An emergency alert warning people in the Bay Area that an estimated 5.7 magnitude earthquake was imminent lit up phones Wednesday morning, but the actual temblor turned out to be a magnitude 4.2.
Dr. Angie Lux, a project scientist for Earthquake Early Warning at the Berkeley Seismological Laboratory, explained why there was a significant difference between the estimated and actual magnitudes.
"Our ShakeAlert earthquake early warning system is trying to calculate the magnitudes as quickly as it can," she said. "We use just a very tiny amount of information to calculate how big the earthquake is. We do this within seconds so we can give you as much warning as possible. The downside to doing that means that sometimes when we create the initial estimate, it comes in high. For this particular event, we did create a magnitude estimate initially of 5.7. Definitely a little high."
Lux said the emergency notification system worked as it should, but there is room for improvement.
"I always like to see us do a little bit better job with the magnitude estimates, so I will say that we definitely overestimated the magnitude there," she said. "However, the good thing about this system is that everything worked exactly as it should, other than that magnitude estimate. Sometimes in order to warn the people who are going to feel the shaking – again, we're getting those magnitude estimates as quickly as we can – sometimes that does mean that we do overestimate."
Wednesday morning's quake hit at 9:29 a.m. and was centered near Isleton in southern Sacramento County, about 12.5 miles northeast of Antioch, the USGS said.
No injuries or damage were immediately reported.
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