US Geological Survey geophysicist Paul Caruso said the new temblor was an aftershock of the 8.1 quake, and after a jolt of that size even buildings left standing can be more vulnerable.
"I heard the alarm and ran downstairs with my family", said Sergio Cedillo, a resident who was watching rescue efforts when the alarm sounded.
The United States Geological Survey reports of a 6.2 magnitude quake that hit Mexico's southern state of Oaxaca, Sputnik reported.
The time is nearing when rescuers will be replaced by bulldozers to clear the rubble, but officials insisted it is still a rescue operation.
Across the city, thousands of rescue workers and special teams using sniffer dogs and heat sensors combed wreckage, while the massive outpouring of support from volunteers sparked global praise for Mexico's spirit.
Another strong natural disaster has hit Mexico - just days after a devastating tremor left 295 people dead.
Saturday's emissions come shortly after three powerful quakes that together have killed more than 400 people.
Patricia Fernandez's 27-year-old nephew, Ivan Colin Fernandez, worked as an accountant in the seven-story building, which pancaked to the ground, taking part of the building next door with it.
"We are going to need a lot of help long term, especially when people forget about the news and time goes by", Espinosa said.
The State Government said in a statement that homes collapsed, but "no human lives were lost". "But we won't give up", the civil protection service, Luis Felipe Puente, vowed.
There were no reports of new damage in the capital and rescue efforts were still the priority, Mexico City's Mayor Miguel Ángel Mancera said on Twitter.
"I think this perhaps facilitated the shaking we saw two days ago", Hayes said.
So far there have been no reports of injuries, fatalities or damage, reports news outlet 24 Horas.
An quake measuring 6.2 on the Richter scale struck southern Mexico on Saturday, spreading fear in the capital as rescue efforts were on after the previous disaster.
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