Eureka, Calif. — The strongest earthquake in the adjacent 48 states in more than 20 years rocked the coast of northern California and southern Oregon, causing widespread damage but no know deaths. A freeway bridge collapsed, hurling two vehicles 30 feet to the ground and injuring six persons. The quake was centered 20 miles northwest of Eureka and had a Ritcher magnitude of 7. Disaster officials were amazed at the relatively light damage. Earthquake experts said the tremor occurred on a branch of the San Andreas fault.