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.
When setting up the alert system, geologists also consider the type of ground through which an earthquake travels, de Groot said. That’s because some soil pushes the seismic waves along more than other soils.
Soft soil absorbs more of the seismic waves, slowing them down; but it also makes them bigger, so you get harder shaking.
On Oct. 17, 1989, part of the Cypress Freeway in Oakland collapsed when the 6.9 Loma Prieta earthquake hit the Bay Area. The part that failed was built on mud, de Groot said.
“We’re always looking at fine tuning” the alert system, he said. “We learn from every earthquake.”
Some people contacted the USGS to ask why they got Tuesday’s alert when they lived so far away from the epicenter.
“We’re trying our best not to over-alert people,” de Groot said. The USGS is working with social scientists to predict people’s responses to the alerts: How too many alerts for small quakes could make people complacent, etc.
Because different earthquakes can travel different distances and shake at different strengths, geologists want to strike a balance between speed and accuracy.
“There’s the chance more people would be alerted and not feel any shaking,” de Groot said, but “ultimately it’s about safety.”
First pioneered in 2006, the ShakeAlert system was first used in 2018 to warn trains and other industries to brace for earthquakes.
It went public in California in 2019, then in Oregon and Washington in 2021.
The California Office of Emergency Services helped fund and set up sensors to detect earthquakes, de Groot said: 1,115 in California.
As of December 2022, ShakeAlert uses a network of 1,675 sensor stations and serves 50 million people from Washington to San Diego.
To see a list of apps for which you can sign up to get alerts in your area go to shakealert.org.
Jessica Skropanic is a features reporter for the Record Searchlight/USA Today Network. She covers science, arts, social issues and entertainment stories. Follow her on Twitter@RS_JSkropanic and onFacebook. Join Jessica in the Get Out! Nor Cal recreation Facebook group. To support and sustain this work,please subscribe today. Thank you.