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Davis, California

Saturday, October 23, 2021

Northern California rattled by earthquakes from east and west

Have you been feeling the earth moving under your feet? It might be more than your imagination.

California is notorious for earthquakes, and small tremors shake the state everyday, but within the past couple weeks the state has experienced a flurry of quakes that have been increasing in magnitude.

Northern California has experienced the most geologic activity lately. The largest earthquake, a magnitude 5.4, struck Tuesday night with an epicenter in Humboldt County, according to the earthquake-mapping website of the U.S. Geological Survey. Previously, the eastern edge of California had been a hot spot with a 4.2 magnitude quake striking just west of Reno early Monday morning, followed by a torrent of mild aftershocks.

More recently, Southern California has been involved, with a 4.2 magnitude quake near Palm Springs on Wednesday night and a 4.4 magnitude quake near Bakersfield early yesterday morning.

According to Dr. Jeff Unruh, an associate research geologist in the UC Davis geology department, California is bounded by fault zones. The Sierra Nevada microplate meets the North American plate along California’s eastern border and the San Andreas fault, where the Pacific plate meets the Sierra Nevada microplate. The fault runs through much of western California.

Despite the recent earthquakes sounding rather ominous, John Marquis of the Southern California Earthquake Center at the University of Southern California said in an e-mail interview that the geological activity of the past week has been of little significance.

“All the recent events have been minor, geologically speaking,” Marquis said.

He went on to say that it is difficult to extrapolate a potential meaning from the recent quakes.

“There is a common misperception that small earthquakes ‘relieve the pressure’ building up at the plate boundary,” he said. “It would take many thousands of earthquakes the size we’ve seen lately to release the stress built up within the crust from the constant motion of the North American and Pacific plates.”

However, this does not mean a large quake is imminent.

“These quakes do not necessarily portend a larger quake in the immediate future. Though the current level of seismicity is higher than the recent average, we have had similar ‘flurries’ of activity in the past that were not followed by any large earthquake,” he said.

The problem was a matter of the time scale involved in earthquake cycles, Unruh said.

“These are very small earthquakes compared to the size of earthquakes that have and will occur in these areas, maybe within our lifetime,” he said. “You would have to have a record of earthquakes for a couple hundred years to say, ‘this frequency of earthquakes is unusual,’ but we just don’t have that kind of record. People’s attention spans just aren’t long enough to account for the periods we’re talking about with earthquakes.”

The threat the recent earthquakes, and earthquakes in general, pose to Davis is relatively low. Unruh cited fault lines SF along the west and east sides of the Sacramento Valley that have generated geological activity in the past, including a large quake in 1892 which leveled Winters and Vacaville, but said that the faults are expected to be inactive a while longer, in that events of the magnitude of 1892 happen only once every several centuries.

“The likelihood of an earthquake in the Central Valley is relatively low. It’s higher than Kansas, but it’s not like living in the San Francisco Bay Area,” Unruh said.

Still, living in California has its risks, and fault activity remains unpredictable, Marquis said.

“The possibility of a large quake in California is always there,” Marquis said, “regardless of the level of background seismicity, so we, as Californians, should always be prepared for a major quake at any time.”

 

J. DANA STUSTER can be reached at city@californiaaggie.com.

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