ShakeAlert sends out earthquake warnings from the U.S. Geological Survey.

Three million people throughout Northern California and Southern Oregon woke up in the wee morning hours to an earthquake alert on their smartphone.

Some received the alert seconds before they felt Tuesday’s earthquake off the coast of Humboldt County: a moderate to strong temblor at 6.4 on the Richter scale, according to the U.S. Geological Survey.

Others who got the alert didn’t feel the earthquake at all ― but they could have, according to the USGS.

“ShakeAlert performed exactly as planned,” said Bob de Groot, early warning system’s operations coordinator at the USGS’s Pasadena office.

When USGS sensors near Fortuna, California, registered the temblor, the 2:34 a.m. alert went out to anyone who might be affected: North to the Northern California-Oregon border, south to the San Francisco Bay Area and San Jose, east past Shasta County and northeast as far as Medford, Oregon.

For an earthquake its size, it was the most widespread ShakeAlert sent since the system went up in California in 2019.

The reason so many people got the warning was the quake had tremendous possibility to affect a widespread area, de Groot said. The shaking from deep strong temblors ― the one in Humboldt County started deep in the Gorda Plate ― have the potential to travel far from their source.

With three tectonic plates bumping and sliding against each other, the area near Fortuna is “pretty complicated,” de Groot said. “There’s a lot of action there.” There were 40 quakes with magnitudes of 6.0 and higher in the past 100 years near Fortuna, one of which ― a 6.2 ― hit in December 2021.

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