Back-to-back earthquakes measuring 7.0 and 5.7 shattered highways and rocked buildings Friday in Anchorage and the surrounding area, sending people running into the streets and briefly triggering a tsunami warning for islands and coastal areas south of the city.

No tsunami arrived, and there were no immediate reports of deaths or serious injuries.

The U.S. Geological Survey said the first and more powerful quake was centred about 12 kilometres north of Anchorage, Alaska's largest city, with a population of about 300,000. People ran from their offices or took cover under desks. The 5.7 aftershock arrived within minutes, followed by a series of smaller quakes.

Items from shelves that came unbolted from a wall are strewn across Anchorage True Value Hardware's stockroom floor. Store owner Tim Craig said no one was injured in the store but hundreds of shelf items hit the floor. He said off-duty staffers as well as customers offered to help clean up. (Dan Joling/Associated Press)

"We just hung onto each other. You couldn't even stand," said Sheila Bailey, who was working at a high school cafeteria in Palmer when the quake struck. "It sounded and felt like the school was breaking apart."

A large section of an off-ramp near the Anchorage airport collapsed, marooning a car on a narrow island of pavement surrounded by deep chasms in the concrete. Several cars crashed at a major intersection in Wasilla, north of Anchorage, during the shaking.

Anchorage Police Chief Justin Doll said he had been told that parts of Glenn Highway, a scenic route that runs northeast out of the city past farms, mountains and glaciers, had "completely disappeared." Traffic in the three lanes heading out of the city was bumper-to-bumper and all but stopped Friday afternoon as emergency vehicles passed on the shoulder.

The quake broke store windows, knocked items off shelves, opened cracks in a two-storey building downtown, disrupted electrical service and disabled traffic lights, snarling traffic. It also threw a full-grown man out of his bathtub.

The U.S. Geological Survey said the first and more powerful quake was centred about 12 kilometres north of Anchorage, Alaska's largest city, with a population of about 300,000.

Flights at the airport were suspended for hours after the quake knocked out telephones and forced the evacuation of the control tower. And the 1,287-kilometre Alaska oil pipeline was shut down while crews were sent to inspect it for damage.

Anchorage's school system cancelled classes and asked parents to pick up their children while it examined buildings for gas leaks or other damage.

Fifteen-year-old Sadie Blake and other members of the Homer High School wrestling team were at an Anchorage school gymnasium waiting for a tournament to start when the bleachers started rocking "like crazy" and the lights went out.

Ceiling is falling down <a href="https://t.co/ZPY6fhEPrp">pic.twitter.com/ZPY6fhEPrp</a> —@Chase_AK

People started running down the bleachers in the dark, trying to get out.

"It was a gym full of screams," said team chaperone Ginny Grimes.

When it was over, Sadie said, there was only one thing she could do: "I started crying."

Jonathan Lettow was waiting with his five-year-old daughter and other children for a school bus near their home in Wasilla when the quake struck. The children got on the ground while Lettow tried to keep them calm.

"It's one of those things where in your head you think, 'OK, it's going to stop,' and you say that to yourself so many times in your head that finally you think, 'OK, maybe this isn't going to stop,"' he said.

Soon after the shaking ended, the school bus pulled up and the children boarded, but the driver stopped at a bridge and refused to go across because of deep cracks in the road, Lettow said.

Former Alaska governor Sarah Palin tweeted that her home was damaged: "Our family is intact — house is not. I imagine that's the case for many, many others." She posted a video of the inside of her parents' home, with broken dishes littering the kitchen floor. A large set of antlers appeared to have fallen off a wall of the living room.

🙏🏼 for Alaska. Our family is intact - house is not... I imagine that’s the case for many, many others. So thankful to be safe; praying for our state following the earthquake. —@SarahPalinUSA

Gov. Bill Walker issued a disaster declaration. He was in an elevator in a high-rise Anchorage office building and said it was a "rough ride" coming down. He described the quake as a 7.2, though it was unclear why his figure differed from that of the USGS.

Thrown from bathtub

In Kenai, southwest of Anchorage, Brandon Slaton was home alone and soaking in his bathtub when the earthquake struck. Slaton, who weighs 209 pounds, said it created a powerful back-and-forth sloshing that threw him out of the tub.

His 120-pound mastiff panicked and tried to run down the stairs, but the house was swaying so much that the dog was thrown into a wall and tumbled down the stairs, Slaton said.

Students stand amid fallen ceiling tiles and other debris at Chugiak High School in Anchorage. (Submitted by Clint Scholtisek)

Slaton ran into his son's room after the shaking stopped. The boy's fish was on the floor, gasping, its tank shattered. Slaton put the fish in a bowl.

"It was anarchy," he said. "There's no pictures left on the walls, there's no power, there's no fish tank left. Everything that's not tied down is broke."

7.2 earthquake here in Anchorage, Alaska. This is a video my dad took from the Minnesota exit ramp from international. 😰😰 <a href="https://t.co/1yOGj3yz9q">pic.twitter.com/1yOGj3yz9q</a> —@sarahh_mars

Alaska was the site of the most powerful earthquake ever recorded in the U.S. The 9.2-magnitude quake on March 27, 1964, was centred about 120 kilometres east of Anchorage. It and the tsunami it triggered claimed about 130 lives.

The state averages 40,000 earthquakes a year, with more large quakes than the 49 other states combined. Southern Alaska has a high risk of earthquakes because the Earth's plates slide past each other under the region.

Alaska has been hit by a number of powerful quakes over 7.0 in recent decades, including a 7.9 last January southeast of Kodiak Island. But it is rare for a quake this big to strike so close to such a heavily populated area.

David Harper was getting some coffee at a store when the low rumble began and intensified into something that sounded "like the building was just going to fall apart." He ran for the exit with other patrons.

"People who were outside were actively hugging each other," he said. "You could tell that it was a bad one."