Friday’s Covelo Earthquakes Highlight a Fault Scientists Still Don’t Fully Understand

The magnitude 4.4 earthquake that shook the Covelo area Friday evening sits on one of the least studied fault systems in Northern California, one that geologists say could someday produce a much bigger quake near the Potter Valley dams PG&E is already working to remove.

Humboldt State University emeritus geology professor Lori Dengler walked through the sequence in an interview with Redheaded Blackbelt Friday evening, just an hour after the main shock hit.

The sequence began a little after 3 p.m. with a magnitude 3.8 earthquake located about 7 – 8 miles north-northwest of Covelo. That quake was felt in the Covelo area, Dengler said, though she hadn’t seen “felt reports” from farther away at that point. Over the next few hours, four smaller quakes followed, all in the magnitude 1.7 to 2.7 range. Dengler said those likely went unnoticed by most people.

Then, at 6:14 p.m., a magnitude 4.4 hit. USGS first estimated it at 4.2 before revising the number upward. “Certainly a 4.4 is strong enough to be felt by many in the Covelo area,” Dengler said, adding that she had “felt reports” from Fort Bragg, Willits and Ukiah as well.

Dengler encouraged anyone in Mendocino, Lake or Southern Humboldt counties who felt shaking to file a report with the USGS, even if the shaking was faint. “You are our instruments right now,” she said, noting the region has relatively few seismic sensors covering that stretch of ground.

Based on what USGS has recorded so far, Dengler said the quakes are likely on or very close to the Bartlett Springs fault. Both the 3.8 and the 4.4 were shallow, roughly six to seven miles below the surface. The 4.4 was a strike-slip earthquake, meaning two blocks of crust slid sideways past each other rather than moving up or down, and appears to trend north-northwest, though the aftershock data isn’t precise enough yet to confirm the fault’s exact orientation.

The Bartlett Springs fault runs roughly from east of Ukiah north toward the Kettenpom and Zenia area, but Dengler said it’s poorly mapped because it runs through dense forest with little exposed ground to study. “There haven’t been any major trench studies done on it,” she said.

That lack of data matters because the fault is being closely watched by PG&E due to its potential to affect the Potter Valley area, where the utility is already in the process of decommissioning the Scott and Cape Horn dams. According to PG&E’s surrender application filed with federal regulators, the company determined the aging hydroelectric project was no longer cost-effective to operate. A 2023 seismic risk assessment of Scott Dam prompted PG&E to leave the dam’s spillway gates permanently open as a precaution, cutting Lake Pillsbury’s storage capacity by about a quarter, or roughly 20,000 acre-feet.

Later reporting from Friends of the Eel River laid out why Scott Dam’s location is such a concern. The dam sits nearly on top of the Bartlett Springs Fault Zone, capable of producing an earthquake in the mid-6 to low-7 magnitude range. Sediment has also piled up behind the dam over the past century, pressing against its upstream face and threatening to eventually block the dam’s only low-level water outlet. Congressman Jared Huffman has said a PG&E engineer once told him that of all the utility’s infrastructure, “Scott Dam is the one that keeps me up at night.”

“Clearly a 4.4 is not going to be damaging,” Dengler said. “Most likely it’s going to go back to sleep. But always have a ‘but’,” referring to the small but real chance a bigger quake could follow.

Whether Friday’s sequence should be called foreshocks leading up to the 4.4, or simply an earthquake swarm with no clear mainshock, remains an open question, Dengler said, adding that only time will tell.

The USGS aftershock forecast puts the odds of another magnitude 3 quake at 12 percent and the odds of another magnitude 4 at 1 percent, Dengler said. The chance of anything larger is under 1 percent.

Dengler also addressed whether Friday’s activity is connected to the magnitude 5.6 earthquake that struck near Willits on June 24, located about 40 miles to the south on the Maacama fault. She said the two are probably not directly related, since they’re on different faults, though both are part of the broader system of faults running parallel to the San Andreas, including the Hayward and Rogers Creek faults to the south. She said she hopes researchers look into whether the two fault systems could be interacting.

One challenge for scientists trying to pin down exactly what’s happening underground is a lack of nearby equipment. Dengler said there are only a few seismic instruments covering that part of Mendocino County, and they sit fairly far from where Friday’s quakes struck, making it hard to get a precise picture of the fault’s shape and behavior. She said PG&E may have some instruments of its own in the area, given the utility’s long-running interest in the Bartlett Springs fault’s proximity to Potter Valley.

Typically, USGS will deploy portable seismic instruments to an area after a magnitude 5 or larger earthquake to better pinpoint aftershock locations, Dengler said, but Friday’s 4.4 falls below that threshold, so it isn’t clear whether that will happen this time. She noted that some portable instruments may still be in the field nearby, left over from monitoring after the June 24 magnitude 5.6 near Willits.

Dengler said she hopes someone deploys additional instruments to the area regardless, given how little is known about the fault. “It would be really interesting to see the detail of the pattern,” she said, adding that interest in monitoring a fault isn’t limited to areas with large populations. Places like Potter Valley, where infrastructure and dam safety are already a concern, draw attention from researchers even when the surrounding area is rural and sparsely instrumented.