The earthquake hit 157 miles (256km) off the Alaska coast and sparked a tsunami warning for Hawaii, Canada and the US West Coast.
People on Kodiak Island have been urged to stay away from the coast after the quake struck just 175 miles away.
Reports on social media suggest the tremors were felt as far south as Seattle and Vancouver.
Environment Canada issued a tsunami warning for large parts of Vancouver Island, including Greater Victoria.
FOX•GETTYAlaska earthquake: Live webcam records the situation develop on Vancouver Island
The National Tsunami Warning Center (NTWC) issued a tsunami warning for the coastal areas of British Columbia.
The NTWC latest update said: "Tsunami warnings mean that a tsunami with significant inundation is expected or occurring.
"Warnings indicate that widespread dangerous coastal flooding accompanied by powerful currents are possible and may continue for several hours after the initial wave arrival."
The quake had a depth of around six miles, according to USGS.
It is already expected to be one of the largest quakes in