There were no immediate reports of damage or injuries. There was no danger of a tsunami, according to the U.S. National Tsunami Warning Center in Palmer, Alaska.
The quake hit at 8:10 p.m. local time and was centered about 25 miles (40 kilometers) southwest of Port Alice, British Columbia, and about 280 miles (450 kilometers) northwest of Seattle, Washington. It occurred at a depth of 7 miles (11.4 kilometers).
It was followed by aftershocks of magnitude 5.0 and 4.2, the USGS said.
Port Hardy resident Jennifer Nickerson said the quake caused lights to sway and the fish tank in the hotel where she works to rock.
In less than two hours, more than 650 people in nearly three dozen cities logged on to the earthquake information site to report feeling the quake.
The online reports came from as far away as Kelowna in south-central British Columbia and the Seattle suburbs of Auburn and Bellevue, said USGS geophysicist Dale Grant in the Golden, Colorado, earthquake information center. None of the online comments reported damage, Grant said.
The quake was the strongest in the area since another 6.6 on Nov. 2, 2004, Grant said.