ANCHORAGE, Alaska (AP) — A 6.6 magnitude earthquake has rocked buildings in Anchorage and caused lamp posts and trees to sway, prompting people to run out of offices and seek shelter under office desks.

The U.S. Geological Survey says the earthquake Friday morning was centered about 7 miles north of Alaska’s largest city.

This is what happened on the 6th floor of the Nesbett Courthouse during the Anchorage #earthquake. Both attorneys jumped under their desks. Evacuated the building after the shaking stopped. pic.twitter.com/dqHGPCv6XO — Heather Hintze (@HeatherHintze) November 30, 2018

An Associated Press reporter working in downtown Anchorage saw cracks in a 2-story building after the quake. It was unclear whether there were injuries.

People went back inside buildings after the earthquake but a smaller aftershock a short time later sent them running back into the streets again.

The National Tsunami Warning Center has issued a tsunami warning for coastal zones of southern Alaska following an earthquake.

The center said Friday that the warning was in effect for parts of the state’s Cook Inlet and the southern Kenai Peninsula.

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

Cereal boxes and packages of batteries littered the floor of a grocery store after the earthquake Tuesday morning that rocked buildings in Alaska’s largest city, and picture frames and mirrors were knocked from living room walls.

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.