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Minor quakes hit San Francisco area

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SAN FRANCISCO, California (CNN) -- Two minor earthquakes shook the Bay area on Sunday, but no damage or injuries were reported.

A 3.77-magnitude quake occurred at 7:57 p.m. (10:57 p.m. EST) and was centered two miles southeast of San Ramon in Contra Costa County, said Sheryl Tankersley, spokeswoman for the Governor's Office of Emergency Services.

County officials described the event as a "small, rolling earthquake," according to Tankersley.

The first quake, which had a preliminary magnitude of 3.9, happened at 6:54 a.m. (9:54 a.m. EST) and was centered 26 miles east of San Francisco City Hall in San Ramon, said Bill Smith, a geophysicist for the U.S. Geological Survey in Golden, Colorado. Its depth of six miles is considered shallow, he said.

Typically, earthquakes of such magnitude do not cause damage, Smith said.

"They get quakes like that very frequently in the Bay area and other parts of California," Smith said. "It would pretty much need to be a 5 before it started knocking things off of shelves."

Callers to local radio stations said the tremor woke them up. A resident of Oakland said it felt like a truck hit her house.

Though minor, the tremor was the strongest along the Calaveras fault since the 1970s, said Dave Schwartz, a geologist with the USGS, who was seven miles north of the epicenter when it struck and slept through it.

The Calaveras is one of three major faults in the Bay area, he said, along with the Hayward and the San Andreas. It has about a 10 percent chance of producing a magnitude-6.7 quake or larger within the next 30 years, he said.

Asked if Sunday's first quake could portend stronger activity in the near future, Schwartz said, "It's certainly something we're going to be watching. Whenever part of the fault has been kind of quiet for a while and then produces an earthquake, we like to keep an eye on it."

If the earthquake's magnitude had been 5 or higher, Schwartz said, "we would have issued some statement saying that in the next 24 hours, there's a certain probability of a larger earthquake. We do that routinely for magnitudes of 5 and above. We don't do it for the smaller ones.

"But since I live here, I'm going to keep an eye on it," he added, "and my earthquake insurance is paid up."

Tankersley said the second earthquake along the same fault line brings additional concerns, noting the emergency services office will be watching to see if a third quake of magnitude 3.5 strikes within 24 hours of the original Sunday morning tremor. Such a quake could signal an increased period of seismic activity.

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