A powerful earthquake sparked a tsunami warning along the coastlines of southern Alaska and British Columbia, but the alert was cancelled when no damaging waves were generated.

The U.S. Geological Survey said the quake had a preliminary reading of 7.5 and struck at 12:50 a.m. PT (11:50 p.m. local time Friday), about 102 kilometres west of Craig, Alaska and some 300 kilometres west-northwest of Prince Rupert, B.C.

The Alaska Tsunami Warning Center had issued a tsunami warning for the southern coast of Alaska, stretching for 765 kilometres to the northern tip of Vancouver Island — but the centre cancelled the warning a few hours later.

The earthquake happened along the same fault line as the one that struck off B.C.'s Haida Gwaii last October, but in a section that has not seen this kind of strong seismic activity for a few hundred years, says CBC meterologist and seismologist Johanna Wagstaffe.The earthquake happened along the same fault line as the one that struck off B.C.'s Haida Gwaii last October, but in a section that has not seen this kind of strong seismic activity for a few hundred years, says CBC meterologist and seismologist Johanna Wagstaffe.

CBC meteorologist Joanna Wagstaffe said a small tsunami wave was generated off Port Alexander, Alaska. In addition, at least two strong aftershocks measuring 4.5 and 4.7 were reported after the initial quake.

"But there are dozens of smaller aftershocks occurring," Wagstaffe said.

"Woke me up from a dead sleep," said one woman describing the initial quake on her Twitter feed from Haida Gwaii, formerly the Queen Charlottetown Islands in B.C., which was hit by a 7.7-magnitude quake last October.

People gather at Sitka High School early Saturday in Sitka, Alaska, following a magnitude 7.5 earthquake. People gather at Sitka High School early Saturday in Sitka, Alaska, following a magnitude 7.5 earthquake. (Daily Sitka Sentinel, James Poulson/Associated Press)

The quake also woke Carol Kulesha, mayor of Queen Charlotte City.

"I was asleep and I heard a clattering sound and the house was shaking, so I definitely felt this one, although it was nothing like the previous one that was 7.7," she said.

Kulesha said the fire department sounded the alarm for people to evacuate to higher ground. After the warning was cancelled, a fire department crew was "sent back to the hills to tell people they can go home now."

'Sounded like a big Mack truck'

"It was scary," said Annette Reaves, who works at the hospital in the town of Sitka, Alaska.

"It shook the staff here at the hospital for about 15 seconds. It wasn't a rolling kind of shake, but it was just a trembling shake. It sounded like a big Mack truck coming through," Reaves said.

She said patients remained calm as they were advised to move away from windows, but officials decided the hospital did not have to be evacuated.\

Before the early Saturday tsunami warning was cancelled, B.C.'s emergency notification centre said a tsunami could impact low-lying coastal areas in northern and central areas.

People evacuated homes in Kincolith, B.C., north of Prince Rupert, near the Alaskan border. People evacuated homes in Kincolith, B.C., north of Prince Rupert, near the Alaskan border. (Courtesy of Adrian Stewart)

The Alaska Tsunami Warning Center had warned that "significant widespread inundation of land is expected," adding that dangerous coastal flooding was possible.

In its cancellation statement, the centre said some areas were seeing just small sea level changes.

When a tsunami warning is issued, people are advised to move to higher ground.

Shortly after the quake struck, a tsunami advisory was issued for the outer west coast of Vancouver Island from Cape Scott to Cape Renfrew and in the Juan de Fuca Strait from Jordan River to Greater Victoria, but Julianne McCaffrey of Emergency Management B.C. told CBC News that advisory was soon cancelled.

An advisory means strong currents are likely and people should stay away from the shore.

Officials in Craig reported on the city's Twitter feed they are "standing down" from the tsunami warning. But they warned people "may experience aftershocks over the next several days."

The Alaska Earthquake Information Center said the quake was widely felt but it received no reports of any damage.

With files from The Associated Press