powerGov. Jerry Brown declared a state of emergency for this region north of San Francisco on Sunday, as emergency workers scrambled to put out fires and treat injuries after a strong earthquake, measured at magnitude 6.0, struck early this morning. Even as the shaking continued — several smaller aftershocks were reported — emergency crews were combing through a number of buildings in downtown Napa, making sure that no one had been trapped inside. Several fires were reported in the area, and a number of homes and buildings, including some in downtown, had structural damage. At least 90 people had been treated at the emergency room of Queen of the Valley Medical Center in Napa, according to Vanessa DeGier, a spokeswoman for the hospital. Most injuries were minor cuts or abrasions caused by falling debris. But three patients remained in critical condition, including a child who had been crushed by a falling fireplace. More