There have been 15 earthquakes of magnitude 7 or stronger since 1973 within 310 miles (500 kilometers) of Tuesday's quake. Weaker buildings collapse with each quake, leaving a cadre of stronger ones that can withstand the shaking.
The quake's epicenter was 200 miles (320 kilometers) south-southeast of Mexico City. Despite the distance, it was felt powerfully in the capital where office towers swayed violently and the streets filled with people fleeing buildings. Some people sat on curbs, head in their hands, to calm themselves.
Mexico City was badly damaged in 1985 when a magnitude-8.0 earthquake killed at least 10,000 people. But experts say Tuesday's quake was smaller and released far less energy.
Mexico City was built on an ancient lakebed and its spongy soil amplifies seismic waves, and the damage they cause. The 1985 quake destroyed 400 buildings and damaged thousands more.
The latest jolt "wouldn't have been nearly as effective at generating those deep bass tones" that caused the damage seen in 1985, USGS seismologist Susan Hough said in an email.
Victor Hugo Espindola Castro of Mexico's national seismological service said another factor was improved building standards following the 1985 quake.
"Many of the buildings that were damaged in 1985 were poorly constructed and from that came the new building regulations, so that now buildings are stronger," he said.
In Mexico City, telephone service was down and some neighborhoods were without power, according to Mayor Marcelo Ebrard, who set up a hotline for people to report damage.
A pedestrian bridge collapsed on an empty transit bus.