Swarm of earthquakes rattles Cobb Mountain, Geysers area

Cobb Mountain has been on the move — literally shaking every hour or two and, often, many times within an hour — rocking people accustomed to frequent seismic movement with an unusually large swarm of earthquakes in recent days.

They included one Tuesday night which, at magnitude 4.4, proved more alarming than most, setting dogs barking and rousing people from sleep around the area.

“I thought the house was going to shake apart,” said Hank Blohm, a 46-year resident of the Lake County town of Cobb and owner of the Mountain Meadow golf course and restaurant. “We have them all the time, related to the geothermal (steamfields), but not to this level.”

“You could hear this one coming like a big truck or train in the distance,” he said. “You hear a sound and then you think, ‘What was that?’ and then the shaking, and it was like, Wow, the duration was much longer than we normally have, and I actually thought it was much bigger than it was.”

The 4.4 magnitude shaker struck around 10:40 p.m. about 4½ miles west-northwest of Cobb, just inside the Sonoma County line.

It was followed in the next 10 minutes by 24 additional shakers, four of which measured magnitude 2.2 to 2.8. Within an hour, a magnitude 3.1 quake struck, one of about 100 tremors that occurred in the Cobb Mountain area in 24 hours.

“It just never stopped jiggling,” said Rich Curtola, another longtime denizen of Cobb. “Everything was wiggling for at least 30 minutes after the major shock.”

There were no immediate reports of damage from the larger quake, though some jars of caramel ice cream topping and tomato sauce fell off shelves at the nearby Hardester’s Market, a clerk there said.

But for residents of the area around Cobb Mountain and the Geysers steamfields, and even those farther flung, the shaker was a wake-up call and a jarring reminder that earthquakes are a continual threat to the region.

Mostly, folks in the area are used to smaller ones, in the magnitude 1.0 to 2.0 range, thanks to the seismically active geysers area, where condensed steam and recycled waste water from Lake and Sonoma counties are injected deep underground, onto hot rocks to generate steam and produce clean energy.

The pressure and temperature differential, and then withdrawal of large volumes of steam, causes the rock to contract, applying stress to small fractures in the rock and inducing earthquakes, the U.S. Geological Survey says.

Usually they’re small and barely perceptible, but larger quakes do occur.

“We get clusters up here, and we easily get, on an average day, 10-plus shakers,” said Curtola, 72. “And most of us don’t even feel them. They’re just rumbles.”

But then Tuesday the 4.4 hit, he said. “I never felt an earthquake like that.”

It was followed by 43 more over the next 20 minutes, he said.

The area is miles from the monster San Andreas Fault and the increasingly worrisome Rodgers Creek Fault, both of which run through Sonoma County, so anything larger than a rare 4.5 or 4.6 magnitude quake, such as occurred in 2013, is unlikely, the USGS said.

Typically, those larger events only come every three years or so “and have been happening like that since the 1990s,” said Alicia Hotovec-Ellis, a volcano seismologist with the USGS.

But it’s an unpredictable phenomenon, and it’s possible to have two magnitude four-plus quakes in a year.

It’s also common to go from several a day to a swarm like the current one, she said.

It doesn’t necessarily mean there’s a change in activity in the geothermal plant, though an effort to check in with operational changes at Calpine did not get a response.

Hotovec-Ellis said the magnitude 4 quakes “are good reminders that California is seismically active, and it’s a good reminder that you should be prepared just in case something bigger than a four happens.”

Allison Rodgers, owner of Mountain High Coffee and Books in Cobb, said they also make one think about earthquake insurance.

And they’re good conversation starters.

“We just got done with the snow,” she said, “so it gives us something new to focus on. New morning talk.”

You can reach Staff Writer Mary Callahan (she/her) at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.