A tsunami warning was issued immediately after the quake. It was later canceled.
The U.S. Geological Survey put the magnitude of the quake at 7.9 on the Richter scale. Major quakes in the range between 7.0 and 7.9 are large enough to damage even well-designed buildings, to cause considerable destruction as far as 150 miles from the epicenter, and to kill as many as 250,000 people.
But Monday's quake, centered undersea 15 miles from Little Sitkin Island, struck in an area of sparsely inhabited islands. On Adak, the 150 residents took refuge in a shelter 600 feet above sea level while the tsunami warning was in effect, City Manager Clayton Lockett told CNN.
At least 17 aftershocks followed the earthquake, some of them as severe as 5.9.
Alaska on the Pacific Ring of Fire is familiar with earthquakes. The Good Friday Earthquake on March 27, 1964, which measured 9.2 on the Richter scale, killed 139 people, flattened much of downtown Anchorage and was followed by tsunamis that caused deaths in Oregon and California.
The quake was the strongest ever recorded in the United States.
Monday's earthquake ties with the 2002 Denali quake as the strongest in the country since 1964.