Seconds before a 6.2 earthquake rattled California, phones got a vital warning

Half a million phones received emergency alerts thanks to system offering a few seconds to take cover.

Dr Lucy Jones, a seismologist, describes how an early warning system would work at a news conference in 2013. Photograph: Reed Saxon/AP

By @GabrielleCanon
Last modified on 2021 Dec 21

In the moments before a 6.2-magnitude earthquake struck the northern California coast on Monday, roughly half a million phones began to buzz. An early-alert system managed by the US Geological Survey sent warnings out before the ground started to shake, giving residents in the sparsely populated area vital time to take cover.

The earthquake brought significant shaking but minimal damage in Humboldt county, about 210 miles north-west of San Francisco, and officials said it was an excellent test of the alert-system. It was the largest magnitude quake that’s occurred since the system, known as ShakeAlert, was officially rolled out across the west coast.

“We got some reports from folks that they got up to 10 seconds’ warning before they felt shaking. That’s pretty darn good,” said Robert de Groot, a ShakeAlert coordinator with the USGS.

USGS ShakeAlert

Replying to @USGS_ShakeAlert

It took 11.4 seconds for the first #ShakeAlert Message data package to be produced by @USGS. The quake was estimated at M4.5. At this level alert delivery to cell phones by @MyShakeApp, @Android, @ReadySanDiego or @QuakeAlertUSA was possible. @Cal_OES @CAGeoSurvey @USGS_Quakes

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ShakeAlert issues warnings through a series of agencies and apps including the MyShakeApp, public wireless emergency alert systems, and the Android operating system, powered by Google. A data package is created from information provided by USGS sensors and – within seconds – shows up on phones. Some apps that provide alerts are available to download but even some who didn’t have an app on their phone were notified. Affected individuals are instructed to drop, cover, and hold on. Having extra seconds to do so can save lives.

This event provided an opportunity for the scientists and system operators to test and improve ShakeAlert so it will be even better when the next big earthquake strikes. “We can run as many simulations and tests as possible but we are really going to learn the most from real earthquakes,” de Groot said. “It’s giving us the chance to use the system and learn how to do a better job of alerting people.”

The earthquake occurred just after noon and was centered off the coast, near a tiny town called Petrolia that’s home to fewer than 1,000 people. The nearest population center, Eureka, is about 45 miles north.

That left only about 25,000 people in the range of strong or very strong shaking, according to the US Geological Survey, though residents as far away as Sacramento and the San Francisco Bay Area reported feeling trembling.

The Humboldt county sheriff’s office of emergency services did not issue any evacuation orders, though a few roads were closed due to rockslides. The US Geological Survey estimated economic losses of less than $10m and no fatalities.

Photos showed shattered store windows, broken bottles fallen into store aisles and tiles that had fallen loose from a commercial building’s ceiling. The area last suffered an earthquake of a similar magnitude in 1993, when one person died, according to the USGS.

California Geological Survey
A M6.2 #earthquake has occurred offshore northern California 24 miles west of Petrolia. Moderate to strong shaking was recorded by the seismic network along the coast. The earthquake was felt in San Francisco by this CGS geologist! We are continuing to monitor this region. (TD)
Shakemap showing shaking intensity of the M6.2 20 Dec 2021 earthquake that occurred offshore of northern California.
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Jane Dexter, manager of the Petrolia General Store, told the San Francisco Chronicle the rumbling and shaking had lasted for about 20 seconds. Glass bottles fell off the shelves at the store, bursting on the floor, but no one was hurt, she said.

“It was bigger than [anything] I’ve felt in a long time out here,” she told the Chronicle.

There have been dozens of aftershocks ranging from 2.6 to 4.5 in magnitude, according to the USGS, through Monday and Tuesday. Though the quake originated off the coast, it did not trigger a tsunami and the National Weather Service announced that there was not a threat of one.

Even as alerts improve, officials emphasized how essential it is for residents and visitors to earthquake-prone areas across the west coast to be prepared. “It is one tool in a larger toolbox,” said De Groot. Everyone should have an emergency earthquake kit, copies of important documents, and plenty of bottled water for each person in the household. “[ShakeAlert] is just an enhancement for them to use to keep safe.”

The Associated Press contributed to this report

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