The quake struck the waters just before 4 a.m. PT.
Coastal areas from British Columbia to California were alerted, but no warning or watch was issued.
There were no immediate reports of damage or injuries.
A 9.2-magnitude earthquake in Alaska in 1964 triggered a tsunami that recorded the only known tsunami deaths on the mainland United States. Fourteen people died in Crescent City, Calif., and four died in Oregon.
Friday's earthquake was originally reported as a 7.1, but it was downgraded to a 6.8.