A powerful earthquake sparked a brief tsunami warning for a lengthy stretch of the coasts of British Columbia early Saturday, but no damaging waves were generated and the warning was cancelled.

The magnitude 7.5 quake did generate a small tsunami, but the Alaska Tsunami Warning Center said the waves didn't pose a threat.

The U.S. Geological Survey said the quake struck at about 12:50 a.m. PT about 100 kilometres west of Craig, Alaska, and some 300 kilometres west-northwest of Prince Rupert, B.C. The tremor struck at a depth of about 10 kilometres.

A tsunami warning quickly followed for about 1,125 kilometres of coastline from Cape Fairweather, Alaska, to the northern tip of Vancouver Island, and an advisory was issued for the B.C. coast as far south as Victoria.

A warning means an area is likely to be hit by a wave, while an advisory means there may be strong currents without widespread inundation.

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.

Seismologist Jana Pursley of the U.S. Geological Survey said the quake was followed by six aftershocks, the strongest of which registered a 5.1 and came nearly four hours after the initial quake.

The Alaska Tsunami Warning Center had initially warned that "significant widespread inundation" of land was expected along with possible coastal flooding.

But it later cancelled the warning, saying the waves were too small to pose a threat, reaching just 15 centimetres above normal sea level.

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 Charlotte 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. (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.

Alex Godin, who was working at a Tim Hortons in Prince Rupert, B.C., when the quake struck, said the tremor was barely noticeable and "felt like a bump."

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. (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.

No connection to South Coast

Saturday’s quake was likely indirectly connected to the 7.7 quake in October off the coast of Haida Gwaii, according to Simon Fraser University professor John Clague.

Clague told CBC News that the earlier earthquake might have set up the stresses that caused the second one.

But he also said these tremors do not necessarily mean a similar quake is imminent for B.C.'s South Coast, because different fault systems are involved.

“It's separate from what's happening on the South Coast because we have a different geologic or tectonic environment. So, I wouldn't worry that these recent earthquakes would trigger an earthquake on the South Coast,” Clague said.

Clague added, however, that although there is no causal relationship between the two systems, there still remains the possibility of a damaging earthquake on the South Coast. There are many faults in the region and a history of at least one catastrophic quake in the last several centuries.

With files from The Associated Press and the CBC's Dan Burritt