Iran Press/ America: The quake, about 24 miles off the coast of the port city of Eureka, hit at 2:34 am (10:00 GMT) on Tuesday, according to the United States Geological Survey (USGS).
The earthquake cut power to more than 70,000 people in Humboldt County according to the outage tracker site, poweroutage.us.
Local media reported the earthquake caused numerous gas leaks, and downed powerlines, and resulted in at least one structure fire in the city of Ferndale.
Small rockslides were also reported and at least one roadway buckled during the quake, which was followed by dozens of small aftershocks.
Humboldt County officials warned of reports of “widespread damages to roads and homes”, and urged residents to check gas and water lines for signs of leaks.
Police in the city of Ferndale also moved to close a bridge that the California Highway Patrol reported had cracks following the quake, KRCR-TV reported.
The earthquake followed a smaller magnitude 3.6 earthquake that struck about 402km (250 miles) near San Francisco on Saturday.
California is regularly shaken by tremors and seismologists have warned an earthquake capable of causing widespread destruction is likely to hit the state in the next 30 years.
A 6.7 magnitude earthquake in 1994 in Northridge, northwest of Los Angeles, left at least 60 people dead and caused an estimated $10bn in damage.
A 6.9 magnitude quake in San Francisco in 1989 claimed the lives of 67 people.
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