Confusion and frustration that followed a big California jolt have highlighted the hurdles an earthquake early warning system here might face, a scientist says.

When a 7.1 quake was felt across much of central and southern California this month, Los Angeles residents took to social media questioning why it hadn't registered on the just-launched ShakeAlertLA app.

Officials had to point out that shaking in LA County hadn't reached a point that was likely to result in major damage, as seismologists warned that lowering the threshold could leave people too accustomed to the alerts.

Unlike Japan, Mexico and now parts of


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