A magnitude-6.2 earthquake struck Wednesday night about 50 miles from Adak.
"To give you an idea of how large that is, you get only about 100 magnitude-6 earthquakes on the globe annually," said seismologist Ken MacPherson of the Alaska Earthquake Center. "So that's a fairly significant earthquake."
Adak residents felt the shallow earthquake, which followed a series of smaller quakes in the same area.
MacPherson also said there have been dozens of aftershocks.
"Some of them are in the magnitude-4 range," he said. "We could see up to a magnitude-5. A general rule of thumb is that the largest aftershock will be about one whole magnitude less than the main shock."
MacPherson said the activity is nothing out of the ordinary in the seismically active Aleutian Islands. There is no tsunami threat.
The quake struck near Takawangha and Tanaga Volcanoes. While there's no evidence the event was volcanic in origin, the Alaska Volcano Observatory said scientists are monitoring for signs of unrest.
As of Thursday afternoon, the observatory has listed the alert levels for Takawangha and Tanaga Volcanoes as "normal."