A moderate earthquake Thursday afternoon shook a wide swath of Northern California, rattling homes and businesses from Redding to Elk Grove and shaking workers in downtown Sacramento high-rises. Less than 12 hours later, an aftershock produced another significant temblor.
Thursday’s 5.5 magnitude quake, centered in the waters of Lake Almanor in northwestern Plumas County struck at 4:19 p.m., according to the U.S. Geological Survey. The aftershock struck at 3:18 a.m. Friday and had a magnitude of 5.2.
The epicenters were about 23 miles southeast of Lassen Peak, 28 miles west-southwest of Susanville and 50 miles northeast of Chico. It was felt about 120 miles south in Sacramento and Elk Grove, north to Redding and east to Reno.
By 7 a.m. Friday, 19 lesser aftershocks were reported, the largest of which registered a magnitude of 3.8, according to seismologists. All but two of them were centered within the reservoir’s waters.
“We were right in the middle of it,” Mark Guillory, owner of Sierra’s Fly & Tackle in Lake Almanor said of Thursday’s temblor. “Crap was falling off the walls and my truck outside was moving.”
Guillory said he wasn’t aware of any damage yet, but said he heard an ambulance roar by shortly after the earthquake.
Small quakes aren’t unusual for the area, but the main shock and large aftershock were the first serious quakes in nearly 10 years — a magnitude-5.7 quake, centered on the eastern shore of the lake, shook the region on May 23, 2013.
“I’m really concerned about fireplaces coming off a house type thing, where people have brick and mortar,” Guillory said.
Guillory said his shop is a wood frame and that when the earthquake hit, fishing lures and equipment started falling off the walls.
“I was talking to the UPS guy and we just stared at each other and said, ‘What the hell was that?’” Guillory said. “’Was that what I think that was? I think it was.’”
Guillory said as he looked outside he could see his ¾-ton pickup truck bouncing up and down in the parking lot.
A short time later, a customer came into his shop and told him there had been rock slides along California State Route 147 along the east side of the lake.
“We’ve got major rock slides going down, like rocks half the size of cars,” he said.
Brian Ferguson, a spokesman for the Governor’s Office of Emergency Services, said Highway 89 was inspected for rock slides Thursday afternoon and there were some reports of localized power outages but no major damage. According to the Chico Enterprise-Record, 911 service was knocked offline for the California Highway Patrol’s Chico dispatch center; CHP confirmed communication lines were re-established.
A Plumas County sheriff’s official said there were no reports of damage or injuries, and that rock slides were not unusual in the area following past year’s fires and storms.
“We have absolutely no reports of any damage to date or any injuries,” said sheriff’s Patrol Commander Carson Wingfield. “We do have county crews inspecting county buildings and the school district is inspecting buildings.”
Wingfield said he felt that initial shock while he was on duty.
“I was inside a truck doing surveillance on a residence and the truck started shaking, so I moved,” Wingfield said. “It’s not like something that has never happened here.
“It’s fairly infrequent, but as far as magnitude it was nothing major.”
A dispatcher at the Lassen County Sheriff’s Office in Susanville said the quake was felt there, also, but that there had not been reports of damage.
Maintenance crews inspected all Plumas Unified School District campuses following Thursday’s earthquake and re-inspected following the overnight temblor, reporting no damage, superintendent William Roderick said in a statement to parents early Friday morning.
All schools opened on their normal schedules Friday, Roderick said.
The Shake Alert system was activated for Thursday’s main shock and Friday’s largest aftershock, USGS officials said in a social media posts. “Since the quake was greater than magnitude 5, Shake Alert-powered alerts were delivered to cellphones by the @MyShakeApp (and FEMA’s) Wireless Emergency Alert system,” the federal agency said in a social media post.
Minor earthquake motion felt at NWS Sacramento. Blinds swaying slightly. #cawx
— NWS Sacramento (@NWSSacramento) May 11, 2023
Donna O’Connell said she and her co-workers felt Thursday’s rumbling at the Big Cove Resort on Lake Almanor Peninsula: “Did we ever.”
She said a few minutes after the temblor that there was no major damage just a few “things that fell off the shelves at the store.” O’Connell said she and her colleagues ran outside as soon as the shaking began and that some people had ventured off to check on their property.
“It wasn’t a rolling one, it was a shaking one,” she said, adding that it felt like the quake rumbled for about 15 seconds, but, “They always feel like they last longer than they really are.”
Letting out a sigh, she said “We’re OK.”
O’Connell said the quake reminded her of the quake that hit along the East Shore in 2013.
Plumas Pines Resort, a motel and restaurant just off the western shore of Lake Almanor and roughly 4 miles from the epicenter, said in a Facebook post it would be closed through at least Friday morning after Thursday’s earthquake caused damage to its inventory.
The motel posted photos showing dozens of shattered wine glasses along with broken glasses and dishes strewn on the floor of its kitchen. It also posted a brief video clip showing its bar area jolted by the temblor; the shaking knocked the surveillance offline after about a second.
Both moderate earthquakes were calculated at a depth of about 3½ miles, according to the USGS.
Jana Pursley, a seismologist for the agency’s National Earthquake Information Center in Rockville, Maryland, said the rupture Thursday happened along a “normal fault,” one in which two tectonic chunks are sliding past one another and “one slid downward” along a northwest-southeast fault system typical of California’s geology.
Such strike-slip faults are common in the state, the most notable of which is the San Andreas Fault. The earthquakes are being produced along the Almanor Fault, a known but not well-documented 25-mile delineation that runs north from Lake Almanor toward Hat Creek.
Together, the Almanor quakes represented the largest to hit the state since a magnitude-6.4 temblor struck Ferndale in Humboldt County in late December, killing two who suffered medical emergencies during the shaking and damaging some structures including the 111-year-old Fernbridge.
Nearly two years ago, a magnitude-6 temblor struck in the eastern Sierras — widely felt in the Central Valley — in 2021. Over the July 4 holiday in 2019, a magnitude-7.1 temblor shook the high desert community of Ridgecrest — it was felt across Southern California and sloshed pools as far north as Sacramento.
But a magnitude-5 quake is generally considered mild, but can cause slight damage to buildings, especially to non-reinforced structures. The area has infrastructure with past concerns.
A 2019 Sacramento Bee story reported that Magalia Dam, a small century-old structure that’s only 103 feet tall, was declared seismically weak two decades ago, restricting how much water it could hold.
Closer to the epicenter is Canyon Dam holding back Lake Almanor, which was highlighted for its cracked spillway in a 2017 Sacramento Bee story. A 2016 annual inspection report compiled by the state Division of Safety of Dams noted some “freeze-thaw” damage to the concrete floor, but inspectors said the chute walls were stable and “the structure remains in satisfactory condition for continued use.”
Ferguson of Cal OES said inspectors were dispatched to check on the dam, which is owned by Pacific Gas and Electric Co., and hydroelectric facilities there.
I'm in Sacramento, and I think we just had an earthquake.
— Matthew Keys (@MatthewKeysLive) May 11, 2023
— True Kathleen (@kathleenjones21) May 11, 2023