Thursday, 02 February 2023

Pillsbury quake area's largest in 30 years

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LAKE PILLSBURY – A 4.8 quake that shook residents of the Lake Pillsbury area awake early Wednesday morning was the area's largest quake since 1977, according to a seismologist.


The earthquake was recorded at 1:42 a.m. by the U.S. Geological Survey.


The series of almost 40 aftershocks that followed the quake included a sizable 3.3 magnitude temblor that occurred at 8:52 p.m. Wednesday.


David Oppenheimer, a seismologist for the U.S. Geological Survey, said the last time there was an earthquake above magnitude 4.5 in the Lake Pillsbury area was Nov. 22, 1977.


That quake, he said, happened nine miles southwest of the lake, rather than nine miles west northwest, the area where Wednesday's quake was centered.


Area residents said they definitely felt it when it happened.


“It just about knocked us out of bed,” said Soda Creek Store owner Nick Uram.


Despite the early morning shaker's magnitude, Uram said items weren't knocked off the shelves at his store, although his home on Lake Pillsbury Ranch was shaken up “pretty good.”


No one coming into his store Wednesday reported any damage, Uram said.


Dixie Offt of Lake Pillsbury Resort & Marina said the resort's full-time caretaker was awakened moments before the quake by his cat.


The caretaker checked the water lines, cabins and marina for the resort – which will open for the season on Memorial Day – and found everything to be all right, said Offt. “We sustained no damage.”


As for the lake and its dams, they also escaped damage, according to David Eisenhauer, a spokesman for Pacific Gas & Electric, which oversees Lake Pillsbury.


As soon as the earthquake occurred, Eisenhauer said, PG&E staff inspected both Cape Horn and Scotts dams and found no problems.


“We're keeping a close eye on all of our facilities up there, but so far everything is looking sturdy,” said Eisenhauer.


Oppenheimer said there is a “persistent band of seismicity” that goes through Lake Pillsbury.


“It's a bit unusual to see behavior like what's happened with this earthquake,” he said.


Particularly unusual, said Oppenheimer, was the quake's aftershock sequence, with nearly 40 smaller quakes occurring throughout the day.


“We don't know exactly why some earthquakes have robust aftershock sequences and others don't,” he said.


An earthquake's behavior is influenced by a variety of factors, said Oppenheimer, including rock type or fluid pressure in the fault zone.


Serpentine, a common rock found in the state's coastal ranges, tends to be associated with faults that creep a lot and have larges number of small earthquakes, said Oppenheimer. “So maybe there's some serpentine in this fault zone.”


Oppenheimer explained that strain in the earth's crush is released through the state's larger faults – such as the San Andreas and Calaveras. The larger faults account for up to 90 percent of overall plate motion. Oppenheimer said the size of an earthquake tends to correlate to the total length of the fault.


Along with those major quakes, there are secondary and tertiary faults, and there are enough of them that seismologists don't even know where they all are because the smaller faults don't break through to the surface.


Such is the case with the fault along which Wednesday's quake took place. There are no mapped faults for the quake's epicenter, Oppenheimer said. “We don't know about these faults until they pop off.”


It's also hard to guess just how big of a quake could ultimately occur there, although this week's quake could be at the fault's upper limits, he added.


“It's not a major player in releasing strain in California,” Oppenheimer said. “Those are the ones that do come to the surface, like the San Andreas fault.”


There are other named faults in that area, said Oppenheimer, such as the Maacama fault. As to concerns about the aftershocks triggering a quake from that fault, Oppenheimer said the probability is “exceedingly low.”


For people worrying about “the big one,” Oppenheimer says it's doubtful that it would occur on the unnamed fault.


“The big one, if you're a seismologist, is a repeat of the 1906 earthquake,” he said, referring to the massive 7.8 earthquake that occurred along the San Andreas fault near San Francisco 101 years ago Wednesday.


Oppenheimer said scientists are learning all the time about the state's seismicity.


“We don't have a very complete picture of earthquake activity in California,” he said. Monitoring only began in the 1930s, reaching current standards in the 1970s.


Seismic activity isn't organized, he said, with some faults not showing activity for hundreds, sometimes thousands, of years.


E-mail Elizabeth Larson at This email address is being protected from spambots. You need JavaScript enabled to view it..


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