According to the U.S. Geological Survey, the quake at 4:42 a.m. posted a 4.2 rating on the Richter scale. The epicenter was near Piedmont. It was felt from Sonoma to San Francisco and throughout the East Bay.
No injuries have been reported in the region, and the damage generally entailed stores in disarray and a few shattered windows.
"There was just stuff all over the floor," said James Velebit, a receiving clerk for the Safeway in Montclair, near the epicenter. "Every aisle was damaged."
Velebit, who was working prior to the store's 6 a.m. opening, said the quake lasted at least 20 seconds -- about as lengthy, he said, as the time it took to run outside.
"This is a cement building, so I really felt it," Velebit said.
Down the street at Albertsons on 1963 Mountain Blvd., similar damage caused a two-hour delay in the store's opening. The Safeway at 1444 Shattuck Ave. in Berkeley suffered a shattered window, which has kept the store closed as of 9 a.m.
The quake sparked a handful of power outages in Oakland, with 4,600 customers losing power after an electrical switch was jolted and damaged. PG&E spokeswoman Susan Simon said most of that outage has since been restored.
On BART, spokesman Linton Johnson said the quake did not cause any detectable damage, though trains were brought
Earthquake experts warn that aftershocks are not out of the question, but the fact that none have been felt since the tremor is a promising sign.
"It's surprising for a 4.2 that we haven't seen any aftershocks," said Steve Walter, a seismologist with USGS who has tracked earthquakes for 25 years.
Walter said that the last time an earthquake of 3.5 or larger hit the Northern Hayward Fault -- as opposed to the Southern Hayward Fault that generates most of the small quakes in the area -- was a 4.0 quake in 1987.
For that stretch of the fault, Walter said, only five such quakes have occurred in the past 50 years.
Walter also dispelled any thoughts that this morning's events lessens the chances of a larger, more catastrophic quake in the future.
"This is a locked fault, so it's not releasing strain through regular activity," he said. "This only released a little bit of strain."
Reach Robert Salonga at email@example.com or 925-943-8013.