An earthquake shook remote northern Lake County at 5:01 a.m. Wednesday, Oct. 28. The U.S. Geological Survey first reported it as magnitude 3.9, then revised it to 4.0, according to the San Francisco Chronicle. The epicenter was about 6 miles north of Lake Pillsbury, near the Lake–Mendocino county line and 27.8 miles northeast of central Ukiah. The Chronicle gives the depth as 6.6 miles and the Press Democrat as about 7.2 miles.
Part of a swarm
The Press Democrat reports this was the largest of 12 earthquakes recorded in the same area this month. Eight came Wednesday between 4:51 a.m. and 9:13 a.m., ranging from magnitude 1.6 to 4.0. Four smaller ones, magnitude 1.5 to 1.8, hit between Oct. 7 and Oct. 17. The USGS called the 4.0 the potential mainshock of the sequence.
Which fault?
The quakes clustered near the Bartlett Springs fault zone. That fault is part of the San Andreas system and runs from Cache Creek in Yolo County toward Lake Berryessa. The Chronicle says the epicenter was 1.6 miles from the fault's Elk Creek section, but it wasn't immediately clear whether the quakes happened on the fault itself. Any small quake has a slight chance of being a foreshock, but the Chronicle puts that risk in the single digits, percentage-wise.
The dams
Lake Pillsbury is the reservoir behind Scott Dam, one of two Eel River dams PG&E is seeking to remove as it decommissions the Potter Valley Project. PG&E spokesperson Megan McFarland told the Press Democrat that crews surveyed the project's infrastructure Wednesday morning and found "there was no impact." No injuries or damage were reported.
Felt a quake? You can report it on the USGS "Did You Feel It?" page, which helps scientists map how strongly it shook.