Expert: Yountville-area fault had been poised for an earthquake

A magnitude-4.1 earthquake that struck Thursday near Yountville in Napa County originated with one of the Bay Area fault lines scientists have said was poised for a significant earthquake.

The Green Valley fault that runs between Vallejo and Fairfield was primed with enough pent-up energy to produce a major earthquake, according to a report released last fall. On Thursday, the fault produced a temblor larger than any aftershock after the August magnitude-6.0 earthquake along the West Napa Fault, said Brad Aagaard, research geophysicist with the U.S. Geological Survey.

Aagaard said Thursday’s 7:53 p.m. shaker could potentially be considered an aftershock.

“Sometimes an aftershock sequence will go on for years,” Aagaard said.

The earthquake originated about 6 miles east-northeast of Yountville and 5.5 miles below the surface. The plate along the west side of the fault moved to the north, and the eastern portion moved to the south, he said.

It was the same type of strike-slip motion found with most faults in the Bay Area, according to Aagaard.

Officials with Napa County fire and the Sheriff’s Office said no damage was reported.

The earthquake was too far below the surface and didn’t have enough energy to generate any visible earth movement for field scientists to document, Aagard said. But they will be investigating the movement of the fault to learn more about the North Bay fault region.

“The little earthquakes that give us data without causing damage are helpful from a scientific point of view,” Aagaard said.

You can reach Staff Writer Julie Johnson at 521-5220 or julie.johnson@pressdemocrat.com. On Twitter @jjpressdem.