It struck some 100 kilometers east-southeast of Adak, one of the Andreanof Islands group that make up part of Alaska's Aleutian Islands.
The second struck shortly afterwards, at 0349 GMT, just five miles further out to sea.
With a harsh climate, Adak is a sparsely populated island home to no more than a few hundred residents.
The Aleutian Islands are frequently rattled by strong quakes. A 6.4-magnitude quake shook the Andreanof Islands in August this year.