4.0 Earthquake Shakes Fremont, Bay Area

Here at KQED News Bay Area Early Morning Earthquake Headquarters -- actually, a humble abode in a university town in the East Bay -- we like to think we're attuned to the Earth's slightest movement.

However, we slept through Tuesday morning's earthquake, which the U.S. Geological Survey now reports as a 4.0-magnitude shake at 2:41 a.m. on the outskirts of Fremont.

The temblor took place on the Hayward Fault, and has been followed by a series of light aftershocks -- the largest of which was measured at magnitude 2.7.

No damage or injuries were reported. The one widespread impact: systemwide delays on BART as crews checked tracks for damage.

The undisturbed quality of our sleep aside, the quake did wake up plenty of people around the Bay Area. The USGS "Did You Feel It?" page records responses from Monterey County north to Sonoma County and from San Francisco as far east as Stockton. (We will note that here at Early Morning Earthquake HQ, there's just one response from our home ZIP code, and that's simply a report that the quake was not felt. We were not the only ones who slept through the shake.)

USGS geologist David Schwartz says the Tuesday temblor is a reminder that the Hayward Fault is due for a much bigger quake.

"The Hayward Fault is the star [of] the Bay Area in terms of its seismic potential," Schwartz told KQED's Ted Goldberg. "And this is just sort of another knock on the door saying, 'I'm here, I'm active, and one day I'm going to produce a large event.' "

The last big, damaging earthquake on the Hayward Fault: Oct. 21, 1868. That quake, with a magnitude estimated from 6.8 to 7.0, caused widespread property damage and killed 30 people. The Bay Area population at the time: about a quarter million -- just 30,000 of whom lived in Alameda and Contra Costa counties.

Our regional population today: 7 million plus, with 2.5 million living in the East Bay counties.