In Coalcoman, Michoacan, near the quake's epicentre, buildings were damaged, but there were not immediate reports of injuries.
"It started slowly and then was really strong and continued and continued until it started to relent," said 16-year-old Carla Cárdenas, a resident of Coalcoman. Cárdenas ran out of her family's hotel and waited with neighbors.
She said the hotel and some homes along the street displayed cracks in walls and segments of facades and roofs had broken off.
"In the hotel, the roof of the parking area boomed and fell to the ground, and there are cracks in the walls on the second foor," Cárdenas said.
She said the town's hospital was seriously damaged, but she had so far not heard of anyone injured.
Mexico's National Civil Defense agency said that based on historic data of tsunamis in Mexico, variations of as much as 82cm were possible in coastal water levels near the epicentre. The US Tsunami Warning Center said that hazardous tsunami waves were possible for coasts within 300 kilometres of the epicentre.
Mexico City Mayor Claudia Sheinbaum tweeted that there were no reports of damage in the capital
Alarms for the new quake came less than an hour after a quake alarms warbled in a nationwide earthquake simulation marking major, deadly quakes that struck on the same date in 1985 and 2017.
"This is a coincidence," that this is the third Sept. 19 earthquake, said U.S. Geological Survey seismologist Paul Earle. "There's no physical reason or statistical bias toward earthquakes in any given month in Mexico."
Nor is there a season or month for big earthquakes anywhere on the globe, Earle said. But there is a predictable thing: People seek and sometimes find coincidences that look like patterns.
"We knew we'd get this question as soon as it happened," Earle said. "Sometimes there are just coincidences."
The quake was not related to or caused by the drill an hour or so earlier, nor was it connected to a damaging temblor in Taiwan the day before, Earle said.
Humberto Garza stood outside a restaurant in Mexico City's Roma neighborhood holding his 3-year son. Like many milling about outside after the earthquake, Garza said that the earthquake alarm sounded so soon after the annual simulation that he was not sure it was real.
"I heard the alarm, but it sounded really far away," he said.
Outside the city's environmental ombudsman's office, dozens of employees waited. Some appeared visibly shaken.
Power was out in parts of the city, including stoplights, snarling the capital's already notorious traffic.
