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The red star marks the epicentre of the strong earthquake that struck west of Port Alice, off the northwest coast of Vancouver Island on Thursday morning. ((Natural Resources Canada))

The 5.8-magnitude earthquake that shook the ocean floor off the northwest coast of Vancouver Island, 191 kilometres west of Port Alice, B.C., early Thursday was just the latest in a swarm of tremors this week.

Dave Ross was still in bed in the small community of Holberg on the northwestern end of Vancouver Island when he was awoken by the subtle shaking. He said there was a little bit of rumbling and just a bit of vibration, but accepted the quake as just a part of life on the West Coast.

Thursday morning's quake was the strongest of more than 100 recorded in the area since Tuesday. In total, there have been 18 quakes of magnitude 4 or higher in the region this week.

'Eventually we will have larger earthquakes on land because they are all part of the system.' — Garry Rogers, earthquake scientist

Natural Resources Canada's Earthquakes Canada section initially said the tremor had a magnitude of 6.1 but later downgraded it to a magnitude 5.8, according to its website.

It said the quake, which struck at 5:37 a.m. PT, was "too small to cause a tsunami."

The U.S. Geological Survey, which first measured the quake as magnitude 6.1 with a depth of 10 kilometres, has also downgraded it to a magnitude 5.8, according to its website.

"There have been no felt reports, but this earthquake may have been felt mildly on Northern Vancouver Island," Earthquakes Canada's website said earlier Thursday morning.

On Wednesday, a magnitude 5 quake struck the ocean floor west of Vancouver Island at 1:17 p.m. PT. There were no reports of damage and there was no risk of a tsunami.

Earthquake scientist Garry Rogers, with the Geological Survey of Canada, said Thursday that swarm activity happens several times a year in the area — an indication that the area's tectonic plates are moving.

"Swarm activity is common for that area … where new oceanic material is being created," Rogers said in a telephone interview.

"[It's] an active volcanic region offshore under the water [and] has lots of small earthquakes. Typically, they occur like this, in swarm activity," he said.

More quakes are expected in the next few days and they could include an even larger one, Rogers said.

"They're all connected to land and eventually we will have larger earthquakes on land because they are all part of the system."

With files from the Canadian Press