A 7.8 magnitude earthquake occurred off the Alaskan Peninsula and triggered a tsunami warning for areas within 300 kilometers of the epicenter.
Sirens have been reported to howl when residents are warned to move inland or to a higher level.
The flat quake occurred at 0612 GMT on Wednesday about 500 miles southwest of Anchorage and about 60 miles southeast of the remote settlement of Perryville, the US Geological Survey said.
"Based on the preliminary earthquake parameters … dangerous tsunami waves are possible for coastlines located 300 km from the earthquake epicenter," said the Pacific Tsunami Warning Center.
A tsunami warning was in effect for the Alaskan Peninsula and Southern Alaska.
"The extent of the tsunami risk is being assessed for other US Pacific coasts and Canada in North America."
Warning sirens sounded in Kodiak, the capital of Kodiak Island, and several thousand people evacuated lower areas, Sergeant Daniel Blizzard told Alaska State Troopers 11 KTVA News.
"People are quite concerned, especially about an earthquake as big as Kodiak," he said.
"There was no wave at all last time. This time we are not sure if there is a wave, but we are preparing as if there is one," said Blizzard.
The first place where a tsunami wave was predicted, Sandy Point, had previously reported "only a very small wave," James Gridley, director of the Palmer National Tsunami Warning Center, told Alaska Public Media (APM).
"We don't expect a massive wave anywhere," he said.
However, the quake was felt hundreds of miles away.
"Bed and curtains went. Felt like a very long quake!" A witness in Homer, Alaska, 400 miles from the epicenter, said on the quake monitoring website, msc-csem.org.
Several aftershocks followed, with the strongest being 5.7.
There were no immediate reports of damage or loss.
The tremor seemed to be a quake in the subduction zone, where two tectonic plates converge, said a seismologist.
"It's the interface, the plate boundary between where the Pacific plate meets North America. It's a very common type of earthquake in the area," Mike West of the Alaska Volcano Observatory told APM.
Alaska is part of the seismically active Pacific fire ring.
The U.S. state was struck by a 9.2 magnitude earthquake, the largest ever in North America, in March 1964. It devastated Anchorage and triggered a tsunami that struck the Gulf of Alaska, the west coast of the United States, and Hawaii.
More than 250 people were killed in the quake and tsunami.
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