Early Wednesday morning, a magnitude‑4.0 tremor rattled the area north of Lake Pillsbury, marking the strongest event in a recent swarm of quakes.
Key points
- A magnitude‑4.0 quake struck north of Lake Pillsbury on Oct. 28, the strongest in a 12‑event swarm this month.
- The earthquakes are centered near the Bartlett Springs fault zone, an active segment of the San Andreas system.
- PG&E’s survey reported no damage to Scott Dam, which is slated for removal pending federal approval.
The U.S. Geological Survey recorded a magnitude 4.0 earthquake at about 5:01 a.m. on Oct. 28, 2026, roughly six miles north of Lake Pillsbury in Lake County, at a depth of approximately 7.2 miles. The agency identified this event as the potential mainshock of the ongoing sequence.
The quake was the largest of 12 earthquakes documented in the region over the past several weeks. Eight of those tremors occurred on Wednesday between 4:51 a.m. and 9:13 a.m., ranging from magnitude 1.6 to 4.0, while four smaller events (magnitudes 1.5‑1.8) were logged between Oct. 7 and Oct. 17.
Sequence of quakes this month
All of the seismic activity has been concentrated 5‑7 miles north of Lake Pillsbury, near the Bartlett Springs fault zone. This fault is a major segment of the broader San Andreas system, extending from Cache Creek in eastern Yolo County to Lake Berryessa east of Napa Valley, and is classified by the USGS as an active fault capable of generating significant earthquakes.
The epicenter lies close to Scott Dam, one of two Eel River dams owned by Pacific Gas and Electric Co. (PG&E) that the utility plans to remove as part of the Potter Valley Project decommissioning. PG&E spokesperson Megan McFarland said a survey conducted Wednesday found “no impact” from the recent seismic activity on the dam or related infrastructure.
Fault zone context
PG&E is seeking federal approval to dismantle the Potter Valley Project, which includes Scott Dam, Cape Horn Dam, and a shuttered hydroelectric plant that divert water from the Eel River to the Russian River watershed. The Federal Energy Regulatory Commission expressed support for the removal proposal earlier this year, though a final decision has not yet been issued.
Reader questions
What is the Bartlett Springs fault zone?
It is a major fault segment of the San Andreas system that runs from Cache Creek in Yolo County to Lake Berryessa, identified by the USGS as active and capable of producing significant earthquakes.
Did the October 28 quake damage Scott Dam or other PG&E infrastructure?
According to PG&E spokesperson Megan McFarland, a survey conducted after the quake found no impact on the dam or related facilities.
What is the status of PG&E’s plan to remove the Potter Valley Project dams?
PG&E is seeking federal approval to dismantle the dams; the Federal Energy Regulatory Commission has signaled support, but a final decision has not yet been made.
Source: pressdemocrat.com
