VANCOUVER -- A strong earthquake struck at sea Thursday off Canada's Pacific coast west of Vancouver Island, but was too small to generate a tsunami, Canadian and US officials reported.

Initial government reports indicated it was a quake of 6.1-magnitude, but the US Geological Survey later downgraded it to 5.8.

The quake occurred at 5:37 a.m. local time (1237 GMT) some 153 kilometers (95 miles) west-southwest of Port Hardy on the northern tip of the island, and 472 kilometers (293 miles) from Vancouver, the largest city in western Canada, said the US agency.

Natural Resources Canada said it was the largest of "a series of earthquakes occurring in the seismically active region" beginning late Monday.

"These earthquakes are too far offshore to be felt on land and too small to generate a tsunami," the Canadian government agency said.

Glen Plummer, spokesman for Emergency Management British Columbia, the province's emergency preparedness agency, said Thursday's pre-dawn earthquake was "nothing to be alarmed about," as it was "not an unusual event."

No reports of casualties or damage had come in to the agency, Plummer said.

Experts say Canada's Pacific Coast is overdue for a major earthquake, but a British Columbia government website on seismic activity points out that the province "experiences an earthquake every day, but only a small number of these quakes are noticeable and even fewer result in damage."