Many buildings in the area are constructed of thick, unreinforced masonry walls, with timber roof beams supporting clay tile roofs.
Scientists are studying how this latest quake happened.
The quake occurred near the point of collision between three tectonic plates, the Cocos, the Caribbean and the North American.
"It's a big quake".
Its epicenter was off the coast of Chiapas, said the United States Geological Survey.
"I don't know how you can make sense of it".
More than 200 people were injured across Mexico, officials said. More than half of the victims were in Juchitan in Oaxaca state.
In Tabasco state, two children were among the dead, officials said.
Rescuers pried several survivors from the rubble in Juchitan Friday, but also discovered others who had died.
The quake also triggered a tsunami warning and the evacuation of thousands of people in coastal communities in Chiapas; however, the warning was later lifted.
The Mexican Seismological Service recorded 721 aftershocks in the wake of Thursday's 8.1-magnitude temblor. A state senator from Chiapas told The Daily Beast, "We could be looking at more than 10,000 people facing crisis".
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The quake struck at 11:49 p.m. Thursday.
The quake also triggered a tsunami warning and the evacuation of thousands of people in coastal communities in Chiapas. No further details have been provided. The massive 8.1 quake killed at least 61 people, authorities said. At least 90 deaths have been affirmed so far and the fatalities are expected to continue to increase.
Authorities say that a hotel in Oaxaca has collapsed, but no one has been reported dead.
Reports indicate a hospital and the town hall are among the collapsed structures.
Details remain sketchy this morning but Oaxaca Governor Alejandro Murat Hinojosa said 17 people died in Juchitán in the Isthmus of Tehuantepec.
The Maldivian government has expressed condolences to Mexican President Enrique Peña Nieto following Thursday's powerful natural disaster that killed dozens. And it's likely to strike land just about a day after the country was hit by a major, magnitude 8.1 natural disaster.
Almost 2,900 people have been evacuated from their homes in Veracruz, and 1,500 more relocated to storm shelters in the neighboring Puebla state, AP reports.
Residents braved hundreds of aftershocks to clear the city's streets, but a number of funeral processions brought traffic to a standstill, halting military convoys that were transporting food, shovels and sledgehammers.
Earlier President Enrique Pena Nieto said more than 770 tremors have been registered in the aftermath of the strongest quake to hit Mexico since 1932.
The Pacific Tsunami Warning Center reported waves of 3.3 feet (1 meter) above the tide level off Salina Cruz, Mexico.