Back-to-back earthquakes rattled northern California Monday morning, triggering an early start to the work week for some in San Francisco and the surrounding Bay Area.
The stronger quake -- at a 4.0 magnitude -- was centered one mile north of El Cerrito in the East Bay and 10 miles north-northwest of Oakland. It struck at a depth of 5.7 miles at 5:33 a.m. PT, according to the U.S. Geological Survey. It came about seven seconds after a 3.5 temblor -- known as the foreshock -- hit in nearly the same location at a depth of five miles.
"There were two earthquakes … and since then there have been a lot of little aftershocks, which is totally to be expected, down in the magnitude 2.0 range,” Don Blakeman, a geophysicist at the USGS' National Earthquake Information Center in Colorado, told msnbc.com.
The USGS website initially appeared to show two quakes, then one temblor in the immediate aftermath -- and some people wrote on Twitter that they felt two.
"It’s always difficult to split quakes this close together," Blakeman said, noting that automatic systems first upload the information, which quake analysts then have to review.
“I think part of the interest today is the fact that it’s been a little while since we had even a 4.0 in the Bay Area," he noted. "I think (it) kind of reminded everybody that California still has earthquakes.”
No damage was reported, the California Highway Patrol's Central Division said on Twitter. Blakeman said they haven't had any direct reports of damage.
"For a magnitude 4.0 quake we’ll probably get some reports of things falling off of people shelves and ... maybe a window cracked here and there, something like that, but shouldn’t be anything major,” he said.
Christine Cosgrove, who lives in Berkeley -- about four miles north of the epicenter, told the San Francisco Chronicle that "a big chunk of our chimney fell down. For us, this was the strongest earthquake we've felt in 22 years in the house. Other items fell off window sills and broke."
The Bay Area Rapid Transit (BART) stopped all trains where they were for at least five minutes as part of standard procedure, said communications director Jim Allison. A later inspection of the 104 miles of track did not turn up any problems, he added.
The shaking sent many to Twitter to comment on the small tremors:
"Here in Mill Valley, heard rumble of quake 1, then a second later, the hard hit of quake 2. Both were sharp pops with little after-roll," wrote Stephen Bové.
"(W)orked better than coffee 5:45am," quipped Maritza Ruiz-Kim.
"My shaky wake up call seemed to last for 30 sec+," said Amanda Walter.
"Was in airport shuttle during the quake. But now everyone else in SF is awake obscenely early, too!" exclaimed Zoelle Egner.
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