By Steve Quinn

JUNEAU Alaska (Reuters) - A magnitude 8.0 earthquake struck deep under the ocean floor near Alaska's Aleutian Islands, triggering shaking that could be felt for vast distances and prompting a tsunami warning, the U.S. Geological Survey and National Tsunami Warning Center said.

The tsunami warning, later downgraded to an advisory, prompted the evacuation of about 150 residents of the town of Adak to higher ground, according to media reports. It was not immediately clear whether the quake caused injuries or damage.

The quake 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.

The warning covered coastal areas of Alaska from Nikolski to Attu, officials said, adding that the level of tsunami danger was being evaluated for other U.S. and Canadian Pacific coastal areas.

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. (1700 ET), about 14 miles (23 km) southeast of Little Sitkin Island, Alaska, at a depth of about 71 miles (114 km), the USGS said.

Tsunamis are waves resulting from undersea quakes that can measure several yards (meters) 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 center said.

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

(Additional reporting by Eric Beech in Washington; Writing by Sharon Bernstein in Sacramento; Editing by Jim Loney amd Will Dunham)