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[Updated at 4:00 p.m. ET] The U.S. Geological Survey has again revised the magnitude of the Mexican quake, down to 7.4.

[Updated at 3:29 p.m. ET] Though the epicenter was about 175 miles from Mexico City, earthquakes are a frightening experience for the city's more than 20 million residents of Mexico City. About 10,000 people perished in a massive quake in 1985.

The city, built on volcanic ash and clay, is particularly vulnerable to temblors.

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[Updated at 3:17 p.m. ET] Local authorities in Guerrero state have reported aftershocks, while residents in Oaxaca and Guerrero states and the eastern state of Veracruz reported that phone service had been knocked out in their areas.

[Updated at 3:12 p.m. ET] Pascal Clemens, a businessman in Acapulco, says he has lived in the city for 17 years, and Tuesday's earthquake was in the top five of those he's experienced in that time.

It was "a pretty strong one," he said.

[Updated at 3:03 p.m. ET] A real estate agent in Acapulco told CNN International that he was in an office building when the quake hit. He said that the area has felt tremors for a while now, “but not strong like this one.”

Footage in Mexico City showed people milling around outside office buildings moments after evacuating.

[Updated at 2:42 p.m. ET]  Tourists and residents told CNN they felt the earthquake in the resort city of Acapulco, which, according to the U.S. Geological Survey, is about 120 miles west of the epicenter.

Mexican President Felipe Calderon said in a Twitter post that there were no immediate reports of serious damage.

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In Mexico City, Mayor Marcelo Ebrard sent tweets saying there was no visible damage in the capital and that the sprawling city's strategic services and water system were functioning.

Regardless, he tweeted, numerous buildings were being evacuated.

[Updated at 2:33 p.m. ET] Eleven miles is about the same depth as the devastating Haitian quake of 2010, according to CNN meteorologist Chad Myers, who said the region can expect aftershocks.

The epicenter was about 15 miles east of Ometepec in the Guerrero region, according to the U.S. Geological Survey.

There were no immediate reports of damage or injuries.

[Updated at 2:28 p.m. ET] Updated information from the U.S. Geological Survey indicates it was a 7.6-magnitude earthquake. It struck at a depth of almost 11 miles.

[Updated at 2:24 p.m. ET] CNN's Nick Parker in Mexico City said the earthquake lasted about 10 or 15 seconds.

"The room I was in rattled. A few vases rattled around," he said.

[Updated at 2:21 p.m. ET] The U.S. Geological Survey is reporting that the temblor that struck the Mexican capital registered a preliminary magnitude of 7.9.

It struck in the Guerrero region, with its epicenter about 92 miles west of Oaxaca and about 120 miles east of Acapulco.

[Posted at 2:16 p.m. ET] Residents rushed into the streets of Mexico City on Tuesday after a strong earthquake hit.