Mexico's National Seismological Service reported that the quake occurred at 7:29 a.m. CDT, with an epicenter near the Oaxacan city of Pinotepa Nacional, close to the Pacific coast, about a seven-hour drive from the heavily populated capital.
"All is functioning normally," Mexico City Mayor Marcel Ebrard said in an early morning tweet, reporting that hospitals, the subway and water systems were all undamaged.
Government officials in surrounding municipalities also said they had discovered no damage after a first round of review.
A more powerful quake in March, with a magnitude of 7.4, remains fresh in the minds of Mexicans. Though Mexico City escaped serious consequences from that quake, it damaged at least 500 homes in the southern state of Guerrero and injured more than 100 people.
This week, Mexico City commemorated the anniversary of the Sept. 19, 1985, earthquake that killed at least 10,000 people.
Those commemorations included a massive disaster drill. In a simulation of a real quake response, thousands of residents poured out of buildings, and emergency response officials deployed to practice routines and test equipment that had been upgraded since the quake in 1985, when Mexicans were scandalized by a botched response from the then-ruling Institutional Revolutionary Party, or PRI.
Saturday's quake was barely felt in some parts of Mexico City, if at all. The head of the capital's civil protection division said there was more perceptible movement in the neighborhoods of Roma and Condesa, both of which were badly damaged in the 1985 quake but have since made a comeback as two of the city's trendiest and most culturally important areas.
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-- Richard Fausset and Cecilia Sanchez
Image: A map shows the approximate location of the epicenter of Saturday morning's earthquake in southern Mexico. Credit: U.S. Geological Survey