ISLETON, Calif. — People across the Sacramento area and the Bay Area received an earthquake alert on their phones just before 9:30 a.m. Wednesday morning.
It's a warning system called ShakeAlert from the U.S. Geological Survey. According to the USGS website, ShakeAlert "monitors for significant earthquakes and issues alerts to warn that strong shaking is expected imminently."
Tim Dawson is the program manager for seismic hazards at the California Geological Survey. He felt Wednesday's magnitude 4.2 earthquake from his office in San Mateo, more than 50 miles from the epicenter southwest of Isleton.
"I had about 10 to 15 seconds to actually take protective action,” Dawson said. "About 15 seconds after I got the alert, the building kind of swayed a little bit and I could feel the waves pass through the building. It was a great experience."
He calls it a great experience because the warning worked. People had time to take cover.
Angie Lux agrees. She’s the project scientist for Earthquake Early Warning at the UC Berkeley Seismology Lab, which developed the MyShake app. It delivers ShakeAlerts across California, Oregon and Washington.
"People felt shaking, people got the alerts, so I would say that this was a success in our books,” she said of Wednesday morning’s warnings.
However, she acknowledges the MyShake alert overstated the strength of the earthquake.
“We did overestimate the initial magnitude; we said it was a 5.7. That was definitely a little high, so I know that caught some people off guard,” Lux said. “But overall, the system performed exactly as it should have done; we located the earthquake really well. Everything happened really fast, which is what our system is designed to do. But because it works so quickly, sometimes we do miss that initial estimate by a little bit."
She said the Seismology Lab “will be looking into this to see an improvement for future detections."
Even though the Sacramento area doesn't typically see earthquakes as frequently or as strong as those elsewhere in the state, Lux and Dawson say people should still take cover when they get a warning.
Near the epicenter in Isleton, Miriam Carbajal was at work when the quake hit.
"It just shook. Like, I was able to see the whole shelves go side-to-side,” she told ABC10. “Then behind me there was some bottles. I heard them falling."
The city manager tells ABC10 he is not aware of any structural damage in the small Delta city.
Dawson said Wednesday’s earthquake likely came from the Midland fault, which extends from Byron on the southern end up to Dixon on the northern end. It's part of something called the Great Valley thrust fault system, a series of faults along the western side of the Sacramento and San Joaquin valleys.
For example, Dawson points to a magnitude 6.3 earthquake that happened just outside Coalinga in 1983. And in 1892, two earthquakes between Vacaville and Winters measured 6.4 and 6.6.
Dawson calls it a good reminder that bigger quakes can happen here — and to take warnings seriously.
Coincidentally, Thursday – Oct. 19 – is the Great ShakeOut, an annual earthquake drill. People with the MyShake app will get a test alert on their phone at 10:19 a.m. It will be clearly marked as a test. Many schools will also be participating in earthquake drills.
Charlotte Fadipe, with the California Earthquake Authority, says people should not run outdoors when they get an alert.
“Drop to the ground, cover your head and hold onto something,” Fadipe said. “The whole idea is to make yourself as small as possible. When an earthquake occurs, we found in California, running out the door, running toward the window or the doorway is not helpful. Most people trip and fall, and a lot of the injuries that actually occur, occur when things fly off the shelves and hit people.”
WATCH ALSO:
