There were no immediate reports of damage or casualties.
The quake was followed by a series of smaller aftershocks with the strongest registering 5.2, according to the agency.
The USGS put the quake's epicenter inside Canada's Yukon Territory in a wilderness preserve 83 kilometers (52 miles) northwest of Skagway, Alaska.
USGS said it struck at 12:31 GMT at a depth of 2.2 kilometers (1.4 miles).
The agency initially measured the quake's intensity at 6.5, but later revised it to 6.2. It also revised the depth from an initial reading of 0.1 kilometers (0.06 miles)
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