Workers clear debris from a collapsed bridge that fell on a bus in Mexico City on Tuesday, following a 7.4-magnitude quake that shook Mexico. There were still no reports of any deaths. [Violeta Schmidt / Reuters]

A woman comforts crying children outside a school in the Roma neighborhood of Mexico City, after they were evacuated during the earthquake. [Alexandre Meneghini / Associated Press]

A powerful earthquake that shook Mexico from its massive capital to its resort-studded southern coast damaged hundreds of homes and sent thousands of panicked people fleeing from swaying office buildings, yet apparently didn't cause a single death.

As of early Wednesday, there were still no reports of deaths from Tuesday's 7.4-magnitude quake centered near the border between the southern states of Oaxaca and Guerrero, even after 10 aftershocks. Interior Secretary Alejandro Poire said on Tuesday night that nine people were injured in Oaxaca and two in Mexico City.

Seismologists and civil protection officials said that where the earthquake hit and how it hit appear to have limited the damage, along with improved construction in the capital following a massive 1985 quake.

There were reports of damaged buildings but none was reported to have collapsed on the Oaxaca side of the border, civil protection spokeswoman Cynthia Tovar said. In Guerrero, home to Acapulco, where little damage was reported, officials say about 800 homes were damaged and 60 collapsed.

Authorities said the absence of tall buildings in the mountainous rural area is one reason for the lack of casualties.

"Another factor to consider is how tested an area has been," said USGS seismologist Susan Hoover.

There have been 15 earthquakes of magnitude 7 or stronger since 1973 within 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 320 km 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 8.0-magnitude 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 e-mail.

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.

About 40 passengers were stranded for a short time on the Mexico City airport air train, but later released. The airport closed briefly but officials said there was no runway damage and they resumed operations.

In the working-class neighborhood of Guerrero, pieces of concrete and plaster fell off some buildings and others had fresh cracks.

"I thought I was having a stroke at first. The earth danced. It was strong," said 42-year-old homemaker Marianela Jimenez. "But it looks like it will end up being some dust and a bad scare."

Groups of women hugged and cried at Mexico City's Angel of Independence monument, where hundreds of people evacuated from office buildings said they never had felt such a strong earthquake.