A second quake of the same magnitude hit in the same vicinity and at the same depth 30 seconds later, the US Geological Survey said.
"A tsunami warning is now in effect which includes the coastal areas of Alaska from Unimak Pass, Alaska (80 miles northeast of Dutch Harbor) to Amchitka Pass, Alaska (125 miles west of Adak)," the West Coast and Alaska Tsunami Warning Center said.
"Those feeling the earth shake, seeing unusual wave action or the water level rising or receding may have only a few minutes before the tsunami arrival and should move immediately," warned the WCATWC.
"Homes and small buildings are not designed to withstand tsunami impacts. Do not stay in these structures."
The quake struck more than 1,000 miles west of the major Alaskan city of Anchorage.