People help a victim after an earthquake hit Mexico City, Mexico, Sept 19, 2017. [Photo/Agencies]

Market stall vendor Edith Lopez, 25, said she was in a taxi a few blocks away when the quake struck Tuesday. She said she saw glass bursting out of the windows of some buildings. She was anxiously trying to locate her children, whom she had left in the care of her disabled mother.

Local media broadcast video of whitecap waves churning the city's normally placid canals of Xochimilco as boats bobbed up and down.

Mexico City's international airport suspended operations and was checking facilities for damage.

Much of Mexico City is built on former lakebed, and the soil can amplify the effects of earthquakes centered hundreds of miles away.

The new quake appeared to be unrelated to the magnitude 8.1 temblor that hit Sept. 7 off Mexico's southern coast and also was felt strongly in the capital.

US Geological Survey seismologist Paul Earle noted the epicenters of the two quakes were 400 miles (650 kilometers) apart and said most aftershocks are within (60 miles) 100 kilometers.

There have been 19 earthquakes of magnitude 6.5 or larger within 150 miles (250 kilometers) of Tuesday's quake over the past century, Earle said.

Earth usually has about 15 to 20 earthquakes this size or larger each year, Earle said.

Initial calculations showed that more than 30 million people would have felt moderate shaking from Tuesday's quake.

AP