SAN DIEGO — When the first of a series of earthquakes rolled through San Diego July 4, some residents questioned why they didn’t get a notification. 

Just a week earlier, most phones countywide buzzed with an alert that tested the Wireless Emergency Alert (WEA) system that would send ShakeAlerts, but the system isn’t ready to go live. 

What is a ShakeAlert? 
Across the country, the U.S. Geological Survey has ground sensors that detect primary and secondary waves. There are 45 in San Diego County. 

When a sensor detects primary waves from a significant earthquake, it triggers an alert to the USGS. A ShakeAlert would then take that data and send a notification to you that secondary waves, which cause shaking, are imminent. 

Are all ShakeAlerts the same?
No, there are three different notification systems. Only one is currently working.

1) ShakeAlert LA – This is the only functioning system and only serves Los Angeles County. It is set to notify residents when shaking above a magnitude 5.0 is imminent within the county. This is an app-based system that requires users to download an app. 

2) ShakeAlert – This is a statewide app that is still in development. It would send notifications based on a user’s location. It would require users to download an app. 

3) ShakeAlert WEA – This is the system that was tested last week. It would send a notification to all phones within a designated area based on the geolocation of the device. It would work similar to an AMBER Alert and would not require users to download any additional programs.

Why are there so many systems? 
"What we found with Wireless Emergency Alerts is that not everyone receives them. Not everyone is going to download the app, so the idea is to create multiple platforms and multiple ways that people can receive messages about earthquakes," said Holly Porter, Director of the County Office of Emergency Services. “During a disaster, just by its very definition, systems are overwhelmed. technology fails. We want people to have all the options available to them to receive a potential alert and warning."

When will San Diego start getting ShakeAlerts? 
The ShakeAlert app should be available statewide in late 2019. The WEA is still undergoing testing. 

What was the WEA test last week? 
"We tested that system to see if it's fast enough. If it could possibly be used to send out an alert for an earthquake early warning," said Porter. "[We’re testing] what platform will be fast enough once those primary earthquake waves are detected by sensors to quickly get an alert to public before secondary waves arrive."

If the system were working, would I have received a ShakeAlert Thursday or Friday?
Likely not because the shaking in San Diego County wasn’t severe enough to warrant notification.

What magnitude earthquake will trigger a ShakeAlert in San Diego? 
That’s still being decided.

Will San Diego have our own ShakeAlert App?
No, we will be part of the two statewide alerting systems.

ShakeAlert LA was set to send an alert to residents during a 5.0 or greater magnitude earthquake. The city later said, based on feedback from residents, it would lower the threshold. 

Some researchers are advocating for a high threshold to avoid desensitizing the public. They argue it should be reserved for earthquakes when lives or property are at risk. Others want a lower threshold, so residents can be prepared for any earthquake that might affect them. 

The threshold for the statewide ShakeAlert is still being debated. 

"It's looking for that sweet spot where we want to alert people when there's protective action recommendation you want them to take when there's possible damage, but not over alert to the point the alerts don't really mean anything," said Porter. "We want the alerts to be meaningful and we want to give people notice of what they can do at that moment to protect themselves."