A magnitude 7.9 earthquake struck deep under the ocean floor near Alaska's Aleutian Islands, triggering shaking that could be felt for vast distances and briefly prompting a tsunami warning, the National Tsunami Warning Center said.

The tsunami warning, later downgraded to an advisory, prompted the evacuation of about 200 residents of the town of Adak to higher ground, city manager Layton Lockett said. The quake did not appear to cause any injuries or damage. 

Alaska earthquake

The undersea earthquake was originally reported to be magnitude 8, but was later downgraded to a magnitude 7.9. The quake was so large that officials predict seismometers around the world will pick up readings of aftershocks, of which dozens were created, for up to 24 hours. (Google Maps)

The earthquake was so large and deep that it triggered dozens of aftershocks within an hour and prompted enough shaking that it will be picked up by seismometers around the world over the next 24 hours, said Mike West, a seismologist who serves as director of the Alaska Earthquake Center.

"When you've got an earthquake that big, it rings the Earth like a bell," West said.

Lockett said he and his staff were in their offices when the earthquake struck.

"Oh, we felt it," he said. "We felt it in length, in duration and in intensity. We were sitting there for about 20 seconds, then we went outside and it kept going and going and going."

The tsunami warning covered coastal areas of Alaska from Nikolski to Attu.

The U.S. National Oceanic and Atmospheric Administration initially warned of widespread, dangerous coastal flooding and powerful currents in the region for hours in the event of a tsunami. The warning was downgraded about two hours after the earthquake hit.

A tsunami advisory, less severe than a warning, was in effect for coastal areas of Alaska from Unimak Pass to Nikolski.

The quake struck shortly before 1 p.m., about 23 kilometres southeast of Little Sitkin Island, Alaska, at a depth of about 114 kilometres, the USGS said.

Tsunamis are waves resulting from undersea quakes that can measure several metres high and can overwhelm coastal areas near and far, NOAA said. It takes a large quake of magnitude 7.0 or higher to produce a tsunami, the centre said.

In 2004, a tsunami produced by a magnitude 9.3 undersea earthquake struck near Indonesia and 240,000 people were killed, the centre noted.

With files from The Associated Press