A massive earthquake off the Alaska coast, a shaker that hit frightening 8.0 on the Richter scale, triggered a tsunami warning Monday for the Aleutian Islands and surrounding areas.
UPDATE, 5:30 PM PDT: The National Weather Service says that the tsunami warning was canceled as of late Monday afternoon local time. The U.S. Geological Survey has also downgraded the magnitude of the Alaska earthquake to 7.9. No injuries have been reported so far, but the mayor of Adak, Alaska ordered his town's 150 residents to evacuate to a shelter 600 feet above sea level.
According to a bulletin from the Pacific Tsunami Warning Center, the size of the earthquake was initially measured at 7.1 but was subsequently increased to 8.0. The earthquake, however, struck too deep to set off tsunami warnings for the West Coast of the United States. The quake was centered west of Alaska's Aleutian Islands in the Pacific Ocean.
Because the earthquake hit deep under the ocean floor, the sea floor itself probably remained mostly stable, which is why there was not a more widespread tsunami warning, according to earth scientist Brent Ward at Simon Fraser University in British Columbia, Canada.
"You need the seafloor movement to generate a tsunami," said Ward. "When you move the seafloor, punching up from the bottom, it displaces the water and that's what generates the wave."
The quake hit about 13 miles southeast of Little Sitkin Island at a depth of about 60 miles. While the Aleutian Islands are not heavily populated, there are a number of small communities there, which reportedly felt Monday's quake.
No reports of damage had come in from the Aleutians as of late Monday afternoon, but because the tsunami warning remained in effect for those areas, residents may have remained indoors or moved to higher ground as they are always advised to do during tsunami warnings. If so, they were likely unable to inspect the area for earthquake damage at the time.
Earthquakes measuring 8.0, nonetheless, are serious events, Ward said.
"Anything over 8.0 we call a great earthquake and we only get about 15 or so of those a year of that magnitude around the world," he told The Vancouver Sun.
The tsunami warning was in effect for Alaska's coastal areas from Nikolski to Attu. Other areas received a tsunami advisory, meaning that while waves may hit, large-scale inundations of water are not expected.
[Images of Tsunami simulation and 1964 Great Alaska Earthquake and Tsunami Via Bing]