The opinions expressed in this piece do not necessarily reflect the views of The Mendocino Voice. We value the diverse opinions of our readers and we welcome robust, wide-ranging thoughts and ideas on subjects relevant to Mendocino County. Opinion pieces can be sent to info@mendovoice.com.
A geological feature of continental significance (the Bartlett Springs Fault, or BSF) runs through Lake Pillsbury approximately 5,000 feet east of Scott Dam, part of PG&E’s Potter Valley Project. The BSF is the easternmost fault in the San Andreas Fault Zone (SAFZ), a major transform-fault boundary between the North American and Pacific tectonic plates.
In northwestern California, the SAFZ is a 60-mile-wide region of fault activity on two major faults that are elements of the SAFZ in addition to the SAF itself. One is the Maacama Fault, located roughly along the course of the Russian River in Mendocino County. The other is the Bartlett Springs Fault, which is currently understood by the U.S. Geological Survey to extend (as a set of interconnected faults having different names) northwest from the vicinity of the Carquinez Strait in Solano County to the Mad River Fault Zone in coastal Humboldt County.
Recent earthquakes associated with the Bartlett Springs Fault and the Maacama Fault in the region that includes the Potter Valley Project demonstrate that both fault systems are currently active and capable of producing seismic effects.
On Aug. 5, a swarm of earthquakes directly associated with the BSF that occurred approximately 19 miles north of Covelo included a moment-magnitude 4.4 quake. On July 11, an earthquake swarm occurred approximately seven miles northeast of Covelo with nearly identical seismic characteristics, including another magnitude 4.4 event, indicating that the dynamics of the BSF include ancillary faults associated with the BSF.
Prior to these BSF-related events, an earthquake swarm occurred on June 24, approximately seven miles north of Redwood Valley, including a magnitude 5.6 quake that was widely felt throughout the Ukiah region. This swarm was associated with the Maacama Fault and associated ancillary faults.
No scientist we know can accurately predict when a seismic event on a known active fault might occur, but geological professionals can estimate a potential magnitude for a major seismic event based on fault length and other factors. Recent USGS studies have identified the Bartlett Springs Fault as capable of producing an earthquake with a magnitude between 6.7 and 7.2.
Earthquakes with such magnitudes are major seismic events. Recent earthquakes in northern Venezuela on June 24 that occurred on tectonic plate-boundary transform faults, like those in the SAFZ, had computed magnitudes of 7.2 and 7.5, resulting in major surface damage and many fatalities.
What this means for Scott Dam
An earthquake in this magnitude range on the Bartlett Springs Fault in the vicinity of Lake Pillsbury would, in addition to other effects, likely affect the stability of Scott Dam, potentially resulting in its failure. We understand that PG&E and its consultants provided geotechnical assessments of the potential effects of BSF seismicity on Scott Dam to the Federal Energy Regulatory Commission. PG&E and FERC classified the information as confidential, and it has not been made publicly available.
Geological science, while complex, clearly indicates that the Bartlett Springs Fault represents a significant potential for the failure of Scott Dam. We have yet to see the internal studies conducted by PG&E for Scott Dam, but we suspect that those studies by geotechnical professionals say the same. That is, seismic risk to Scott Dam seems likely to be a relevant factor in PG&E’s determination to abandon the PVP. We believe that this information is important for informing discussions about the future of the Potter Valley Project and should be made available for public consideration.
Bob Schneider of Davis cofounded CalWild (formerly the California Wilderness Coalition) and Tuleyome. Chad Roberts of Davis is a conservation ecologist.
I realize this is an opinion piece, but as a co-founder of CalWild and a conservation ecologist, the authors obviously have a stake in removing the Potter Valley Project dams. They are not experts in seismology or dam construction. Their opinion should be taken in that context.
If a geologist and a phd ecologist seemingly just stating facts and asking for more information strikes you biased, I’m honestly not sure you’re prepared for the truth.
Hopefully everyone can agree with the last 2 sentances. Then let the chips fall where they may: “That is, seismic risk to Scott Dam seems likely to be a relevant factor in PG&E’s determination to abandon the PVP. We believe that this information is important for informing discussions about the future of the Potter Valley Project and should be made available for public consideration.”
How do you get permits to rebuild a dam with siesmic risks so high that PG&E is legally required keep the fill levels low to not place stress on the dam?
As the Executive Director of CalWild, let me be clear: this piece was not vetted by us, does not represent us, and is not in any way associated with CalWild, which is neutral on the question of removal of these dams. This is necessary so we can work with people on all sides of that question to build a community vision for the future of the watershed, which we have been doing since 2023.
Bob Schneider was a founder of ours, but at this point he is simply a member. He does not speak for us.