Magnitude 5.6 Earthquake in Mendocino County Alerts Bay Area

What that felt like depended heavily on distance. Close to the epicenter, Cochran said, people experienced “very strong to severe shaking,” and the kind that is “quite frightening” and impossible to ignore.

Farther south, in the northern Bay Area, most people likely felt nothing, and those who did felt only a faint tremor.

“You might wonder, oh, did a truck just drive by or was that an earthquake,” she said.

The quake struck along the Maacama Fault, a long vertical fracture running between Santa Rosa and Laytonville. Cochran said it is a well-studied fault capable of producing far larger earthquakes, “probably up to north of a magnitude 7.”

By late morning, the 5.6 had been followed by three aftershocks of magnitude 2.5 or greater, all within the first hour, Cochran said. More are expected.

She put the odds of another magnitude 4 or larger quake in the coming days at about 40%, and the chance of one magnitude 5 or above — similar to Wednesday’s — at roughly 7%. There is also a small chance, about 1 in 100, of a magnitude 6 or larger event within the next week.

For people who got an alert on their phones, the warning came fast. Cochran said the ShakeAlert system detected the quake within five and a half seconds, with an initial magnitude estimate of 5.7 nearly exact and a location that was “essentially spot on.” Alerts went out across a wide region.

Cochran said the morning was a reminder to prepare.

“We all in California live in earthquake country,” she said, urging people to store food and water and secure shelves and bookcases so nothing falls during strong shaking.

Through MyShake and ShakeAlert, she added, residents can get seconds of warning before the next one arrives.