People watch for a tsunami at Ocean Beach in San Francisco on Thursday, Dec. 5, 2024. (Juliana Yamada/KQED)
Nearly 200 aftershocks have rattled Northern California since a 7.0 magnitude earthquake struck off the coast of Humboldt County on Thursday morning, according to the U.S. Geological Survey.
And experts say more aftershocks are likely. But that’s par for the course — especially in this region.
“The bottom line is we live in earthquake country,” said Robert de Groot, operations coordinator of ShakeAlert, an earthquake early warning system operated by the U.S. Geological Survey.
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Continued aftershocks of 3.0 or 4.0 magnitude are more than 99% and 98% certain, respectively, for at least the next week, according to de Groot.
“The chances of earthquakes from this event begin to dwindle over time and they decrease in terms of number,” he said, noting that larger aftershocks are unlikely. There’s only about a 5% chance that anything nearing a 6.0 magnitude aftershock could happen, he said.
The epicenter of the initial 7.0 temblor which struck at 10:44 a.m. about 40 miles off the Humboldt coast, is on the active Mendocino Fault, where the Pacific, Juan de Fuca and North American tectonic plates meet — and where the San Andreas Fault ends.
The earthquake prompted a tsunami warning that reached more than 5 million people, from Southern Oregon to Santa Cruz County. The warning, which was canceled just over an hour later, prompted BART to temporarily stop service in the Transbay Tube and drove thousands of Northern California residents to temporarily evacuate in search of higher ground.
Three minutes later, a 4.1 magnitude earthquake hit near Lake and Sonoma counties.
The ShakeAlert system alerted around 4.5 million people, some as far as Salinas in Monterey County, de Groot said.
De Groot said aftershocks are not likely to happen along the San Andreas Fault — which extends some 750 miles south through California — because they typically happen closer to the epicenter of the initial earthquake.
“Think of it as parts of that fault that didn't move quite as far as it should’ve, so it’s not like you’re going to have earthquakes in this region and all of a sudden have an earthquake that’s down in Hayward or in Berkeley,” he said. “That’s way too far away.”
Aftershocks since a 7.0 magnitude 7.0 earthquake (orange) struck off the coast of Humboldt County at 10:44 a.m. Thursday. Fault lines are marked in red. Source: USGS (Matthew Green/KQED)
But Roland Bürgmann, a geophysics professor at UC Berkeley, says it’s still important to keep an eye on the San Andreas Fault right now.
“The San Andreas Fault ends right where the aftershocks end from this last event,” said Bürgmann, emphasizing that the location of the earthquake's epicenter remains “very active.” “So that means the San Andreas Fault is feeling a lot of pressure right now. It got a lot of shaking and changes and stress.”
Justin Rubinstein, a research geophysicist with the USGS, said the agency creates forecasts that estimate the probabilities of certain magnitude aftershocks.
Right now, he says, the probability of at least one aftershock with a magnitude of 5.0 over the next week is 35%.
“I think we can certainly expect aftershocks to last for weeks or months,” he said.
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