FAIRFIELD — Fairfield and Suisun City got a gentle shaking Friday morning from an earthquake that is apparently part of an ongoing series of quakes, though most are too small to be felt.
It was a magnitude of 3.1 on the Richter scale, enough to be noticed, not enough to do damage. But it wasn’t along the Green Valley fault, the area’s most notorious earthquake fault.
The U.S. Geological Survey’s online earthquake map shows the epicenter near downtown Fairfield and Suisun City. The quake happened at 9:38 a.m. at a depth of about eight miles.
It is far from the first time a quake has happened beneath Fairfield. Steve Walter, a staff seismologist at the USGS in Menlo Park, said several might happen annually, most of them small. Instruments have measured them for several decades, he said.
“There’s a cloud of deeper earthquakes we see under the Fairfield area,” Walter said.
By “cloud,” he means that the quakes, when plotted, don’t define a fault line. That’s in contrast to quakes that are plotted along the Hayward fault, he said.
Also, the quakes are as much as nine miles to 11 miles deep. That’s deeper than most quakes in the region, he said.
“It’s a bit of a mystery what’s going on in this area,” Walter said.
This mystery that has been out of the spotlight. Fairfield looked at earthquakes in the region when it did a 2001 environmental study for its General Plan update. The study mentions the Green Valley fault and the nearby Cordelia fault, but nothing about an earthquake “cloud.”
The Association of Bay Area Governments has long warned that a major quake could happen in the Fairfield-Suisun area along the Green Valley fault. Its report on earthquake scenarios makes no mention of a series of smaller quakes beneath Fairfield.
Walter sounded no alarm bells over Friday’s mild 3.1 quake.
“There’s no reason to think people need to be more worried based on that earthquake,” Walter said.
The USGS website includes a map of quakes in the region since 1900. The entire Bay Area is almost buried beneath the dots.
Reach Barry Eberling at 427-6929 or firstname.lastname@example.org. Follow him on Twitter at www.twitter.com/beberlingdr.