Consecutive earthquakes measuring 7.0 and 5.7 have shattered highways and rocked buildings in Anchorage, Alaska 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 US 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.
The quake broke store windows, knocked items off shelves, opened cracks in a two-storey, downtown building, disrupted electrical service and disabled traffic lights, snarling traffic. It also threw a full-grown man out of his bathtub.
Flights at the airport were suspended for hours after the quake knocked out telephones and forced the evacuation of the control tower. And the Alaska oil pipeline was shut down while crews were sent to inspect it for damage.
Governor Bill Walker issued a disaster declaration. He was in a lift in a high-rise Anchorage office building and said it was a "rough ride" coming down.
Alaska was the site of the most powerful earthquake ever recorded in the US The 9.2-magnitude quake on March 27, 1964, was centred about 120km 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 in January southeast of Kodiak Island.
But it is rare for a quake this big to strike so close to such a heavily populated area.