The U.S. Coast Guard is warning people in far southern Alaska to take precautions after a tsunami warning was issued for the region.

The warning was sparked by a strong earthquake Saturday night that shook off the west coast of Canada. The U.S. Geological Survey said the 7.7-magnitude earthquake hit in the Queen Charlotte Islands area, followed by a 5.8-magnitude aftershock several minutes later.

The National Weather Service issued a warning for coastal areas of southern Alaska including the small communities of Craig and Sitka. The U.S. Coast Guard in Alaska said it's trying to warn everyone with a boat on the water to prepare for a potential tsunami.

Lt. Bernard Auth of the Juneau Command Center said the Coast Guard was also working with local authorities to alert people in coastal towns to take precautions.

Lucy Jones, a USGS seismologist, said the earthquake likely would not generate a large tsunami.

"This isn't that big of an earthquake on tsunami scales," she said. "The really big tsunamis are usually up in the high 8s and 9s."

She said the earthquake occurred along a "fairly long" fault - "a plate 200 kilometers long" in a subduction zone, where one plate slips underneath another. Such quakes lift the sea floor and tend to cause tsunamis, she said.