What ShakeAlert warning system got wrong — and right — about last week’s 6.0 quake

California’s new ShakeAlert earthquake warning system got three big things wrong about last week’s Sierra Nevada temblor.

But that doesn’t mean it won’t work for us.

The fledgling system, built to alert people about imminent risk, was tested on Thursday evening in the toughest of places: a hotspot of geologic turbulence in a rural and remote region with very few sensors.

Its performance in the Bay Area and other metropolitan areas is likely to be far better, especially as it gains more experience, said Doug Given, ShakeAlert Program national coordinator at the U.S. Geological Survey.

“We’ve prioritized the higher population areas in our build-up strategy,” he said. “These rural earthquakes gave the system difficulty.”

The detection stations “are sparse near the earthquake,” he said. “The system is not yet built out for the coverage that we have planned.”

A similar problem was created by last May’s earthquake in rural north Lake Tahoe. The system sent out a warning of a magnitude 6.0 quake; in reality, it was 4.7. The rupture happened about 18 miles northwest of the system’s original reports.

“The system has to be super fast. There’s always this trade off between speed and accuracy,” he said. “And we’re making those calculations initially with very, very limited information. We’re trying to characterize an earthquake with, in some cases, less than one second of ground motion data from a single location.”

On Thursday evening, ShakeAlert was fast, but not accurate.

The system initially got the earthquake’s magnitude wrong. Alerts were for a 4.8-magnitude quake; later, it was determined there were two separate earthquakes roughly 25 seconds apart in the Sierra Nevada’s Antelope Valley. One was determined to be a 6.0magnitude event, while the other, about three miles away and southwest of Walker, was a 5.2.

There was some confusion immediately following the shaking as the 5.2 quake — also initially reported as a 4.8 — was removed from the U.S. Geological Survey’s Latest Earthquakes list, then readded with a more accurate location and upgraded several hours later.

The system initially reported the rupture happened in Farmington, a little more than 130 miles south of where the two quakes actually occured — the Sierra Nevada’s Antelope Valley, near Coleville.

And at first ShakeAlert presumed it was detecting three different small earthquakes, rather than two large ones.

Such details didn’t matter to some California residents, who welcomed the warning in time to take action.

“It worked. The whole point is to give you a little notice,” said a Lodi-based wine industry consultant who asked that his name not be used. He rushed from his office to the safety of a bathroom when the system’s app issued a startling emergency alert. “It’s really worth it.”

“I was just working at my computer and I saw the phone shake and buzz, then a notification popped up. My computer is right next to a window and the glass could have shattered,” he said. With his daughter, ”we both got down on the floor next to the sink and we started feeling the rolling waves.”

After more than a decade in development, the system is finally a reality for tens of millions of West Coast residents. Public use started in October 2019. There are three apps available: the U.S. Geological Survey’s ShakeAlert, University of California, Berkeley’s MyShake and Early Warning Labs’s QuakeAlertUSA. California authorities can also issue alerts via text message, through the Amber Alert-style Wireless Emergency System, which does not require downloading an app or having a smartphone.

The earthquake warnings are possible because when a fault slips, it generates two kinds of waves. The initial waves travel fast but are weak. It’s the second set of waves that are so damaging.

When seismic sensors detect the first waves, they quickly send alerts to monitoring centers in Seattle, Menlo Park, Berkeley and Pasadena, California. Within about 5 seconds, computer algorithms analyze the data to rapidly identify the epicenter and strength of the earthquake and decide whether the temblor will be powerful enough to warrant an alert.

The system detects earthquakes as low as magnitude 3.5. Alerts are sent when a magnitude hits 4.5. The Amber Alert-style alarm goes off when a magnitude reaches 5.