Story highlights Preliminary measures put the quake's strength at 8.0, later downgraded to 7.9

An initial tsunami warning was canceled

The 150 residents of Adak moved to a shelter on higher ground

Adak is on an island off the Alaskan mainland

An initial tsunami advisory has been canceled for coastal Alaska. One had been issued after a magnitude 7.9 earthquake struck Monday about 15 miles from Little Sitkin Island, Alaska.

The quake had a depth of 71 miles, according to seismologists.

The town of Adak evacuated its 150 residents to a shelter about 600 feet above sea level after the initial tsunami warning, City Manager Layton Lockett told CNN.

There were no initial reports of quake damage in Adak or on Shemya Island, said Ian Dickson, website manager for the Alaska Earthquake Information Center.

Dickson said there were 17 measurable aftershocks in the two hours after the quake.

First estimates put the quake at magnitude 8.0. The strength was subsequently set by the U.S. Geological Survey at 7.9 magnitude.