Back-to-back earthquakes measuring 7.0 and 5.8 rocked buildings and buckled roads Friday morning in Anchorage, prompting people to run from their offices or take cover under desks.

The U.S. Geological Survey said the first and more powerful quake was centered about 7 miles (12 kilometers) north of Anchorage, Alaska's largest city.

Officials canceled a tsunami warning for coastal areas of southern Alaska. National Tsunami Warning Center senior technician Michael Burgy said officials monitored gauges to see if any underwater landslides would generate tsunami waves. Because there were none, the warning was canceled. 

Cracks could be seen in a two-story downtown Anchorage building. It was unclear whether there were injuries.

The operators of the 800-mile (1,290-kilometer) long trans-Alaska pipeline said they shut the system down as a precaution following the quakes.

Photographs posted to social media sites showed damage that included collapsed ceiling tiles at an Anchorage high school and buckled roadway pavement in places.

The quake knocked CNN affiliate KTUU off the air. Items fell from shelves at the station, news director Tracy Sabo told CNN.

The US Geological Survey has reported at least four aftershocks following the first quake. The largest, registering 5.8, was located in the city of Anchorage.

The NOAA alert said that "for other US and Canadian Pacific coasts in North America, the level of tsunami danger is being evaluated. Further information will be provided in supplementary messages."

Alaska averages 40,000 earthquakes per year, with more large quakes than the other 49 states combined.

Southern Alaska has a high risk of earthquakes due to tectonic plates sliding past each other under the region. According to the U.S. Geological Survey, the Pacific plate is sliding northwestward and plunges beneath the North American plate in southern Alaska, the Alaska Peninsula and the Aleutian Islands.

On March 27, 1964, Alaska was hit by a magnitude 9.2 earthquake, the strongest recorded in U.S. history, centered about 75 miles (120 kilometers) east of Anchorage. The quake, which lasted about 4½ minutes, and the tsunami it triggered claimed about 130 lives.

Anchorage lawyer Justin Capp says he was getting ready for work when he felt the shaking start.

He grabbed on to the doorframe in the hallway and the door slammed into his hands, scraping his fingers and hand.

Capp says he's lived in Anchorage eight years and that Tuesday's quake was the worst he had experienced.

Another lawyer, Hank Graper, was driving when the quake struck. He first thought his vehicle had a flat tire, then thought it was exploding. He realized it was an earthquake after he saw traffic poles swaying.

Graper called it the most "violent" earthquake he's experience in his 20 years in Anchorage.

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