The earthquake struck at 11:53 a.m. local time (4:53 p.m. Eastern time) near Little Sitkin Island in the far western part of the Aleutian Island chain, just west of the International Date Line.
According to the U.S. Geological Survey, the main jolt was followed by aftershocks measuring magnitude 6.0 and 5.8, occurring 18 and 37 minutes after the mainshock, respectively. Smaller aftershocks of 4.8 and 5.1 followed within 90 minutes of the mainshock.
The National Tsunami Warning Center in Palmer, Alaska, issued a tsunami warning for a portion of the Aleutians stretching from Nikolski to Attu. "Widespread dangerous coastal flooding accompanied by powerful currents are possible and may continue for hours after tsunami arrival," the NTWC bulletin said.
"We're seeing water leave our bay, so we do have everybody up on the Bering Hill area, where our primary evacuation center is at," City Manager Layton Lockett told The Associated Press by telephone as he gathered some last paperwork before heading out himself to join about 300 residents at the center.
The agency downgraded the bulletin to a less-serious tsunami advisory at 1:44 p.m. local time, just under two hours after the quake.
The first tsunami measurement came from Amchitka, Alaska, about 25 miles from the quake's epicenter. A tsunami of 0.6 foot (about 7 inches) was recorded at 12:36 p.m. local time. The NTWC said that the depth of the earthquake, some 71 miles below the earth's surface, would mitigate the extent of the tsunami. Tsunami heights are expected to be less than 1 foot across the Aleutians, according to the NTWC bulletin downgrading the earlier warning.