert.
But the demand is clear. The MyShake app had more than 100,000 new downloads since the quake.
And MyShake isn’t the only option.
There’s also a San Diego County app that works across the state and QuakeAlertUSA.
Google will also send shake alerts, but only to phones using Android.
If multiple sensors detect an earthquake of 4.5 or greater, it’ll alert users based on their settings.
The USGS said the system worked Tuesday.
“We saw a lot of effective alerts go out to people throughout the San Jose region,” said Robert De Groot of the USGS’s ShakeAlert operations team.
Another reason you may not get the alert is how close you are to the quake.
If you’re on top of the fault, it’ll be too late -- like many in East San Jose Tuesday.
Others may be too far away.
“Only certain people will get those alerts. It’s people who are within the intensity three region or larger,” said De Groot. “Intensity three is sort of akin to a truck passing by your house.”
The teams of scientists developing the ShakeAlert continue to compile data from every quake to see if there are other precursors to speed up the warning time.
Still, experts said it’s good to prepare and check to make sure your app is ready before the next one hits.
