The state’s new ShakeAlert earthquake warning system got three big things wrong about this week’s Antelope Valley 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 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,” he added. “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: called it a 4.8; later, it was upgraded to a 6.0.

The system thought the rupture happened about 31 miles south of where it actually did. And at first it presumed it was detecting three different small earthquakes, rather than one large one.

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, UC 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. 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.

When complete, the system will have a network of 1,675 seismic sensors. Currently 773 sensors – about half of the goal – are in place, due to funding constraints.

Thursday’s rupture took place in the Antelope Valley, a high and lonesome 15-mile-long valley in the eastern Sierra Nevada stretching from Mono County, California to Douglas County, Nevada. It’s an agricultural region, home to only 1,500 people, flanked on either side by the lofty 10,000-foot Sierra Nevada mountains.

There are few sensors in that region of California, because not many people are threatened. It’s a formidable terrain, and sensors aren’t allowed in wilderness areas. A reading may be influenced by whether a sensor is sitting on hard rock or soft soil. On Thursday, one of the existing sensors may not have been working right.

“This meant that you don’t have stations nearby where the earthquake occurs, then you have to wait for the waves that are propagated to hit the stations that you do have,” said Given.

This also created a 25-second delay. “I took quite a while for the system to get enough information to declare an earthquake. In a densely instrumented area, we typically can issue an alert in under 5 seconds,” he said.

Because those sensors are clustered down near Mammoth Mountain, and the system triangulates signals, it got the location wrong.

Those stations – whose readings are combined to create an average — skewed the magnitude. “One anomalously high or low station can pull the average in one direction or the other,” said Given.

Over the vast distance, the waves propagated across other stations in the state’s network. This confused the system and it interpreted the Antelope Valley event as several earthquakes, not one.

“This is a work in progress,” said Given. “The station coverage is not complete. The algorithms are still being improved.”

“The system worked, but we’re not going to be done until it’s perfect,” he said. “And it’s never going to be perfect.”

Correspondent Bailey Bedford contributed to this article.