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Calaveras Fault: Latest temblor a good reminder of creeping menace

The Calaveras Fault doesn’t have the same celebrity status as its more famous siblings, the San Andreas and Hayward Faults.

But Tuesday’s earthquake was a reminder of the powerful pent-up energy of this 100-mile stretch of the East and South Bay landscape, where large sections of the earth’s crust scrape past one another in opposite directions.

“The Calaveras Fault is right up there, with those two other faults, in terms of its ability to generate earthquakes,” said Richard Allen, director of the Berkeley Seismology Lab.

The epicenter, estimated to be four miles below Joseph D. Grant County Park, was located in an area notorious for seismic restlessness. A much larger 6.2 quake occurred in nearly the same spot in 1984, followed by a 5.4 earthquake in 2007, and a 4.1 temblor in 2017.

The Calaveras Fault runs roughly from Hollister, near Gilroy, to Danville and beyond, and is linked to the Hayward Fault. The U.S. Geological Survey estimates a 7.4% likelihood of a magnitude 6.7 or greater earthquake on the Calaveras Fault in the next 30 years.

While unsettling, it contributes to the stunning beauty of the region. The fault lies along the long linear base of Pleasanton Ridge, for instance, and is part of the geological wonder of the Livermore Valley, Kilkare Canyon, Sunol Ridge and other noted East Bay landscapes.

The town of Hollister bears charming evidence of its motion. Along downtown streets, curbs are bent, sidewalks are fractured and porch posts lean.

Along with the nearby Hayward Fault, “they create this beautiful topography that we all enjoy here in the Bay Area,” said Allen.

The nine counties of the Bay Area are home to dozens of major faults — from the San Andreas, Hayward and Calaveras to the smaller Concord-Green Valley, the San Gregorio and the Rodgers Creek.

They are all located within the active boundary between the Pacific and the North American tectonic plates. The Pacific plate slowly and continually slides northwest past the North American plate.

Much of the time, the Calaveras fault is just creeping, because the zone is full of slippery rocks and minerals. This lets it release energy by sliding.

On average, it slides around 0.2 inches every year. While this is bothersome – stressing pipelines and cracking sidewalks, for instance – it’s rarely catastrophic.

However, the accumulated creep on the fault doesn’t match the growing stress of the crustal tension, so it sometimes ruptures. This makes the region notorious for its seismic risk.

Two years ago, it was the site of an unsettling earthquake “swarm,” when more than 35 quakes ranging as high as a magnitude-3.5 were recorded over a two-month period in 2020.

It’s the reason why the Calaveras Reservoir — located on the border of Santa Clara and Alameda counties — just got a $720 million seismic makeover. The old dam was only 1,500 feet from the Calaveras Fault, and was at risk of liquefaction during to a big earthquake. The new dam has a more water-tight foundation, so can be safely filled to capacity.

It’s less of a household name because it’s tucked on the east side of the East Bay Hills and runs under fewer cities, said Austin Elliott, earthquake geologist with the U.S. Geological Survey.

It’s a “strike-slip” fault, he said. Its eastern side is move southward and its western side is moving northward. It’s part of the larger field of plate tectonics — the Earth’s outer shell is broken up into giant puzzle pieces, or plates, that glide atop a conveyor belt of hot, weak rock.

Like the San Andreas and Hayward Faults, the Calaveras Fault “must accommodate the northward motion of the Pacific Plate next to North America,” said Elliott.

“So it’s not uncommon for this portion of the Calaveras fault to produce some of the more notable earthquakes in the Bay Area,” he said.

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Calaveras Fault: Latest temblor a good reminder of creeping menace


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