Schwarzenegger signed the emergency proclamation on Tuesday after meeting with officials in Paso Robles.
He toured the town centre where a landmark two-storey clock tower and building collapsed, crushing two women who were standing outside.
Small aftershocks rippled through the area on Tuesday morning, the largest coming a day after the major quake struck at 1916 GMT on Monday.
In addition to the two deaths, the magnitude 6.5 earthquake injured more than 40 people, cut electricity to thousands, and shook houses and skyscrapers from San Francisco to Los Angeles.
A total of 82 commercial buildings in Paso Robles were declared unsafe by building inspectors on Tuesday, and inspection teams were continuing to examine the damage.
The state emergency declaration provides funds for San Luis Obispo County to pay emergency workers, clear debris and repair damage to roads and power and sewage lines.
Schwarzenegger has pledged to
rebuild the shattered town
Paso Robles Mayor Frank Mecham said the town would also seek federal disaster assistance, adding that officials may not know the full scope and cost of the damage for several days.
Meanwhile, Schwarzenegger thanked emergency workers and pledged to rebuild the shattered business district. He warned the disaster was a lesson to other building owners who have not yet made their properties earthquake proof.
"The greatest tragedy is that time will not undo the loss of life," Schwarzenegger told a crowd of emergency workers and citizens who turned out for a glimpse of the action star-turned-governor.
"I want to give my heartfelt thanks to all the rescue workers. We have already begun the task of recovering and rebuilding."
Looking at the crumpled building where the two women died, he evoked cheers by pledging "this will be your Main Street once again".
The quake marks the second disaster Schwarzenegger has had to respond to since he was elected in October this year.
As governor-elect he toured wildfire devastated neighbourhoods in Southern California with outgoing Governor Gray Davis.
"A very important part of being governor is being in touch with communities, especially after a disaster," Schwarzenegger said. "I want to be the governor of the people."
Monday's quake measured 6.5 on
the Richter scale
Schwarzenegger, a pro-business Republican, had pledged not to raise taxes to plug the state's budget deficit unless a natural disaster occurred.
During his brief statement, Schwarzenegger urged Californians to "go and enjoy the holidays" despite the earthquake and the elevated terror risk announced earlier in the week by the Bush administration.
Monday's quake was the strongest to hit California since 1999, when a 7.1-magnitude temblor struck the state's southern desert.
The US Geological Survey said the quake would produce hundreds of aftershocks over the next days, weeks and even years.