Small tsunami waves of less than 1 foot were reported in Alaska, the center said.
[Previous story, published at 7:11 a.m. ET]
A magnitude-7.9 earthquakedetected in the Gulf of Alaska has triggered tsunami warnings in Alaska and tsunami watches across several Western states.
A tsunami warning is in effect for southeast and south Alaska, including the Alaska Peninsula and Aleutian Islands, as well asBritish Columbia in Canada. A tsunami watch is in effect for California, Oregon and Washington, according to the Tsunami Warning Center.
As of 2:29 a.m. (6:29 a.m. ET), water in the harbor near Kodiak was receding, the city's police department tweeted, after earlier urging residents to get at least 100 feet above sea level.
"Citizens should remain in place and wait for further updates," Kodiak police said.
Receding water can be a dangerous precursor to a significant tsunami, CNN meteorologist Dave Hennen said.
"Think of a sloshing bathtub with a water level dropping, only to rise extremely quickly," he said. "The tsunami would come in waves. In some occasions, the first wave is not the largest."
The San Francisco Department of Emergency Management warned residents within three blocks of the Pacific Coast or within five blocks of the San Francisco Bay to prepare to evacuate, "so that you are ready if evacuation is needed," the agency tweeted.
The earthquake struck about 175 miles southeast of Kodiak, Alaska, shortly after midnight Alaska local time, according to preliminary figures from the United States Geological Survey. The quake had a depth of about 15 miles, according to the USGS. Previously, the earthquake had been measured at magnitude 8.2.
'Whole town is evacuating'
Nathaniel Moore was on a boat in Kodiak when the quake hit. He said he felt it "shake really good for a minute." He and others on the commercial fishing vessel quickly got to shore and headed for higher ground amid the tsunami warning.
"The whole town is evacuating," he told CNN early Tuesday.
Tsunami sirens sounded in Kodiak, and police warned: "This is not a drill."
Wendy Bliss Snipes described the quake as "a slow roller, so it was felt for at least a minute before the real rolling started. Nothing fell off the walls, and I didn't have to wake my kiddo."
Heather Rand, who was in Anchorage, Alaska, told CNN that the earthquake felt like the longest she had ever experienced.
"It was a very long, slow build up. Creepy, more than anything. Definitely the longest, and I was born here," Rand said. She reported no damage besides cracks in the drywall.
CORRECTION: This story has been updated to correctly attribute a quotation from an Alaska resident.