LA has an earthquake warning system. Did it work during July 4th’s 6.4 magnitude event?

Los Angeles will lower the level at which earthquake warnings get sent to residents, city officials said after a 6.4 magnitude quake on Thursday morning, July 4, failed to trigger an alert.

The early warning system did work, U.S. Geological Survey seismologist Robert Graves said during a televised news conference, in that it detected the temblor before it happened. But the ShakeAlert app wasn’t set off – it would have sent alerts to people’s cellphones – because it’s set to warn people when the shaking in Los Angeles County is expected to exceed magnitude 5.0, the city’s Twitter account said.

“We hear you and will lower the alert threshold,” the city account assured residents, after some people complained and called the app useless.

The ShakeAlert system doesn’t predict earthquakes, but its sensors can rapidly detect the first wave of seismic energy that helps scientists estimate a quake’s location and strength, according to the ShakeAlert website. The app system launched earlier this year.

“The instruments pick that up and say, ‘Oh, earthquake,’ and they’re able to calculate is this a big one or a little one and where is it,” said Mike Gardner, a Riverside city councilman who chairs the state Seismic Safety Commission.

That information allows officials to warn people in potential danger zones before the most damaging wave of shaking arrives.

Having advance notice can give local governments and businesses time to turn off heavy machinery and gas lines, slow down trains and get vehicles away from bridges and tunnels.

Graves said that had a public alert been triggered, L.A.-area app users would have gotten up to 45 seconds of warning. Officials setting the threshold try to balance the public desire to know with the risk of desensitizing or unnecessarily alarming people.

Getting frequent alerts can convince people the system works, but lots of false alarms can cause distrust, Caltech seismologist Lucy Jones said at the news conference.

The question officials must answer is: “What’s the psychological benefit of known it’s coming and getting a few seconds’ warning,” Jones said.

Gardner said the 2019-20 state budget, which Gov. Gavin Newsom recently signed, includes enough funding – $16.3 million – to finish building out the statewide early warning system.