The 11:16 a.m. quake — its magnitude measured at 6.5 — hurled the roof of Paso Robles' 1892 clock tower building into a street, crushing a row of parked cars in this San Luis Obispo County town about 20 miles east of the epicenter.
Firefighters dug through the debris in front of a row of stores in Paso Robles, which appeared to be hardest hit by the jolt and is situated in a region dotted with wineries and horse ranches.
It was the largest earthquake to hit the state since 1999, reports CBS Elizabeth Sanchez. Scientists say Californians should expect more tremors in the immediate future.
"What we'll see over the next weeks and months is diminishing frequency and size of aftershocks," said Ross Stein, with the U.S. Geological Survey.
Nick Sherwin, 61, who operated Pan Jewelers in the collapsed building, said he had ordered five employees and eight customers out but "the big jolt hit" when he was about 10 feet from the door.
"My roof basically jumped onto the street and landed on cars with people in them," Sherwin said as he watched firefighters recover the bodies. The cars "are crushed like little toys, nothing left."
Three people were confirmed dead by early afternoon, said Ron Alsop, emergency services coordinator in the largely rural county of about 250,000 people.
The main shock was centered about 11 miles north of the coastal town of Cambria and was immediately followed by at least five aftershocks of magnitude larger than 3.3. The biggest was estimated at 4.7, according to the U.S. Geological Survey.
The quake rocked the federal courthouse in San Francisco, 165 miles to the northwest of the epicenter, and sent the building's upper floors swaying for about 30 seconds. It was felt as a sustained rolling motion in downtown Los Angeles, 185 miles southeast. It was also felt in much of the Central Valley.
"It was pretty sharp," said Sharyn Conn, receptionist at the oceanside Cypress Cove Inn in Cambria. "It really went on and on. I just got everyone under the door frames and rode it out."
Several people were reported hurt by falling barrels at a winery, San Luis Obispo County authorities said.
Other than Paso Robles, damage appeared minor elsewhere in the region.
About 10,000 homes and businesses were without power in the San Luis Obispo area, said John Nelson, spokesman for Pacific Gas and Electric.
Phone service became spotty as the system quickly overloaded.
Cambria is a town of 6,200 on the northern coast of San Luis Obispo County, where some 250,000 people live. The area's major landmark is Hearst Castle at San Simeon, the estate of the late publisher William Randolph Hearst.
Hearst Castle reported no injuries and no immediate signs of any serious damage, but was evacuated as a precaution, said Roy Stearns, spokesman for the state Department of Parks and Recreation. The castle is particularly popular this time of year because it is decorated with the Hearst Christmas ornaments.
The only known damage was a blown transformer in the campground below the hill, Stearns said. But a crew was being organized to go through each of the castle's 150 rooms to look more carefully.
The quake was felt in the control room of the Diablo Canyon nuclear power plant operated by PG&E. Nelson said that there appeared to be no damage to the plant and that it was functioning normally, but officials would conduct a "walk-through" to be sure.