The quake hit at about 4:27 pm PST (0600 IST on Sunday) about 35 kilometers from Ferndale, the US Geological Survey said. No injuries have been reported.
Pacific Gas & Electric Co spokesman J D Guidi said power outages were widespread across most of Humboldt County, affecting about 25,000 customers.
Several traffic lights have fallen and numerous residents have reported water, gas and sewer leaks, Humboldt County Office of Emergency Services spokeswoman Jo Wattle said.
"People have chimneys down, and we're hearing about minor property damage and lots of glassware broken," Wattle said.
"People are really shaken up. It was shaking pretty good, then it had a big jolt to it at the end."
According to the USGS, the quake hit at a depth of nearly 16 kilometers. Seven aftershocks followed in the two hours after the quake, the biggest registering at 6:21 pm PST (0745 IST on Sunday) at a magnitude of 4.5.
Police in Ferndale said the earthquake caused stucco to fall off City Hall and broke shop windows, strewing the historic downtown streets with glass shards.
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