People ran into the streets as alarms sounded after the magnitude 6.1 quake hit the state of Oaxaca, some 520km (323 miles) south of Mexico City.
A strong 6.2 magnitude quake shook Mexico on Saturday, causing panic in traumatized Mexico City, where rescuers trying to free people trapped from this week's earlier natural disaster had to suspend work.
U.S. 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.
It was strong enough to activate seismic alarms in Mexico City, which is still recovering from Tuesday's event. The Federal Police agency said the bridge already been closed due to damage after the September 7 quake.
It was not immediately clear if there were any new injuries or damage from the latest quake.
Mexican officials said the temblor on Saturday morning was likely an aftershock of the September 7 quake.
Just days after a deadly quake which killed almost 300 people in Mexico City, the region is again hit with a second round of tremors. Forty-nine-year-old Jesus Gonzalez fell from a third-story balcony of a building where he was working when Tuesday's magnitude 7.1 quake hit, and he's awaiting surgery.
Alejandra Castellanos was on the second floor of a hotel in a central neighborhood and ran down the stairs and outside with her husband.
Mexico is still reeling from the previous natural calamity as it had caused serious damage to lives and property in the region.
For Patricia Fernandez Romero, who spent the morning on a yellow folding stool under a handwritten list with the names of the 46 missing, it was remembering how badly her 27-year-old son, Ivan Colin Fernandez, sang and realizing how much she wanted to hear him again.
He said that two people had died from heart attacks - local media said the victims were women aged 58 and 83.
Lidia Albarran, whose niece was buried in the collapse of an office building a block away, heard the alarm and anxious that the latest quake could endanger those under the pile of rubble.
US President Donald Trump had offered search-and-rescue assistance to Mexican authorities, saying "God bless the people of Mexico City".
As quake rescue operations stretched into Day 5, Mexico City residents throughout the city held out hope that dozens still missing might be found alive. More than half of the dead -157 - perished in the capital, while another 73 died in the state of Morelos, 45 in Puebla, 13 in Mexico State, six in Guerrero and one in Oaxaca.
Rescue operations are still under way to save those are feared trapped under rubble.
"It's that you get to a point when you're so tense, when they don't come out to give us information", she said.